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-
- Debugging on Linux for s/390 & z/Architecture
- by
- Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
- Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation
- Best viewed with fixed width fonts
-
-Overview of Document:
-=====================
-This document is intended to give a good overview of how to debug
-Linux for s/390 & z/Architecture. It isn't intended as a complete reference & not a
-tutorial on the fundamentals of C & assembly. It doesn't go into
-390 IO in any detail. It is intended to complement the documents in the
-reference section below & any other worthwhile references you get.
-
-It is intended like the Enterprise Systems Architecture/390 Reference Summary
-to be printed out & used as a quick cheat sheet self help style reference when
-problems occur.
-
-Contents
-========
-Register Set
-Address Spaces on Intel Linux
-Address Spaces on Linux for s/390 & z/Architecture
-The Linux for s/390 & z/Architecture Kernel Task Structure
-Register Usage & Stackframes on Linux for s/390 & z/Architecture
-A sample program with comments
-Compiling programs for debugging on Linux for s/390 & z/Architecture
-Figuring out gcc compile errors
-Debugging Tools
-objdump
-strace
-Performance Debugging
-Debugging under VM
-s/390 & z/Architecture IO Overview
-Debugging IO on s/390 & z/Architecture under VM
-GDB on s/390 & z/Architecture
-Stack chaining in gdb by hand
-Examining core dumps
-ldd
-Debugging modules
-The proc file system
-Starting points for debugging scripting languages etc.
-SysRq
-References
-Special Thanks
-
-Register Set
-============
-The current architectures have the following registers.
-
-16 General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing.
-
-16 Control registers, 32 bit on s/390 64 bit on z/Architecture, ( cr0-cr15 kernel usage only ) used for memory management,
-interrupt control,debugging control etc.
-
-16 Access registers ( ar0-ar15 ) 32 bit on s/390 & z/Architecture
-not used by normal programs but potentially could
-be used as temporary storage. Their main purpose is their 1 to 1
-association with general purpose registers and are used in
-the kernel for copying data between kernel & user address spaces.
-Access register 0 ( & access register 1 on z/Architecture ( needs 64 bit
-pointer ) ) is currently used by the pthread library as a pointer to
-the current running threads private area.
-
-16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating
-point format compliant on G5 upwards & a Floating point control reg (FPC)
-4 64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines.
-Note:
-Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines,
-( provided the kernel is configured for this ).
-
-
-The PSW is the most important register on the machine it
-is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of
-a program counter (pc), condition code register,memory space designator.
-In IBM standard notation I am counting bit 0 as the MSB.
-It has several advantages over a normal program counter
-in that you can change address translation & program counter
-in a single instruction. To change address translation,
-e.g. switching address translation off requires that you
-have a logical=physical mapping for the address you are
-currently running at.
-
- Bit Value
-s/390 z/Architecture
-0 0 Reserved ( must be 0 ) otherwise specification exception occurs.
-
-1 1 Program Event Recording 1 PER enabled,
- PER is used to facilitate debugging e.g. single stepping.
-
-2-4 2-4 Reserved ( must be 0 ).
-
-5 5 Dynamic address translation 1=DAT on.
-
-6 6 Input/Output interrupt Mask
-
-7 7 External interrupt Mask used primarily for interprocessor signalling &
- clock interrupts.
-
-8-11 8-11 PSW Key used for complex memory protection mechanism not used under linux
-
-12 12 1 on s/390 0 on z/Architecture
-
-13 13 Machine Check Mask 1=enable machine check interrupts
-
-14 14 Wait State set this to 1 to stop the processor except for interrupts & give
- time to other LPARS used in CPU idle in the kernel to increase overall
- usage of processor resources.
-
-15 15 Problem state ( if set to 1 certain instructions are disabled )
- all linux user programs run with this bit 1
- ( useful info for debugging under VM ).
-
-16-17 16-17 Address Space Control
-
- 00 Primary Space Mode when DAT on
- The linux kernel currently runs in this mode, CR1 is affiliated with
- this mode & points to the primary segment table origin etc.
-
- 01 Access register mode this mode is used in functions to
- copy data between kernel & user space.
-
- 10 Secondary space mode not used in linux however CR7 the
- register affiliated with this mode is & this & normally
- CR13=CR7 to allow us to copy data between kernel & user space.
- We do this as follows:
- We set ar2 to 0 to designate its
- affiliated gpr ( gpr2 )to point to primary=kernel space.
- We set ar4 to 1 to designate its
- affiliated gpr ( gpr4 ) to point to secondary=home=user space
- & then essentially do a memcopy(gpr2,gpr4,size) to
- copy data between the address spaces, the reason we use home space for the
- kernel & don't keep secondary space free is that code will not run in
- secondary space.
-
- 11 Home Space Mode all user programs run in this mode.
- it is affiliated with CR13.
-
-18-19 18-19 Condition codes (CC)
-
-20 20 Fixed point overflow mask if 1=FPU exceptions for this event
- occur ( normally 0 )
-
-21 21 Decimal overflow mask if 1=FPU exceptions for this event occur
- ( normally 0 )
-
-22 22 Exponent underflow mask if 1=FPU exceptions for this event occur
- ( normally 0 )
-
-23 23 Significance Mask if 1=FPU exceptions for this event occur
- ( normally 0 )
-
-24-31 24-30 Reserved Must be 0.
-
- 31 Extended Addressing Mode
- 32 Basic Addressing Mode
- Used to set addressing mode
- PSW 31 PSW 32
- 0 0 24 bit
- 0 1 31 bit
- 1 1 64 bit
-
-32 1=31 bit addressing mode 0=24 bit addressing mode (for backward
- compatibility), linux always runs with this bit set to 1
-
-33-64 Instruction address.
- 33-63 Reserved must be 0
- 64-127 Address
- In 24 bits mode bits 64-103=0 bits 104-127 Address
- In 31 bits mode bits 64-96=0 bits 97-127 Address
- Note: unlike 31 bit mode on s/390 bit 96 must be zero
- when loading the address with LPSWE otherwise a
- specification exception occurs, LPSW is fully backward
- compatible.
-
-
-Prefix Page(s)
---------------
-This per cpu memory area is too intimately tied to the processor not to mention.
-It exists between the real addresses 0-4096 on s/390 & 0-8192 z/Architecture & is exchanged
-with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set
-prefix instruction in linux'es startup.
-This page is mapped to a different prefix for each processor in an SMP configuration
-( assuming the os designer is sane of course :-) ).
-Bytes 0-512 ( 200 hex ) on s/390 & 0-512,4096-4544,4604-5119 currently on z/Architecture
-are used by the processor itself for holding such information as exception indications &
-entry points for exceptions.
-Bytes after 0xc00 hex are used by linux for per processor globals on s/390 & z/Architecture
-( there is a gap on z/Architecture too currently between 0xc00 & 1000 which linux uses ).
-The closest thing to this on traditional architectures is the interrupt
-vector table. This is a good thing & does simplify some of the kernel coding
-however it means that we now cannot catch stray NULL pointers in the
-kernel without hard coded checks.
-
-
-
-Address Spaces on Intel Linux
-=============================
-
-The traditional Intel Linux is approximately mapped as follows forgive
-the ascii art.
-0xFFFFFFFF 4GB Himem *****************
- * *
- * Kernel Space *
- * *
- ***************** ****************
-User Space Himem (typically 0xC0000000 3GB )* User Stack * * *
- ***************** * *
- * Shared Libs * * Next Process *
- ***************** * to *
- * * <== * Run * <==
- * User Program * * *
- * Data BSS * * *
- * Text * * *
- * Sections * * *
-0x00000000 ***************** ****************
-
-Now it is easy to see that on Intel it is quite easy to recognise a kernel address
-as being one greater than user space himem ( in this case 0xC0000000).
-& addresses of less than this are the ones in the current running program on this
-processor ( if an smp box ).
-If using the virtual machine ( VM ) as a debugger it is quite difficult to
-know which user process is running as the address space you are looking at
-could be from any process in the run queue.
-
-The limitation of Intels addressing technique is that the linux
-kernel uses a very simple real address to virtual addressing technique
-of Real Address=Virtual Address-User Space Himem.
-This means that on Intel the kernel linux can typically only address
-Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines
-can typically use.
-They can lower User Himem to 2GB or lower & thus be
-able to use 2GB of RAM however this shrinks the maximum size
-of User Space from 3GB to 2GB they have a no win limit of 4GB unless
-they go to 64 Bit.
-
-
-On 390 our limitations & strengths make us slightly different.
-For backward compatibility we are only allowed use 31 bits (2GB)
-of our 32 bit addresses, however, we use entirely separate address
-spaces for the user & kernel.
-
-This means we can support 2GB of non Extended RAM on s/390, & more
-with the Extended memory management swap device &
-currently 4TB of physical memory currently on z/Architecture.
-
-
-Address Spaces on Linux for s/390 & z/Architecture
-==================================================
-
-Our addressing scheme is as follows
-
-
-Himem 0x7fffffff 2GB on s/390 ***************** ****************
-currently 0x3ffffffffff (2^42)-1 * User Stack * * *
-on z/Architecture. ***************** * *
- * Shared Libs * * *
- ***************** * *
- * * * Kernel *
- * User Program * * *
- * Data BSS * * *
- * Text * * *
- * Sections * * *
-0x00000000 ***************** ****************
-
-This also means that we need to look at the PSW problem state bit
-or the addressing mode to decide whether we are looking at
-user or kernel space.
-
-Virtual Addresses on s/390 & z/Architecture
-===========================================
-
-A virtual address on s/390 is made up of 3 parts
-The SX ( segment index, roughly corresponding to the PGD & PMD in linux terminology )
-being bits 1-11.
-The PX ( page index, corresponding to the page table entry (pte) in linux terminology )
-being bits 12-19.
-The remaining bits BX (the byte index are the offset in the page )
-i.e. bits 20 to 31.
-
-On z/Architecture in linux we currently make up an address from 4 parts.
-The region index bits (RX) 0-32 we currently use bits 22-32
-The segment index (SX) being bits 33-43
-The page index (PX) being bits 44-51
-The byte index (BX) being bits 52-63
-
-Notes:
-1) s/390 has no PMD so the PMD is really the PGD also.
-A lot of this stuff is defined in pgtable.h.
-
-2) Also seeing as s/390's page indexes are only 1k in size
-(bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k )
-to make the best use of memory by updating 4 segment indices
-entries each time we mess with a PMD & use offsets
-0,1024,2048 & 3072 in this page as for our segment indexes.
-On z/Architecture our page indexes are now 2k in size
-( bits 12-19 x 8 bytes per pte ) we do a similar trick
-but only mess with 2 segment indices each time we mess with
-a PMD.
-
-3) As z/Architecture supports up to a massive 5-level page table lookup we
-can only use 3 currently on Linux ( as this is all the generic kernel
-currently supports ) however this may change in future
-this allows us to access ( according to my sums )
-4TB of virtual storage per process i.e.
-4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes,
-enough for another 2 or 3 of years I think :-).
-to do this we use a region-third-table designation type in
-our address space control registers.
-
-
-The Linux for s/390 & z/Architecture Kernel Task Structure
-==========================================================
-Each process/thread under Linux for S390 has its own kernel task_struct
-defined in linux/include/linux/sched.h
-The S390 on initialisation & resuming of a process on a cpu sets
-the __LC_KERNEL_STACK variable in the spare prefix area for this cpu
-(which we use for per-processor globals).
-
-The kernel stack pointer is intimately tied with the task structure for
-each processor as follows.
-
- s/390
- ************************
- * 1 page kernel stack *
- * ( 4K ) *
- ************************
- * 1 page task_struct *
- * ( 4K ) *
-8K aligned ************************
-
- z/Architecture
- ************************
- * 2 page kernel stack *
- * ( 8K ) *
- ************************
- * 2 page task_struct *
- * ( 8K ) *
-16K aligned ************************
-
-What this means is that we don't need to dedicate any register or global variable
-to point to the current running process & can retrieve it with the following
-very simple construct for s/390 & one very similar for z/Architecture.
-
-static inline struct task_struct * get_current(void)
-{
- struct task_struct *current;
- __asm__("lhi %0,-8192\n\t"
- "nr %0,15"
- : "=r" (current) );
- return current;
-}
-
-i.e. just anding the current kernel stack pointer with the mask -8192.
-Thankfully because Linux doesn't have support for nested IO interrupts
-& our devices have large buffers can survive interrupts being shut for
-short amounts of time we don't need a separate stack for interrupts.
-
-
-
-
-Register Usage & Stackframes on Linux for s/390 & z/Architecture
-=================================================================
-Overview:
----------
-This is the code that gcc produces at the top & the bottom of
-each function. It usually is fairly consistent & similar from
-function to function & if you know its layout you can probably
-make some headway in finding the ultimate cause of a problem
-after a crash without a source level debugger.
-
-Note: To follow stackframes requires a knowledge of C or Pascal &
-limited knowledge of one assembly language.
-
-It should be noted that there are some differences between the
-s/390 & z/Architecture stack layouts as the z/Architecture stack layout didn't have
-to maintain compatibility with older linkage formats.
-
-Glossary:
----------
-alloca:
-This is a built in compiler function for runtime allocation
-of extra space on the callers stack which is obviously freed
-up on function exit ( e.g. the caller may choose to allocate nothing
-of a buffer of 4k if required for temporary purposes ), it generates
-very efficient code ( a few cycles ) when compared to alternatives
-like malloc.
-
-automatics: These are local variables on the stack,
-i.e they aren't in registers & they aren't static.
-
-back-chain:
-This is a pointer to the stack pointer before entering a
-framed functions ( see frameless function ) prologue got by
-dereferencing the address of the current stack pointer,
- i.e. got by accessing the 32 bit value at the stack pointers
-current location.
-
-base-pointer:
-This is a pointer to the back of the literal pool which
-is an area just behind each procedure used to store constants
-in each function.
-
-call-clobbered: The caller probably needs to save these registers if there
-is something of value in them, on the stack or elsewhere before making a
-call to another procedure so that it can restore it later.
-
-epilogue:
-The code generated by the compiler to return to the caller.
-
-frameless-function
-A frameless function in Linux for s390 & z/Architecture is one which doesn't
-need more than the register save area ( 96 bytes on s/390, 160 on z/Architecture )
-given to it by the caller.
-A frameless function never:
-1) Sets up a back chain.
-2) Calls alloca.
-3) Calls other normal functions
-4) Has automatics.
-
-GOT-pointer:
-This is a pointer to the global-offset-table in ELF
-( Executable Linkable Format, Linux'es most common executable format ),
-all globals & shared library objects are found using this pointer.
-
-lazy-binding
-ELF shared libraries are typically only loaded when routines in the shared
-library are actually first called at runtime. This is lazy binding.
-
-procedure-linkage-table
-This is a table found from the GOT which contains pointers to routines
-in other shared libraries which can't be called to by easier means.
-
-prologue:
-The code generated by the compiler to set up the stack frame.
-
-outgoing-args:
-This is extra area allocated on the stack of the calling function if the
-parameters for the callee's cannot all be put in registers, the same
-area can be reused by each function the caller calls.
-
-routine-descriptor:
-A COFF executable format based concept of a procedure reference
-actually being 8 bytes or more as opposed to a simple pointer to the routine.
-This is typically defined as follows
-Routine Descriptor offset 0=Pointer to Function
-Routine Descriptor offset 4=Pointer to Table of Contents
-The table of contents/TOC is roughly equivalent to a GOT pointer.
-& it means that shared libraries etc. can be shared between several
-environments each with their own TOC.
-
-
-static-chain: This is used in nested functions a concept adopted from pascal
-by gcc not used in ansi C or C++ ( although quite useful ), basically it
-is a pointer used to reference local variables of enclosing functions.
-You might come across this stuff once or twice in your lifetime.
-
-e.g.
-The function below should return 11 though gcc may get upset & toss warnings
-about unused variables.
-int FunctionA(int a)
-{
- int b;
- FunctionC(int c)
- {
- b=c+1;
- }
- FunctionC(10);
- return(b);
-}
-
-
-s/390 & z/Architecture Register usage
-=====================================
-r0 used by syscalls/assembly call-clobbered
-r1 used by syscalls/assembly call-clobbered
-r2 argument 0 / return value 0 call-clobbered
-r3 argument 1 / return value 1 (if long long) call-clobbered
-r4 argument 2 call-clobbered
-r5 argument 3 call-clobbered
-r6 argument 4 saved
-r7 pointer-to arguments 5 to ... saved
-r8 this & that saved
-r9 this & that saved
-r10 static-chain ( if nested function ) saved
-r11 frame-pointer ( if function used alloca ) saved
-r12 got-pointer saved
-r13 base-pointer saved
-r14 return-address saved
-r15 stack-pointer saved
-
-f0 argument 0 / return value ( float/double ) call-clobbered
-f2 argument 1 call-clobbered
-f4 z/Architecture argument 2 saved
-f6 z/Architecture argument 3 saved
-The remaining floating points
-f1,f3,f5 f7-f15 are call-clobbered.
-
-Notes:
-------
-1) The only requirement is that registers which are used
-by the callee are saved, e.g. the compiler is perfectly
-capable of using r11 for purposes other than a frame a
-frame pointer if a frame pointer is not needed.
-2) In functions with variable arguments e.g. printf the calling procedure
-is identical to one without variable arguments & the same number of
-parameters. However, the prologue of this function is somewhat more
-hairy owing to it having to move these parameters to the stack to
-get va_start, va_arg & va_end to work.
-3) Access registers are currently unused by gcc but are used in
-the kernel. Possibilities exist to use them at the moment for
-temporary storage but it isn't recommended.
-4) Only 4 of the floating point registers are used for
-parameter passing as older machines such as G3 only have only 4
-& it keeps the stack frame compatible with other compilers.
-However with IEEE floating point emulation under linux on the
-older machines you are free to use the other 12.
-5) A long long or double parameter cannot be have the
-first 4 bytes in a register & the second four bytes in the
-outgoing args area. It must be purely in the outgoing args
-area if crossing this boundary.
-6) Floating point parameters are mixed with outgoing args
-on the outgoing args area in the order the are passed in as parameters.
-7) Floating point arguments 2 & 3 are saved in the outgoing args area for
-z/Architecture
-
-
-Stack Frame Layout
-------------------
-s/390 z/Architecture
-0 0 back chain ( a 0 here signifies end of back chain )
-4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats )
-8 16 glue used in other s/390 linkage formats for saved routine descriptors etc.
-12 24 glue used in other s/390 linkage formats for saved routine descriptors etc.
-16 32 scratch area
-20 40 scratch area
-24 48 saved r6 of caller function
-28 56 saved r7 of caller function
-32 64 saved r8 of caller function
-36 72 saved r9 of caller function
-40 80 saved r10 of caller function
-44 88 saved r11 of caller function
-48 96 saved r12 of caller function
-52 104 saved r13 of caller function
-56 112 saved r14 of caller function
-60 120 saved r15 of caller function
-64 128 saved f4 of caller function
-72 132 saved f6 of caller function
-80 undefined
-96 160 outgoing args passed from caller to callee
-96+x 160+x possible stack alignment ( 8 bytes desirable )
-96+x+y 160+x+y alloca space of caller ( if used )
-96+x+y+z 160+x+y+z automatics of caller ( if used )
-0 back-chain
-
-A sample program with comments.
-===============================
-
-Comments on the function test
------------------------------
-1) It didn't need to set up a pointer to the constant pool gpr13 as it isn't used
-( :-( ).
-2) This is a frameless function & no stack is bought.
-3) The compiler was clever enough to recognise that it could return the
-value in r2 as well as use it for the passed in parameter ( :-) ).
-4) The basr ( branch relative & save ) trick works as follows the instruction
-has a special case with r0,r0 with some instruction operands is understood as
-the literal value 0, some risc architectures also do this ). So now
-we are branching to the next address & the address new program counter is
-in r13,so now we subtract the size of the function prologue we have executed
-+ the size of the literal pool to get to the top of the literal pool
-0040037c int test(int b)
-{ # Function prologue below
- 40037c: 90 de f0 34 stm %r13,%r14,52(%r15) # Save registers r13 & r14
- 400380: 0d d0 basr %r13,%r0 # Set up pointer to constant pool using
- 400382: a7 da ff fa ahi %r13,-6 # basr trick
- return(5+b);
- # Huge main program
- 400386: a7 2a 00 05 ahi %r2,5 # add 5 to r2
-
- # Function epilogue below
- 40038a: 98 de f0 34 lm %r13,%r14,52(%r15) # restore registers r13 & 14
- 40038e: 07 fe br %r14 # return
-}
-
-Comments on the function main
------------------------------
-1) The compiler did this function optimally ( 8-) )
-
-Literal pool for main.
-400390: ff ff ff ec .long 0xffffffec
-main(int argc,char *argv[])
-{ # Function prologue below
- 400394: 90 bf f0 2c stm %r11,%r15,44(%r15) # Save necessary registers
- 400398: 18 0f lr %r0,%r15 # copy stack pointer to r0
- 40039a: a7 fa ff a0 ahi %r15,-96 # Make area for callee saving
- 40039e: 0d d0 basr %r13,%r0 # Set up r13 to point to
- 4003a0: a7 da ff f0 ahi %r13,-16 # literal pool
- 4003a4: 50 00 f0 00 st %r0,0(%r15) # Save backchain
-
- return(test(5)); # Main Program Below
- 4003a8: 58 e0 d0 00 l %r14,0(%r13) # load relative address of test from
- # literal pool
- 4003ac: a7 28 00 05 lhi %r2,5 # Set first parameter to 5
- 4003b0: 4d ee d0 00 bas %r14,0(%r14,%r13) # jump to test setting r14 as return
- # address using branch & save instruction.
-
- # Function Epilogue below
- 4003b4: 98 bf f0 8c lm %r11,%r15,140(%r15)# Restore necessary registers.
- 4003b8: 07 fe br %r14 # return to do program exit
-}
-
-
-Compiler updates
-----------------
-
-main(int argc,char *argv[])
-{
- 4004fc: 90 7f f0 1c stm %r7,%r15,28(%r15)
- 400500: a7 d5 00 04 bras %r13,400508 <main+0xc>
- 400504: 00 40 04 f4 .long 0x004004f4
- # compiler now puts constant pool in code to so it saves an instruction
- 400508: 18 0f lr %r0,%r15
- 40050a: a7 fa ff a0 ahi %r15,-96
- 40050e: 50 00 f0 00 st %r0,0(%r15)
- return(test(5));
- 400512: 58 10 d0 00 l %r1,0(%r13)
- 400516: a7 28 00 05 lhi %r2,5
- 40051a: 0d e1 basr %r14,%r1
- # compiler adds 1 extra instruction to epilogue this is done to
- # avoid processor pipeline stalls owing to data dependencies on g5 &
- # above as register 14 in the old code was needed directly after being loaded
- # by the lm %r11,%r15,140(%r15) for the br %14.
- 40051c: 58 40 f0 98 l %r4,152(%r15)
- 400520: 98 7f f0 7c lm %r7,%r15,124(%r15)
- 400524: 07 f4 br %r4
-}
-
-
-Hartmut ( our compiler developer ) also has been threatening to take out the
-stack backchain in optimised code as this also causes pipeline stalls, you
-have been warned.
-
-64 bit z/Architecture code disassembly
---------------------------------------
-
-If you understand the stuff above you'll understand the stuff
-below too so I'll avoid repeating myself & just say that
-some of the instructions have g's on the end of them to indicate
-they are 64 bit & the stack offsets are a bigger,
-the only other difference you'll find between 32 & 64 bit is that
-we now use f4 & f6 for floating point arguments on 64 bit.
-00000000800005b0 <test>:
-int test(int b)
-{
- return(5+b);
- 800005b0: a7 2a 00 05 ahi %r2,5
- 800005b4: b9 14 00 22 lgfr %r2,%r2 # downcast to integer
- 800005b8: 07 fe br %r14
- 800005ba: 07 07 bcr 0,%r7
-
-
-}
-
-00000000800005bc <main>:
-main(int argc,char *argv[])
-{
- 800005bc: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15)
- 800005c2: b9 04 00 1f lgr %r1,%r15
- 800005c6: a7 fb ff 60 aghi %r15,-160
- 800005ca: e3 10 f0 00 00 24 stg %r1,0(%r15)
- return(test(5));
- 800005d0: a7 29 00 05 lghi %r2,5
- # brasl allows jumps > 64k & is overkill here bras would do fune
- 800005d4: c0 e5 ff ff ff ee brasl %r14,800005b0 <test>
- 800005da: e3 40 f1 10 00 04 lg %r4,272(%r15)
- 800005e0: eb bf f0 f8 00 04 lmg %r11,%r15,248(%r15)
- 800005e6: 07 f4 br %r4
-}
-
-
-
-Compiling programs for debugging on Linux for s/390 & z/Architecture
-====================================================================
--gdwarf-2 now works it should be considered the default debugging
-format for s/390 & z/Architecture as it is more reliable for debugging
-shared libraries, normal -g debugging works much better now
-Thanks to the IBM java compiler developers bug reports.
-
-This is typically done adding/appending the flags -g or -gdwarf-2 to the
-CFLAGS & LDFLAGS variables Makefile of the program concerned.
-
-If using gdb & you would like accurate displays of registers &
- stack traces compile without optimisation i.e make sure
-that there is no -O2 or similar on the CFLAGS line of the Makefile &
-the emitted gcc commands, obviously this will produce worse code
-( not advisable for shipment ) but it is an aid to the debugging process.
-
-This aids debugging because the compiler will copy parameters passed in
-in registers onto the stack so backtracing & looking at passed in
-parameters will work, however some larger programs which use inline functions
-will not compile without optimisation.
-
-Debugging with optimisation has since much improved after fixing
-some bugs, please make sure you are using gdb-5.0 or later developed
-after Nov'2000.
-
-Figuring out gcc compile errors
-===============================
-If you are getting a lot of syntax errors compiling a program & the problem
-isn't blatantly obvious from the source.
-It often helps to just preprocess the file, this is done with the -E
-option in gcc.
-What this does is that it runs through the very first phase of compilation
-( compilation in gcc is done in several stages & gcc calls many programs to
-achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp).
-The c preprocessor does the following, it joins all the files #included together
-recursively ( #include files can #include other files ) & also the c file you wish to compile.
-It puts a fully qualified path of the #included files in a comment & it
-does macro expansion.
-This is useful for debugging because
-1) You can double check whether the files you expect to be included are the ones
-that are being included ( e.g. double check that you aren't going to the i386 asm directory ).
-2) Check that macro definitions aren't clashing with typedefs,
-3) Check that definitions aren't being used before they are being included.
-4) Helps put the line emitting the error under the microscope if it contains macros.
-
-For convenience the Linux kernel's makefile will do preprocessing automatically for you
-by suffixing the file you want built with .i ( instead of .o )
-
-e.g.
-from the linux directory type
-make arch/s390/kernel/signal.i
-this will build
-
-s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
--fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -E arch/s390/kernel/signal.c
-> arch/s390/kernel/signal.i
-
-Now look at signal.i you should see something like.
-
-
-# 1 "/home1/barrow/linux/include/asm/types.h" 1
-typedef unsigned short umode_t;
-typedef __signed__ char __s8;
-typedef unsigned char __u8;
-typedef __signed__ short __s16;
-typedef unsigned short __u16;
-
-If instead you are getting errors further down e.g.
-unknown instruction:2515 "move.l" or better still unknown instruction:2515
-"Fixme not implemented yet, call Martin" you are probably are attempting to compile some code
-meant for another architecture or code that is simply not implemented, with a fixme statement
-stuck into the inline assembly code so that the author of the file now knows he has work to do.
-To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler )
-use the -S option.
-Again for your convenience the Linux kernel's Makefile will hold your hand &
-do all this donkey work for you also by building the file with the .s suffix.
-e.g.
-from the Linux directory type
-make arch/s390/kernel/signal.s
-
-s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
--fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -S arch/s390/kernel/signal.c
--o arch/s390/kernel/signal.s
-
-
-This will output something like, ( please note the constant pool & the useful comments
-in the prologue to give you a hand at interpreting it ).
-
-.LC54:
- .string "misaligned (__u16 *) in __xchg\n"
-.LC57:
- .string "misaligned (__u32 *) in __xchg\n"
-.L$PG1: # Pool sys_sigsuspend
-.LC192:
- .long -262401
-.LC193:
- .long -1
-.LC194:
- .long schedule-.L$PG1
-.LC195:
- .long do_signal-.L$PG1
- .align 4
-.globl sys_sigsuspend
- .type sys_sigsuspend,@function
-sys_sigsuspend:
-# leaf function 0
-# automatics 16
-# outgoing args 0
-# need frame pointer 0
-# call alloca 0
-# has varargs 0
-# incoming args (stack) 0
-# function length 168
- STM 8,15,32(15)
- LR 0,15
- AHI 15,-112
- BASR 13,0
-.L$CO1: AHI 13,.L$PG1-.L$CO1
- ST 0,0(15)
- LR 8,2
- N 5,.LC192-.L$PG1(13)
-
-Adding -g to the above output makes the output even more useful
-e.g. typing
-make CC:="s390-gcc -g" kernel/sched.s
-
-which compiles.
-s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce -S kernel/sched.c -o kernel/sched.s
-
-also outputs stabs ( debugger ) info, from this info you can find out the
-offsets & sizes of various elements in structures.
-e.g. the stab for the structure
-struct rlimit {
- unsigned long rlim_cur;
- unsigned long rlim_max;
-};
-is
-.stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0
-from this stab you can see that
-rlimit_cur starts at bit offset 0 & is 32 bits in size
-rlimit_max starts at bit offset 32 & is 32 bits in size.
-
-
-Debugging Tools:
-================
-
-objdump
-=======
-This is a tool with many options the most useful being ( if compiled with -g).
-objdump --source <victim program or object file> > <victims debug listing >
-
-
-The whole kernel can be compiled like this ( Doing this will make a 17MB kernel
-& a 200 MB listing ) however you have to strip it before building the image
-using the strip command to make it a more reasonable size to boot it.
-
-A source/assembly mixed dump of the kernel can be done with the line
-objdump --source vmlinux > vmlinux.lst
-Also, if the file isn't compiled -g, this will output as much debugging information
-as it can (e.g. function names). This is very slow as it spends lots
-of time searching for debugging info. The following self explanatory line should be used
-instead if the code isn't compiled -g, as it is much faster:
-objdump --disassemble-all --syms vmlinux > vmlinux.lst
-
-As hard drive space is valuable most of us use the following approach.
-1) Look at the emitted psw on the console to find the crash address in the kernel.
-2) Look at the file System.map ( in the linux directory ) produced when building
-the kernel to find the closest address less than the current PSW to find the
-offending function.
-3) use grep or similar to search the source tree looking for the source file
- with this function if you don't know where it is.
-4) rebuild this object file with -g on, as an example suppose the file was
-( /arch/s390/kernel/signal.o )
-5) Assuming the file with the erroneous function is signal.c Move to the base of the
-Linux source tree.
-6) rm /arch/s390/kernel/signal.o
-7) make /arch/s390/kernel/signal.o
-8) watch the gcc command line emitted
-9) type it in again or alternatively cut & paste it on the console adding the -g option.
-10) objdump --source arch/s390/kernel/signal.o > signal.lst
-This will output the source & the assembly intermixed, as the snippet below shows
-This will unfortunately output addresses which aren't the same
-as the kernel ones you should be able to get around the mental arithmetic
-by playing with the --adjust-vma parameter to objdump.
-
-
-
-
-static inline void spin_lock(spinlock_t *lp)
-{
- a0: 18 34 lr %r3,%r4
- a2: a7 3a 03 bc ahi %r3,956
- __asm__ __volatile(" lhi 1,-1\n"
- a6: a7 18 ff ff lhi %r1,-1
- aa: 1f 00 slr %r0,%r0
- ac: ba 01 30 00 cs %r0,%r1,0(%r3)
- b0: a7 44 ff fd jm aa <sys_sigsuspend+0x2e>
- saveset = current->blocked;
- b4: d2 07 f0 68 mvc 104(8,%r15),972(%r4)
- b8: 43 cc
- return (set->sig[0] & mask) != 0;
-}
-
-6) If debugging under VM go down to that section in the document for more info.
-
-
-I now have a tool which takes the pain out of --adjust-vma
-& you are able to do something like
-make /arch/s390/kernel/traps.lst
-& it automatically generates the correctly relocated entries for
-the text segment in traps.lst.
-This tool is now standard in linux distro's in scripts/makelst
-
-strace:
--------
-Q. What is it ?
-A. It is a tool for intercepting calls to the kernel & logging them
-to a file & on the screen.
-
-Q. What use is it ?
-A. You can use it to find out what files a particular program opens.
-
-
-
-Example 1
----------
-If you wanted to know does ping work but didn't have the source
-strace ping -c 1 127.0.0.1
-& then look at the man pages for each of the syscalls below,
-( In fact this is sometimes easier than looking at some spaghetti
-source which conditionally compiles for several architectures ).
-Not everything that it throws out needs to make sense immediately.
-
-Just looking quickly you can see that it is making up a RAW socket
-for the ICMP protocol.
-Doing an alarm(10) for a 10 second timeout
-& doing a gettimeofday call before & after each read to see
-how long the replies took, & writing some text to stdout so the user
-has an idea what is going on.
-
-socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3
-getuid() = 0
-setuid(0) = 0
-stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
-stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory)
-stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
-getpid() = 353
-setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0
-setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0
-fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0
-mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000
-ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0
-write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes
-) = 42
-sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0
-sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0
-gettimeofday({948904719, 138951}, NULL) = 0
-sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET,
-sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64
-sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
-sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
-alarm(10) = 0
-recvfrom(3, "E\0\0T\0005\0\0@\1|r\177\0\0\1\177"..., 192, 0,
-{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
-gettimeofday({948904719, 160224}, NULL) = 0
-recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0,
-{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
-gettimeofday({948904719, 166952}, NULL) = 0
-write(1, "64 bytes from 127.0.0.1: icmp_se"...,
-5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms
-
-Example 2
----------
-strace passwd 2>&1 | grep open
-produces the following output
-open("/etc/ld.so.cache", O_RDONLY) = 3
-open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory)
-open("/lib/libc.so.5", O_RDONLY) = 3
-open("/dev", O_RDONLY) = 3
-open("/var/run/utmp", O_RDONLY) = 3
-open("/etc/passwd", O_RDONLY) = 3
-open("/etc/shadow", O_RDONLY) = 3
-open("/etc/login.defs", O_RDONLY) = 4
-open("/dev/tty", O_RDONLY) = 4
-
-The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input
-through the pipe for each line containing the string open.
-
-
-Example 3
----------
-Getting sophisticated
-telnetd crashes & I don't know why
-
-Steps
------
-1) Replace the following line in /etc/inetd.conf
-telnet stream tcp nowait root /usr/sbin/in.telnetd -h
-with
-telnet stream tcp nowait root /blah
-
-2) Create the file /blah with the following contents to start tracing telnetd
-#!/bin/bash
-/usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h
-3) chmod 700 /blah to make it executable only to root
-4)
-killall -HUP inetd
-or ps aux | grep inetd
-get inetd's process id
-& kill -HUP inetd to restart it.
-
-Important options
------------------
--o is used to tell strace to output to a file in our case t1 in the root directory
--f is to follow children i.e.
-e.g in our case above telnetd will start the login process & subsequently a shell like bash.
-You will be able to tell which is which from the process ID's listed on the left hand side
-of the strace output.
--p<pid> will tell strace to attach to a running process, yup this can be done provided
- it isn't being traced or debugged already & you have enough privileges,
-the reason 2 processes cannot trace or debug the same program is that strace
-becomes the parent process of the one being debugged & processes ( unlike people )
-can have only one parent.
-
-
-However the file /t1 will get big quite quickly
-to test it telnet 127.0.0.1
-
-now look at what files in.telnetd execve'd
-413 execve("/usr/sbin/in.telnetd", ["/usr/sbin/in.telnetd", "-h"], [/* 17 vars */]) = 0
-414 execve("/bin/login", ["/bin/login", "-h", "localhost", "-p"], [/* 2 vars */]) = 0
-
-Whey it worked!.
-
-
-Other hints:
-------------
-If the program is not very interactive ( i.e. not much keyboard input )
-& is crashing in one architecture but not in another you can do
-an strace of both programs under as identical a scenario as you can
-on both architectures outputting to a file then.
-do a diff of the two traces using the diff program
-i.e.
-diff output1 output2
-& maybe you'll be able to see where the call paths differed, this
-is possibly near the cause of the crash.
-
-More info
----------
-Look at man pages for strace & the various syscalls
-e.g. man strace, man alarm, man socket.
-
-
-Performance Debugging
-=====================
-gcc is capable of compiling in profiling code just add the -p option
-to the CFLAGS, this obviously affects program size & performance.
-This can be used by the gprof gnu profiling tool or the
-gcov the gnu code coverage tool ( code coverage is a means of testing
-code quality by checking if all the code in an executable in exercised by
-a tester ).
-
-
-Using top to find out where processes are sleeping in the kernel
-----------------------------------------------------------------
-To do this copy the System.map from the root directory where
-the linux kernel was built to the /boot directory on your
-linux machine.
-Start top
-Now type fU<return>
-You should see a new field called WCHAN which
-tells you where each process is sleeping here is a typical output.
-
- 6:59pm up 41 min, 1 user, load average: 0.00, 0.00, 0.00
-28 processes: 27 sleeping, 1 running, 0 zombie, 0 stopped
-CPU states: 0.0% user, 0.1% system, 0.0% nice, 99.8% idle
-Mem: 254900K av, 45976K used, 208924K free, 0K shrd, 28636K buff
-Swap: 0K av, 0K used, 0K free 8620K cached
-
- PID USER PRI NI SIZE RSS SHARE WCHAN STAT LIB %CPU %MEM TIME COMMAND
- 750 root 12 0 848 848 700 do_select S 0 0.1 0.3 0:00 in.telnetd
- 767 root 16 0 1140 1140 964 R 0 0.1 0.4 0:00 top
- 1 root 8 0 212 212 180 do_select S 0 0.0 0.0 0:00 init
- 2 root 9 0 0 0 0 down_inte SW 0 0.0 0.0 0:00 kmcheck
-
-The time command
-----------------
-Another related command is the time command which gives you an indication
-of where a process is spending the majority of its time.
-e.g.
-time ping -c 5 nc
-outputs
-real 0m4.054s
-user 0m0.010s
-sys 0m0.010s
-
-Debugging under VM
-==================
-
-Notes
------
-Addresses & values in the VM debugger are always hex never decimal
-Address ranges are of the format <HexValue1>-<HexValue2> or <HexValue1>.<HexValue2>
-e.g. The address range 0x2000 to 0x3000 can be described as 2000-3000 or 2000.1000
-
-The VM Debugger is case insensitive.
-
-VM's strengths are usually other debuggers weaknesses you can get at any resource
-no matter how sensitive e.g. memory management resources,change address translation
-in the PSW. For kernel hacking you will reap dividends if you get good at it.
-
-The VM Debugger displays operators but not operands, probably because some
-of it was written when memory was expensive & the programmer was probably proud that
-it fitted into 2k of memory & the programmers & didn't want to shock hardcore VM'ers by
-changing the interface :-), also the debugger displays useful information on the same line &
-the author of the code probably felt that it was a good idea not to go over
-the 80 columns on the screen.
-
-As some of you are probably in a panic now this isn't as unintuitive as it may seem
-as the 390 instructions are easy to decode mentally & you can make a good guess at a lot
-of them as all the operands are nibble ( half byte aligned ) & if you have an objdump listing
-also it is quite easy to follow, if you don't have an objdump listing keep a copy of
-the s/390 Reference Summary & look at between pages 2 & 7 or alternatively the
-s/390 principles of operation.
-e.g. even I can guess that
-0001AFF8' LR 180F CC 0
-is a ( load register ) lr r0,r15
-
-Also it is very easy to tell the length of a 390 instruction from the 2 most significant
-bits in the instruction ( not that this info is really useful except if you are trying to
-make sense of a hexdump of code ).
-Here is a table
-Bits Instruction Length
-------------------------------------------
-00 2 Bytes
-01 4 Bytes
-10 4 Bytes
-11 6 Bytes
-
-
-
-
-The debugger also displays other useful info on the same line such as the
-addresses being operated on destination addresses of branches & condition codes.
-e.g.
-00019736' AHI A7DAFF0E CC 1
-000198BA' BRC A7840004 -> 000198C2' CC 0
-000198CE' STM 900EF068 >> 0FA95E78 CC 2
-
-
-
-Useful VM debugger commands
----------------------------
-
-I suppose I'd better mention this before I start
-to list the current active traces do
-Q TR
-there can be a maximum of 255 of these per set
-( more about trace sets later ).
-To stop traces issue a
-TR END.
-To delete a particular breakpoint issue
-TR DEL <breakpoint number>
-
-The PA1 key drops to CP mode so you can issue debugger commands,
-Doing alt c (on my 3270 console at least ) clears the screen.
-hitting b <enter> comes back to the running operating system
-from cp mode ( in our case linux ).
-It is typically useful to add shortcuts to your profile.exec file
-if you have one ( this is roughly equivalent to autoexec.bat in DOS ).
-file here are a few from mine.
-/* this gives me command history on issuing f12 */
-set pf12 retrieve
-/* this continues */
-set pf8 imm b
-/* goes to trace set a */
-set pf1 imm tr goto a
-/* goes to trace set b */
-set pf2 imm tr goto b
-/* goes to trace set c */
-set pf3 imm tr goto c
-
-
-
-Instruction Tracing
--------------------
-Setting a simple breakpoint
-TR I PSWA <address>
-To debug a particular function try
-TR I R <function address range>
-TR I on its own will single step.
-TR I DATA <MNEMONIC> <OPTIONAL RANGE> will trace for particular mnemonics
-e.g.
-TR I DATA 4D R 0197BC.4000
-will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000
-if you were inclined you could add traces for all branch instructions &
-suffix them with the run prefix so you would have a backtrace on screen
-when a program crashes.
-TR BR <INTO OR FROM> will trace branches into or out of an address.
-e.g.
-TR BR INTO 0 is often quite useful if a program is getting awkward & deciding
-to branch to 0 & crashing as this will stop at the address before in jumps to 0.
-TR I R <address range> RUN cmd d g
-single steps a range of addresses but stays running &
-displays the gprs on each step.
-
-
-
-Displaying & modifying Registers
---------------------------------
-D G will display all the gprs
-Adding a extra G to all the commands is necessary to access the full 64 bit
-content in VM on z/Architecture obviously this isn't required for access registers
-as these are still 32 bit.
-e.g. DGG instead of DG
-D X will display all the control registers
-D AR will display all the access registers
-D AR4-7 will display access registers 4 to 7
-CPU ALL D G will display the GRPS of all CPUS in the configuration
-D PSW will display the current PSW
-st PSW 2000 will put the value 2000 into the PSW &
-cause crash your machine.
-D PREFIX displays the prefix offset
-
-
-Displaying Memory
------------------
-To display memory mapped using the current PSW's mapping try
-D <range>
-To make VM display a message each time it hits a particular address & continue try
-D I<range> will disassemble/display a range of instructions.
-ST addr 32 bit word will store a 32 bit aligned address
-D T<range> will display the EBCDIC in an address ( if you are that way inclined )
-D R<range> will display real addresses ( without DAT ) but with prefixing.
-There are other complex options to display if you need to get at say home space
-but are in primary space the easiest thing to do is to temporarily
-modify the PSW to the other addressing mode, display the stuff & then
-restore it.
-
-
-
-Hints
------
-If you want to issue a debugger command without halting your virtual machine with the
-PA1 key try prefixing the command with #CP e.g.
-#cp tr i pswa 2000
-also suffixing most debugger commands with RUN will cause them not
-to stop just display the mnemonic at the current instruction on the console.
-If you have several breakpoints you want to put into your program &
-you get fed up of cross referencing with System.map
-you can do the following trick for several symbols.
-grep do_signal System.map
-which emits the following among other things
-0001f4e0 T do_signal
-now you can do
-
-TR I PSWA 0001f4e0 cmd msg * do_signal
-This sends a message to your own console each time do_signal is entered.
-( As an aside I wrote a perl script once which automatically generated a REXX
-script with breakpoints on every kernel procedure, this isn't a good idea
-because there are thousands of these routines & VM can only set 255 breakpoints
-at a time so you nearly had to spend as long pruning the file down as you would
-entering the msg's by hand ),however, the trick might be useful for a single object file.
-On linux'es 3270 emulator x3270 there is a very useful option under the file ment
-Save Screens In File this is very good of keeping a copy of traces.
-
-From CMS help <command name> will give you online help on a particular command.
-e.g.
-HELP DISPLAY
-
-Also CP has a file called profile.exec which automatically gets called
-on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session
-CP has a feature similar to doskey, it may be useful for you to
-use profile.exec to define some keystrokes.
-e.g.
-SET PF9 IMM B
-This does a single step in VM on pressing F8.
-SET PF10 ^
-This sets up the ^ key.
-which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly into some 3270 consoles.
-SET PF11 ^-
-This types the starting keystrokes for a sysrq see SysRq below.
-SET PF12 RETRIEVE
-This retrieves command history on pressing F12.
-
-
-Sometimes in VM the display is set up to scroll automatically this
-can be very annoying if there are messages you wish to look at
-to stop this do
-TERM MORE 255 255
-This will nearly stop automatic screen updates, however it will
-cause a denial of service if lots of messages go to the 3270 console,
-so it would be foolish to use this as the default on a production machine.
-
-
-Tracing particular processes
-----------------------------
-The kernel's text segment is intentionally at an address in memory that it will
-very seldom collide with text segments of user programs ( thanks Martin ),
-this simplifies debugging the kernel.
-However it is quite common for user processes to have addresses which collide
-this can make debugging a particular process under VM painful under normal
-circumstances as the process may change when doing a
-TR I R <address range>.
-Thankfully after reading VM's online help I figured out how to debug
-I particular process.
-
-Your first problem is to find the STD ( segment table designation )
-of the program you wish to debug.
-There are several ways you can do this here are a few
-1) objdump --syms <program to be debugged> | grep main
-To get the address of main in the program.
-tr i pswa <address of main>
-Start the program, if VM drops to CP on what looks like the entry
-point of the main function this is most likely the process you wish to debug.
-Now do a D X13 or D XG13 on z/Architecture.
-On 31 bit the STD is bits 1-19 ( the STO segment table origin )
-& 25-31 ( the STL segment table length ) of CR13.
-now type
-TR I R STD <CR13's value> 0.7fffffff
-e.g.
-TR I R STD 8F32E1FF 0.7fffffff
-Another very useful variation is
-TR STORE INTO STD <CR13's value> <address range>
-for finding out when a particular variable changes.
-
-An alternative way of finding the STD of a currently running process
-is to do the following, ( this method is more complex but
-could be quite convenient if you aren't updating the kernel much &
-so your kernel structures will stay constant for a reasonable period of
-time ).
-
-grep task /proc/<pid>/status
-from this you should see something like
-task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68
-This now gives you a pointer to the task structure.
-Now make CC:="s390-gcc -g" kernel/sched.s
-To get the task_struct stabinfo.
-( task_struct is defined in include/linux/sched.h ).
-Now we want to look at
-task->active_mm->pgd
-on my machine the active_mm in the task structure stab is
-active_mm:(4,12),672,32
-its offset is 672/8=84=0x54
-the pgd member in the mm_struct stab is
-pgd:(4,6)=*(29,5),96,32
-so its offset is 96/8=12=0xc
-
-so we'll
-hexdump -s 0xf160054 /dev/mem | more
-i.e. task_struct+active_mm offset
-to look at the active_mm member
-f160054 0fee cc60 0019 e334 0000 0000 0000 0011
-hexdump -s 0x0feecc6c /dev/mem | more
-i.e. active_mm+pgd offset
-feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010
-we get something like
-now do
-TR I R STD <pgd|0x7f> 0.7fffffff
-i.e. the 0x7f is added because the pgd only
-gives the page table origin & we need to set the low bits
-to the maximum possible segment table length.
-TR I R STD 0f2c007f 0.7fffffff
-on z/Architecture you'll probably need to do
-TR I R STD <pgd|0x7> 0.ffffffffffffffff
-to set the TableType to 0x1 & the Table length to 3.
-
-
-
-Tracing Program Exceptions
---------------------------
-If you get a crash which says something like
-illegal operation or specification exception followed by a register dump
-You can restart linux & trace these using the tr prog <range or value> trace option.
-
-
-
-The most common ones you will normally be tracing for is
-1=operation exception
-2=privileged operation exception
-4=protection exception
-5=addressing exception
-6=specification exception
-10=segment translation exception
-11=page translation exception
-
-The full list of these is on page 22 of the current s/390 Reference Summary.
-e.g.
-tr prog 10 will trace segment translation exceptions.
-tr prog on its own will trace all program interruption codes.
-
-Trace Sets
-----------
-On starting VM you are initially in the INITIAL trace set.
-You can do a Q TR to verify this.
-If you have a complex tracing situation where you wish to wait for instance
-till a driver is open before you start tracing IO, but know in your
-heart that you are going to have to make several runs through the code till you
-have a clue whats going on.
-
-What you can do is
-TR I PSWA <Driver open address>
-hit b to continue till breakpoint
-reach the breakpoint
-now do your
-TR GOTO B
-TR IO 7c08-7c09 inst int run
-or whatever the IO channels you wish to trace are & hit b
-
-To got back to the initial trace set do
-TR GOTO INITIAL
-& the TR I PSWA <Driver open address> will be the only active breakpoint again.
-
-
-Tracing linux syscalls under VM
--------------------------------
-Syscalls are implemented on Linux for S390 by the Supervisor call instruction (SVC) there 256
-possibilities of these as the instruction is made up of a 0xA opcode & the second byte being
-the syscall number. They are traced using the simple command.
-TR SVC <Optional value or range>
-the syscalls are defined in linux/arch/s390/include/asm/unistd.h
-e.g. to trace all file opens just do
-TR SVC 5 ( as this is the syscall number of open )
-
-
-SMP Specific commands
----------------------
-To find out how many cpus you have
-Q CPUS displays all the CPU's available to your virtual machine
-To find the cpu that the current cpu VM debugger commands are being directed at do
-Q CPU to change the current cpu VM debugger commands are being directed at do
-CPU <desired cpu no>
-
-On a SMP guest issue a command to all CPUs try prefixing the command with cpu all.
-To issue a command to a particular cpu try cpu <cpu number> e.g.
-CPU 01 TR I R 2000.3000
-If you are running on a guest with several cpus & you have a IO related problem
-& cannot follow the flow of code but you know it isn't smp related.
-from the bash prompt issue
-shutdown -h now or halt.
-do a Q CPUS to find out how many cpus you have
-detach each one of them from cp except cpu 0
-by issuing a
-DETACH CPU 01-(number of cpus in configuration)
-& boot linux again.
-TR SIGP will trace inter processor signal processor instructions.
-DEFINE CPU 01-(number in configuration)
-will get your guests cpus back.
-
-
-Help for displaying ascii textstrings
--------------------------------------
-On the very latest VM Nucleus'es VM can now display ascii
-( thanks Neale for the hint ) by doing
-D TX<lowaddr>.<len>
-e.g.
-D TX0.100
-
-Alternatively
-=============
-Under older VM debuggers ( I love EBDIC too ) you can use this little program I wrote which
-will convert a command line of hex digits to ascii text which can be compiled under linux &
-you can copy the hex digits from your x3270 terminal to your xterm if you are debugging
-from a linuxbox.
-
-This is quite useful when looking at a parameter passed in as a text string
-under VM ( unless you are good at decoding ASCII in your head ).
-
-e.g. consider tracing an open syscall
-TR SVC 5
-We have stopped at a breakpoint
-000151B0' SVC 0A05 -> 0001909A' CC 0
-
-D 20.8 to check the SVC old psw in the prefix area & see was it from userspace
-( for the layout of the prefix area consult P18 of the s/390 390 Reference Summary
-if you have it available ).
-V00000020 070C2000 800151B2
-The problem state bit wasn't set & it's also too early in the boot sequence
-for it to be a userspace SVC if it was we would have to temporarily switch the
-psw to user space addressing so we could get at the first parameter of the open in
-gpr2.
-Next do a
-D G2
-GPR 2 = 00014CB4
-Now display what gpr2 is pointing to
-D 00014CB4.20
-V00014CB4 2F646576 2F636F6E 736F6C65 00001BF5
-V00014CC4 FC00014C B4001001 E0001000 B8070707
-Now copy the text till the first 00 hex ( which is the end of the string
-to an xterm & do hex2ascii on it.
-hex2ascii 2F646576 2F636F6E 736F6C65 00
-outputs
-Decoded Hex:=/ d e v / c o n s o l e 0x00
-We were opening the console device,
-
-You can compile the code below yourself for practice :-),
-/*
- * hex2ascii.c
- * a useful little tool for converting a hexadecimal command line to ascii
- *
- * Author(s): Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
- * (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation.
- */
-#include <stdio.h>
-
-int main(int argc,char *argv[])
-{
- int cnt1,cnt2,len,toggle=0;
- int startcnt=1;
- unsigned char c,hex;
-
- if(argc>1&&(strcmp(argv[1],"-a")==0))
- startcnt=2;
- printf("Decoded Hex:=");
- for(cnt1=startcnt;cnt1<argc;cnt1++)
- {
- len=strlen(argv[cnt1]);
- for(cnt2=0;cnt2<len;cnt2++)
- {
- c=argv[cnt1][cnt2];
- if(c>='0'&&c<='9')
- c=c-'0';
- if(c>='A'&&c<='F')
- c=c-'A'+10;
- if(c>='a'&&c<='f')
- c=c-'a'+10;
- switch(toggle)
- {
- case 0:
- hex=c<<4;
- toggle=1;
- break;
- case 1:
- hex+=c;
- if(hex<32||hex>127)
- {
- if(startcnt==1)
- printf("0x%02X ",(int)hex);
- else
- printf(".");
- }
- else
- {
- printf("%c",hex);
- if(startcnt==1)
- printf(" ");
- }
- toggle=0;
- break;
- }
- }
- }
- printf("\n");
-}
-
-
-
-
-Stack tracing under VM
-----------------------
-A basic backtrace
------------------
-
-Here are the tricks I use 9 out of 10 times it works pretty well,
-
-When your backchain reaches a dead end
---------------------------------------
-This can happen when an exception happens in the kernel & the kernel is entered twice
-if you reach the NULL pointer at the end of the back chain you should be
-able to sniff further back if you follow the following tricks.
-1) A kernel address should be easy to recognise since it is in
-primary space & the problem state bit isn't set & also
-The Hi bit of the address is set.
-2) Another backchain should also be easy to recognise since it is an
-address pointing to another address approximately 100 bytes or 0x70 hex
-behind the current stackpointer.
-
-
-Here is some practice.
-boot the kernel & hit PA1 at some random time
-d g to display the gprs, this should display something like
-GPR 0 = 00000001 00156018 0014359C 00000000
-GPR 4 = 00000001 001B8888 000003E0 00000000
-GPR 8 = 00100080 00100084 00000000 000FE000
-GPR 12 = 00010400 8001B2DC 8001B36A 000FFED8
-Note that GPR14 is a return address but as we are real men we are going to
-trace the stack.
-display 0x40 bytes after the stack pointer.
-
-V000FFED8 000FFF38 8001B838 80014C8E 000FFF38
-V000FFEE8 00000000 00000000 000003E0 00000000
-V000FFEF8 00100080 00100084 00000000 000FE000
-V000FFF08 00010400 8001B2DC 8001B36A 000FFED8
-
-
-Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if
-you look above at our stackframe & also agrees with GPR14.
-
-now backchain
-d 000FFF38.40
-we now are taking the contents of SP to get our first backchain.
-
-V000FFF38 000FFFA0 00000000 00014995 00147094
-V000FFF48 00147090 001470A0 000003E0 00000000
-V000FFF58 00100080 00100084 00000000 001BF1D0
-V000FFF68 00010400 800149BA 80014CA6 000FFF38
-
-This displays a 2nd return address of 80014CA6
-
-now do d 000FFFA0.40 for our 3rd backchain
-
-V000FFFA0 04B52002 0001107F 00000000 00000000
-V000FFFB0 00000000 00000000 FF000000 0001107F
-V000FFFC0 00000000 00000000 00000000 00000000
-V000FFFD0 00010400 80010802 8001085A 000FFFA0
-
-
-our 3rd return address is 8001085A
-
-as the 04B52002 looks suspiciously like rubbish it is fair to assume that the kernel entry routines
-for the sake of optimisation don't set up a backchain.
-
-now look at System.map to see if the addresses make any sense.
-
-grep -i 0001b3 System.map
-outputs among other things
-0001b304 T cpu_idle
-so 8001B36A
-is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it )
-
-
-grep -i 00014 System.map
-produces among other things
-00014a78 T start_kernel
-so 0014CA6 is start_kernel+some hex number I can't add in my head.
-
-grep -i 00108 System.map
-this produces
-00010800 T _stext
-so 8001085A is _stext+0x5a
-
-Congrats you've done your first backchain.
-
-
-
-s/390 & z/Architecture IO Overview
-==================================
-
-I am not going to give a course in 390 IO architecture as this would take me quite a
-while & I'm no expert. Instead I'll give a 390 IO architecture summary for Dummies if you have
-the s/390 principles of operation available read this instead. If nothing else you may find a few
-useful keywords in here & be able to use them on a web search engine like altavista to find
-more useful information.
-
-Unlike other bus architectures modern 390 systems do their IO using mostly
-fibre optics & devices such as tapes & disks can be shared between several mainframes,
-also S390 can support up to 65536 devices while a high end PC based system might be choking
-with around 64. Here is some of the common IO terminology
-
-Subchannel:
-This is the logical number most IO commands use to talk to an IO device there can be up to
-0x10000 (65536) of these in a configuration typically there is a few hundred. Under VM
-for simplicity they are allocated contiguously, however on the native hardware they are not
-they typically stay consistent between boots provided no new hardware is inserted or removed.
-Under Linux for 390 we use these as IRQ's & also when issuing an IO command (CLEAR SUBCHANNEL,
-HALT SUBCHANNEL,MODIFY SUBCHANNEL,RESUME SUBCHANNEL,START SUBCHANNEL,STORE SUBCHANNEL &
-TEST SUBCHANNEL ) we use this as the ID of the device we wish to talk to, the most
-important of these instructions are START SUBCHANNEL ( to start IO ), TEST SUBCHANNEL ( to check
-whether the IO completed successfully ), & HALT SUBCHANNEL ( to kill IO ), a subchannel
-can have up to 8 channel paths to a device this offers redundancy if one is not available.
-
-
-Device Number:
-This number remains static & Is closely tied to the hardware, there are 65536 of these
-also they are made up of a CHPID ( Channel Path ID, the most significant 8 bits )
-& another lsb 8 bits. These remain static even if more devices are inserted or removed
-from the hardware, there is a 1 to 1 mapping between Subchannels & Device Numbers provided
-devices aren't inserted or removed.
-
-Channel Control Words:
-CCWS are linked lists of instructions initially pointed to by an operation request block (ORB),
-which is initially given to Start Subchannel (SSCH) command along with the subchannel number
-for the IO subsystem to process while the CPU continues executing normal code.
-These come in two flavours, Format 0 ( 24 bit for backward )
-compatibility & Format 1 ( 31 bit ). These are typically used to issue read & write
-( & many other instructions ) they consist of a length field & an absolute address field.
-For each IO typically get 1 or 2 interrupts one for channel end ( primary status ) when the
-channel is idle & the second for device end ( secondary status ) sometimes you get both
-concurrently, you check how the IO went on by issuing a TEST SUBCHANNEL at each interrupt,
-from which you receive an Interruption response block (IRB). If you get channel & device end
-status in the IRB without channel checks etc. your IO probably went okay. If you didn't you
-probably need a doctor to examine the IRB & extended status word etc.
-If an error occurs, more sophisticated control units have a facility known as
-concurrent sense this means that if an error occurs Extended sense information will
-be presented in the Extended status word in the IRB if not you have to issue a
-subsequent SENSE CCW command after the test subchannel.
-
-
-TPI( Test pending interrupt) can also be used for polled IO but in multitasking multiprocessor
-systems it isn't recommended except for checking special cases ( i.e. non looping checks for
-pending IO etc. ).
-
-Store Subchannel & Modify Subchannel can be used to examine & modify operating characteristics
-of a subchannel ( e.g. channel paths ).
-
-Other IO related Terms:
-Sysplex: S390's Clustering Technology
-QDIO: S390's new high speed IO architecture to support devices such as gigabit ethernet,
-this architecture is also designed to be forward compatible with up & coming 64 bit machines.
-
-
-General Concepts
-
-Input Output Processors (IOP's) are responsible for communicating between
-the mainframe CPU's & the channel & relieve the mainframe CPU's from the
-burden of communicating with IO devices directly, this allows the CPU's to
-concentrate on data processing.
-
-IOP's can use one or more links ( known as channel paths ) to talk to each
-IO device. It first checks for path availability & chooses an available one,
-then starts ( & sometimes terminates IO ).
-There are two types of channel path: ESCON & the Parallel IO interface.
-
-IO devices are attached to control units, control units provide the
-logic to interface the channel paths & channel path IO protocols to
-the IO devices, they can be integrated with the devices or housed separately
-& often talk to several similar devices ( typical examples would be raid
-controllers or a control unit which connects to 1000 3270 terminals ).
-
-
- +---------------------------------------------------------------+
- | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ |
- | | CPU | | CPU | | CPU | | CPU | | Main | | Expanded | |
- | | | | | | | | | | Memory | | Storage | |
- | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ |
- |---------------------------------------------------------------+
- | IOP | IOP | IOP |
- |---------------------------------------------------------------
- | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C |
- ----------------------------------------------------------------
- || ||
- || Bus & Tag Channel Path || ESCON
- || ====================== || Channel
- || || || || Path
- +----------+ +----------+ +----------+
- | | | | | |
- | CU | | CU | | CU |
- | | | | | |
- +----------+ +----------+ +----------+
- | | | | |
-+----------+ +----------+ +----------+ +----------+ +----------+
-|I/O Device| |I/O Device| |I/O Device| |I/O Device| |I/O Device|
-+----------+ +----------+ +----------+ +----------+ +----------+
- CPU = Central Processing Unit
- C = Channel
- IOP = IP Processor
- CU = Control Unit
-
-The 390 IO systems come in 2 flavours the current 390 machines support both
-
-The Older 360 & 370 Interface,sometimes called the Parallel I/O interface,
-sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers
-Interface (OEMI).
-
-This byte wide Parallel channel path/bus has parity & data on the "Bus" cable
-& control lines on the "Tag" cable. These can operate in byte multiplex mode for
-sharing between several slow devices or burst mode & monopolize the channel for the
-whole burst. Up to 256 devices can be addressed on one of these cables. These cables are
-about one inch in diameter. The maximum unextended length supported by these cables is
-125 Meters but this can be extended up to 2km with a fibre optic channel extended
-such as a 3044. The maximum burst speed supported is 4.5 megabytes per second however
-some really old processors support only transfer rates of 3.0, 2.0 & 1.0 MB/sec.
-One of these paths can be daisy chained to up to 8 control units.
-
-
-ESCON if fibre optic it is also called FICON
-Was introduced by IBM in 1990. Has 2 fibre optic cables & uses either leds or lasers
-for communication at a signaling rate of up to 200 megabits/sec. As 10bits are transferred
-for every 8 bits info this drops to 160 megabits/sec & to 18.6 Megabytes/sec once
-control info & CRC are added. ESCON only operates in burst mode.
-
-ESCONs typical max cable length is 3km for the led version & 20km for the laser version
-known as XDF ( extended distance facility ). This can be further extended by using an
-ESCON director which triples the above mentioned ranges. Unlike Bus & Tag as ESCON is
-serial it uses a packet switching architecture the standard Bus & Tag control protocol
-is however present within the packets. Up to 256 devices can be attached to each control
-unit that uses one of these interfaces.
-
-Common 390 Devices include:
-Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters,
-Consoles 3270 & 3215 ( a teletype emulated under linux for a line mode console ).
-DASD's direct access storage devices ( otherwise known as hard disks ).
-Tape Drives.
-CTC ( Channel to Channel Adapters ),
-ESCON or Parallel Cables used as a very high speed serial link
-between 2 machines. We use 2 cables under linux to do a bi-directional serial link.
-
-
-Debugging IO on s/390 & z/Architecture under VM
-===============================================
-
-Now we are ready to go on with IO tracing commands under VM
-
-A few self explanatory queries:
-Q OSA
-Q CTC
-Q DISK ( This command is CMS specific )
-Q DASD
-
-
-
-
-
-
-Q OSA on my machine returns
-OSA 7C08 ON OSA 7C08 SUBCHANNEL = 0000
-OSA 7C09 ON OSA 7C09 SUBCHANNEL = 0001
-OSA 7C14 ON OSA 7C14 SUBCHANNEL = 0002
-OSA 7C15 ON OSA 7C15 SUBCHANNEL = 0003
-
-If you have a guest with certain privileges you may be able to see devices
-which don't belong to you. To avoid this, add the option V.
-e.g.
-Q V OSA
-
-Now using the device numbers returned by this command we will
-Trace the io starting up on the first device 7c08 & 7c09
-In our simplest case we can trace the
-start subchannels
-like TR SSCH 7C08-7C09
-or the halt subchannels
-or TR HSCH 7C08-7C09
-MSCH's ,STSCH's I think you can guess the rest
-
-Ingo's favourite trick is tracing all the IO's & CCWS & spooling them into the reader of another
-VM guest so he can ftp the logfile back to his own machine.I'll do a small bit of this & give you
- a look at the output.
-
-1) Spool stdout to VM reader
-SP PRT TO (another vm guest ) or * for the local vm guest
-2) Fill the reader with the trace
-TR IO 7c08-7c09 INST INT CCW PRT RUN
-3) Start up linux
-i 00c
-4) Finish the trace
-TR END
-5) close the reader
-C PRT
-6) list reader contents
-RDRLIST
-7) copy it to linux4's minidisk
-RECEIVE / LOG TXT A1 ( replace
-8)
-filel & press F11 to look at it
-You should see something like:
-
-00020942' SSCH B2334000 0048813C CC 0 SCH 0000 DEV 7C08
- CPA 000FFDF0 PARM 00E2C9C4 KEY 0 FPI C0 LPM 80
- CCW 000FFDF0 E4200100 00487FE8 0000 E4240100 ........
- IDAL 43D8AFE8
- IDAL 0FB76000
-00020B0A' I/O DEV 7C08 -> 000197BC' SCH 0000 PARM 00E2C9C4
-00021628' TSCH B2354000 >> 00488164 CC 0 SCH 0000 DEV 7C08
- CCWA 000FFDF8 DEV STS 0C SCH STS 00 CNT 00EC
- KEY 0 FPI C0 CC 0 CTLS 4007
-00022238' STSCH B2344000 >> 00488108 CC 0 SCH 0000 DEV 7C08
-
-If you don't like messing up your readed ( because you possibly booted from it )
-you can alternatively spool it to another readers guest.
-
-
-Other common VM device related commands
----------------------------------------------
-These commands are listed only because they have
-been of use to me in the past & may be of use to
-you too. For more complete info on each of the commands
-use type HELP <command> from CMS.
-detaching devices
-DET <devno range>
-ATT <devno range> <guest>
-attach a device to guest * for your own guest
-READY <devno> cause VM to issue a fake interrupt.
-
-The VARY command is normally only available to VM administrators.
-VARY ON PATH <path> TO <devno range>
-VARY OFF PATH <PATH> FROM <devno range>
-This is used to switch on or off channel paths to devices.
-
-Q CHPID <channel path ID>
-This displays state of devices using this channel path
-D SCHIB <subchannel>
-This displays the subchannel information SCHIB block for the device.
-this I believe is also only available to administrators.
-DEFINE CTC <devno>
-defines a virtual CTC channel to channel connection
-2 need to be defined on each guest for the CTC driver to use.
-COUPLE devno userid remote devno
-Joins a local virtual device to a remote virtual device
-( commonly used for the CTC driver ).
-
-Building a VM ramdisk under CMS which linux can use
-def vfb-<blocksize> <subchannel> <number blocks>
-blocksize is commonly 4096 for linux.
-Formatting it
-format <subchannel> <driver letter e.g. x> (blksize <blocksize>
-
-Sharing a disk between multiple guests
-LINK userid devno1 devno2 mode password
-
-
-
-GDB on S390
-===========
-N.B. if compiling for debugging gdb works better without optimisation
-( see Compiling programs for debugging )
-
-invocation
-----------
-gdb <victim program> <optional corefile>
-
-Online help
------------
-help: gives help on commands
-e.g.
-help
-help display
-Note gdb's online help is very good use it.
-
-
-Assembly
---------
-info registers: displays registers other than floating point.
-info all-registers: displays floating points as well.
-disassemble: disassembles
-e.g.
-disassemble without parameters will disassemble the current function
-disassemble $pc $pc+10
-
-Viewing & modifying variables
------------------------------
-print or p: displays variable or register
-e.g. p/x $sp will display the stack pointer
-
-display: prints variable or register each time program stops
-e.g.
-display/x $pc will display the program counter
-display argc
-
-undisplay : undo's display's
-
-info breakpoints: shows all current breakpoints
-
-info stack: shows stack back trace ( if this doesn't work too well, I'll show you the
-stacktrace by hand below ).
-
-info locals: displays local variables.
-
-info args: display current procedure arguments.
-
-set args: will set argc & argv each time the victim program is invoked.
-
-set <variable>=value
-set argc=100
-set $pc=0
-
-
-
-Modifying execution
--------------------
-step: steps n lines of sourcecode
-step steps 1 line.
-step 100 steps 100 lines of code.
-
-next: like step except this will not step into subroutines
-
-stepi: steps a single machine code instruction.
-e.g. stepi 100
-
-nexti: steps a single machine code instruction but will not step into subroutines.
-
-finish: will run until exit of the current routine
-
-run: (re)starts a program
-
-cont: continues a program
-
-quit: exits gdb.
-
-
-breakpoints
-------------
-
-break
-sets a breakpoint
-e.g.
-
-break main
-
-break *$pc
-
-break *0x400618
-
-Here's a really useful one for large programs
-rbr
-Set a breakpoint for all functions matching REGEXP
-e.g.
-rbr 390
-will set a breakpoint with all functions with 390 in their name.
-
-info breakpoints
-lists all breakpoints
-
-delete: delete breakpoint by number or delete them all
-e.g.
-delete 1 will delete the first breakpoint
-delete will delete them all
-
-watch: This will set a watchpoint ( usually hardware assisted ),
-This will watch a variable till it changes
-e.g.
-watch cnt, will watch the variable cnt till it changes.
-As an aside unfortunately gdb's, architecture independent watchpoint code
-is inconsistent & not very good, watchpoints usually work but not always.
-
-info watchpoints: Display currently active watchpoints
-
-condition: ( another useful one )
-Specify breakpoint number N to break only if COND is true.
-Usage is `condition N COND', where N is an integer and COND is an
-expression to be evaluated whenever breakpoint N is reached.
-
-
-
-User defined functions/macros
------------------------------
-define: ( Note this is very very useful,simple & powerful )
-usage define <name> <list of commands> end
-
-examples which you should consider putting into .gdbinit in your home directory
-define d
-stepi
-disassemble $pc $pc+10
-end
-
-define e
-nexti
-disassemble $pc $pc+10
-end
-
-
-Other hard to classify stuff
-----------------------------
-signal n:
-sends the victim program a signal.
-e.g. signal 3 will send a SIGQUIT.
-
-info signals:
-what gdb does when the victim receives certain signals.
-
-list:
-e.g.
-list lists current function source
-list 1,10 list first 10 lines of current file.
-list test.c:1,10
-
-
-directory:
-Adds directories to be searched for source if gdb cannot find the source.
-(note it is a bit sensitive about slashes)
-e.g. To add the root of the filesystem to the searchpath do
-directory //
-
-
-call <function>
-This calls a function in the victim program, this is pretty powerful
-e.g.
-(gdb) call printf("hello world")
-outputs:
-$1 = 11
-
-You might now be thinking that the line above didn't work, something extra had to be done.
-(gdb) call fflush(stdout)
-hello world$2 = 0
-As an aside the debugger also calls malloc & free under the hood
-to make space for the "hello world" string.
-
-
-
-hints
------
-1) command completion works just like bash
-( if you are a bad typist like me this really helps )
-e.g. hit br <TAB> & cursor up & down :-).
-
-2) if you have a debugging problem that takes a few steps to recreate
-put the steps into a file called .gdbinit in your current working directory
-if you have defined a few extra useful user defined commands put these in
-your home directory & they will be read each time gdb is launched.
-
-A typical .gdbinit file might be.
-break main
-run
-break runtime_exception
-cont
-
-
-stack chaining in gdb by hand
------------------------------
-This is done using a the same trick described for VM
-p/x (*($sp+56))&0x7fffffff get the first backchain.
-
-For z/Architecture
-Replace 56 with 112 & ignore the &0x7fffffff
-in the macros below & do nasty casts to longs like the following
-as gdb unfortunately deals with printed arguments as ints which
-messes up everything.
-i.e. here is a 3rd backchain dereference
-p/x *(long *)(***(long ***)$sp+112)
-
-
-this outputs
-$5 = 0x528f18
-on my machine.
-Now you can use
-info symbol (*($sp+56))&0x7fffffff
-you might see something like.
-rl_getc + 36 in section .text telling you what is located at address 0x528f18
-Now do.
-p/x (*(*$sp+56))&0x7fffffff
-This outputs
-$6 = 0x528ed0
-Now do.
-info symbol (*(*$sp+56))&0x7fffffff
-rl_read_key + 180 in section .text
-now do
-p/x (*(**$sp+56))&0x7fffffff
-& so on.
-
-Disassembling instructions without debug info
----------------------------------------------
-gdb typically complains if there is a lack of debugging
-symbols in the disassemble command with
-"No function contains specified address." To get around
-this do
-x/<number lines to disassemble>xi <address>
-e.g.
-x/20xi 0x400730
-
-
-
-Note: Remember gdb has history just like bash you don't need to retype the
-whole line just use the up & down arrows.
-
-
-
-For more info
--------------
-From your linuxbox do
-man gdb or info gdb.
-
-core dumps
-----------
-What a core dump ?,
-A core dump is a file generated by the kernel ( if allowed ) which contains the registers,
-& all active pages of the program which has crashed.
-From this file gdb will allow you to look at the registers & stack trace & memory of the
-program as if it just crashed on your system, it is usually called core & created in the
-current working directory.
-This is very useful in that a customer can mail a core dump to a technical support department
-& the technical support department can reconstruct what happened.
-Provided they have an identical copy of this program with debugging symbols compiled in &
-the source base of this build is available.
-In short it is far more useful than something like a crash log could ever hope to be.
-
-In theory all that is missing to restart a core dumped program is a kernel patch which
-will do the following.
-1) Make a new kernel task structure
-2) Reload all the dumped pages back into the kernel's memory management structures.
-3) Do the required clock fixups
-4) Get all files & network connections for the process back into an identical state ( really difficult ).
-5) A few more difficult things I haven't thought of.
-
-
-
-Why have I never seen one ?.
-Probably because you haven't used the command
-ulimit -c unlimited in bash
-to allow core dumps, now do
-ulimit -a
-to verify that the limit was accepted.
-
-A sample core dump
-To create this I'm going to do
-ulimit -c unlimited
-gdb
-to launch gdb (my victim app. ) now be bad & do the following from another
-telnet/xterm session to the same machine
-ps -aux | grep gdb
-kill -SIGSEGV <gdb's pid>
-or alternatively use killall -SIGSEGV gdb if you have the killall command.
-Now look at the core dump.
-./gdb core
-Displays the following
-GNU gdb 4.18
-Copyright 1998 Free Software Foundation, Inc.
-GDB is free software, covered by the GNU General Public License, and you are
-welcome to change it and/or distribute copies of it under certain conditions.
-Type "show copying" to see the conditions.
-There is absolutely no warranty for GDB. Type "show warranty" for details.
-This GDB was configured as "s390-ibm-linux"...
-Core was generated by `./gdb'.
-Program terminated with signal 11, Segmentation fault.
-Reading symbols from /usr/lib/libncurses.so.4...done.
-Reading symbols from /lib/libm.so.6...done.
-Reading symbols from /lib/libc.so.6...done.
-Reading symbols from /lib/ld-linux.so.2...done.
-#0 0x40126d1a in read () from /lib/libc.so.6
-Setting up the environment for debugging gdb.
-Breakpoint 1 at 0x4dc6f8: file utils.c, line 471.
-Breakpoint 2 at 0x4d87a4: file top.c, line 2609.
-(top-gdb) info stack
-#0 0x40126d1a in read () from /lib/libc.so.6
-#1 0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402
-#2 0x528ed0 in rl_read_key () at input.c:381
-#3 0x5167e6 in readline_internal_char () at readline.c:454
-#4 0x5168ee in readline_internal_charloop () at readline.c:507
-#5 0x51692c in readline_internal () at readline.c:521
-#6 0x5164fe in readline (prompt=0x7ffff810 "\177ÿøx\177ÿ÷Ø\177ÿøxÀ")
- at readline.c:349
-#7 0x4d7a8a in command_line_input (prompt=0x564420 "(gdb) ", repeat=1,
- annotation_suffix=0x4d6b44 "prompt") at top.c:2091
-#8 0x4d6cf0 in command_loop () at top.c:1345
-#9 0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635
-
-
-LDD
-===
-This is a program which lists the shared libraries which a library needs,
-Note you also get the relocations of the shared library text segments which
-help when using objdump --source.
-e.g.
- ldd ./gdb
-outputs
-libncurses.so.4 => /usr/lib/libncurses.so.4 (0x40018000)
-libm.so.6 => /lib/libm.so.6 (0x4005e000)
-libc.so.6 => /lib/libc.so.6 (0x40084000)
-/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
-
-
-Debugging shared libraries
-==========================
-Most programs use shared libraries, however it can be very painful
-when you single step instruction into a function like printf for the
-first time & you end up in functions like _dl_runtime_resolve this is
-the ld.so doing lazy binding, lazy binding is a concept in ELF where
-shared library functions are not loaded into memory unless they are
-actually used, great for saving memory but a pain to debug.
-To get around this either relink the program -static or exit gdb type
-export LD_BIND_NOW=true this will stop lazy binding & restart the gdb'ing
-the program in question.
-
-
-
-Debugging modules
-=================
-As modules are dynamically loaded into the kernel their address can be
-anywhere to get around this use the -m option with insmod to emit a load
-map which can be piped into a file if required.
-
-The proc file system
-====================
-What is it ?.
-It is a filesystem created by the kernel with files which are created on demand
-by the kernel if read, or can be used to modify kernel parameters,
-it is a powerful concept.
-
-e.g.
-
-cat /proc/sys/net/ipv4/ip_forward
-On my machine outputs
-0
-telling me ip_forwarding is not on to switch it on I can do
-echo 1 > /proc/sys/net/ipv4/ip_forward
-cat it again
-cat /proc/sys/net/ipv4/ip_forward
-On my machine now outputs
-1
-IP forwarding is on.
-There is a lot of useful info in here best found by going in & having a look around,
-so I'll take you through some entries I consider important.
-
-All the processes running on the machine have their own entry defined by
-/proc/<pid>
-So lets have a look at the init process
-cd /proc/1
-
-cat cmdline
-emits
-init [2]
-
-cd /proc/1/fd
-This contains numerical entries of all the open files,
-some of these you can cat e.g. stdout (2)
-
-cat /proc/29/maps
-on my machine emits
-
-00400000-00478000 r-xp 00000000 5f:00 4103 /bin/bash
-00478000-0047e000 rw-p 00077000 5f:00 4103 /bin/bash
-0047e000-00492000 rwxp 00000000 00:00 0
-40000000-40015000 r-xp 00000000 5f:00 14382 /lib/ld-2.1.2.so
-40015000-40016000 rw-p 00014000 5f:00 14382 /lib/ld-2.1.2.so
-40016000-40017000 rwxp 00000000 00:00 0
-40017000-40018000 rw-p 00000000 00:00 0
-40018000-4001b000 r-xp 00000000 5f:00 14435 /lib/libtermcap.so.2.0.8
-4001b000-4001c000 rw-p 00002000 5f:00 14435 /lib/libtermcap.so.2.0.8
-4001c000-4010d000 r-xp 00000000 5f:00 14387 /lib/libc-2.1.2.so
-4010d000-40111000 rw-p 000f0000 5f:00 14387 /lib/libc-2.1.2.so
-40111000-40114000 rw-p 00000000 00:00 0
-40114000-4011e000 r-xp 00000000 5f:00 14408 /lib/libnss_files-2.1.2.so
-4011e000-4011f000 rw-p 00009000 5f:00 14408 /lib/libnss_files-2.1.2.so
-7fffd000-80000000 rwxp ffffe000 00:00 0
-
-
-Showing us the shared libraries init uses where they are in memory
-& memory access permissions for each virtual memory area.
-
-/proc/1/cwd is a softlink to the current working directory.
-/proc/1/root is the root of the filesystem for this process.
-
-/proc/1/mem is the current running processes memory which you
-can read & write to like a file.
-strace uses this sometimes as it is a bit faster than the
-rather inefficient ptrace interface for peeking at DATA.
-
-
-cat status
-
-Name: init
-State: S (sleeping)
-Pid: 1
-PPid: 0
-Uid: 0 0 0 0
-Gid: 0 0 0 0
-Groups:
-VmSize: 408 kB
-VmLck: 0 kB
-VmRSS: 208 kB
-VmData: 24 kB
-VmStk: 8 kB
-VmExe: 368 kB
-VmLib: 0 kB
-SigPnd: 0000000000000000
-SigBlk: 0000000000000000
-SigIgn: 7fffffffd7f0d8fc
-SigCgt: 00000000280b2603
-CapInh: 00000000fffffeff
-CapPrm: 00000000ffffffff
-CapEff: 00000000fffffeff
-
-User PSW: 070de000 80414146
-task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68
-User GPRS:
-00000400 00000000 0000000b 7ffffa90
-00000000 00000000 00000000 0045d9f4
-0045cafc 7ffffa90 7fffff18 0045cb08
-00010400 804039e8 80403af8 7ffff8b0
-User ACRS:
-00000000 00000000 00000000 00000000
-00000001 00000000 00000000 00000000
-00000000 00000000 00000000 00000000
-00000000 00000000 00000000 00000000
-Kernel BackChain CallChain BackChain CallChain
- 004b7ca8 8002bd0c 004b7d18 8002b92c
- 004b7db8 8005cd50 004b7e38 8005d12a
- 004b7f08 80019114
-Showing among other things memory usage & status of some signals &
-the processes'es registers from the kernel task_structure
-as well as a backchain which may be useful if a process crashes
-in the kernel for some unknown reason.
-
-Some driver debugging techniques
-================================
-debug feature
--------------
-Some of our drivers now support a "debug feature" in
-/proc/s390dbf see s390dbf.txt in the linux/Documentation directory
-for more info.
-e.g.
-to switch on the lcs "debug feature"
-echo 5 > /proc/s390dbf/lcs/level
-& then after the error occurred.
-cat /proc/s390dbf/lcs/sprintf >/logfile
-the logfile now contains some information which may help
-tech support resolve a problem in the field.
-
-
-
-high level debugging network drivers
-------------------------------------
-ifconfig is a quite useful command
-it gives the current state of network drivers.
-
-If you suspect your network device driver is dead
-one way to check is type
-ifconfig <network device>
-e.g. tr0
-You should see something like
-tr0 Link encap:16/4 Mbps Token Ring (New) HWaddr 00:04:AC:20:8E:48
- inet addr:9.164.185.132 Bcast:9.164.191.255 Mask:255.255.224.0
- UP BROADCAST RUNNING MULTICAST MTU:2000 Metric:1
- RX packets:246134 errors:0 dropped:0 overruns:0 frame:0
- TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
- collisions:0 txqueuelen:100
-
-if the device doesn't say up
-try
-/etc/rc.d/init.d/network start
-( this starts the network stack & hopefully calls ifconfig tr0 up ).
-ifconfig looks at the output of /proc/net/dev & presents it in a more presentable form
-Now ping the device from a machine in the same subnet.
-if the RX packets count & TX packets counts don't increment you probably
-have problems.
-next
-cat /proc/net/arp
-Do you see any hardware addresses in the cache if not you may have problems.
-Next try
-ping -c 5 <broadcast_addr> i.e. the Bcast field above in the output of
-ifconfig. Do you see any replies from machines other than the local machine
-if not you may have problems. also if the TX packets count in ifconfig
-hasn't incremented either you have serious problems in your driver
-(e.g. the txbusy field of the network device being stuck on )
-or you may have multiple network devices connected.
-
-
-chandev
--------
-There is a new device layer for channel devices, some
-drivers e.g. lcs are registered with this layer.
-If the device uses the channel device layer you'll be
-able to find what interrupts it uses & the current state
-of the device.
-See the manpage chandev.8 &type cat /proc/chandev for more info.
-
-
-
-Starting points for debugging scripting languages etc.
-======================================================
-
-bash/sh
-
-bash -x <scriptname>
-e.g. bash -x /usr/bin/bashbug
-displays the following lines as it executes them.
-+ MACHINE=i586
-+ OS=linux-gnu
-+ CC=gcc
-+ CFLAGS= -DPROGRAM='bash' -DHOSTTYPE='i586' -DOSTYPE='linux-gnu' -DMACHTYPE='i586-pc-linux-gnu' -DSHELL -DHAVE_CONFIG_H -I. -I. -I./lib -O2 -pipe
-+ RELEASE=2.01
-+ PATCHLEVEL=1
-+ RELSTATUS=release
-+ MACHTYPE=i586-pc-linux-gnu
-
-perl -d <scriptname> runs the perlscript in a fully interactive debugger
-<like gdb>.
-Type 'h' in the debugger for help.
-
-for debugging java type
-jdb <filename> another fully interactive gdb style debugger.
-& type ? in the debugger for help.
-
-
-
-SysRq
-=====
-This is now supported by linux for s/390 & z/Architecture.
-To enable it do compile the kernel with
-Kernel Hacking -> Magic SysRq Key Enabled
-echo "1" > /proc/sys/kernel/sysrq
-also type
-echo "8" >/proc/sys/kernel/printk
-To make printk output go to console.
-On 390 all commands are prefixed with
-^-
-e.g.
-^-t will show tasks.
-^-? or some unknown command will display help.
-The sysrq key reading is very picky ( I have to type the keys in an
- xterm session & paste them into the x3270 console )
-& it may be wise to predefine the keys as described in the VM hints above
-
-This is particularly useful for syncing disks unmounting & rebooting
-if the machine gets partially hung.
-
-Read Documentation/sysrq.txt for more info
-
-References:
-===========
-Enterprise Systems Architecture Reference Summary
-Enterprise Systems Architecture Principles of Operation
-Hartmut Penners s390 stack frame sheet.
-IBM Mainframe Channel Attachment a technology brief from a CISCO webpage
-Various bits of man & info pages of Linux.
-Linux & GDB source.
-Various info & man pages.
-CMS Help on tracing commands.
-Linux for s/390 Elf Application Binary Interface
-Linux for z/Series Elf Application Binary Interface ( Both Highly Recommended )
-z/Architecture Principles of Operation SA22-7832-00
-Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the
-Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05
-
-Special Thanks
-==============
-Special thanks to Neale Ferguson who maintains a much
-prettier HTML version of this page at
-http://linuxvm.org/penguinvm/
-Bob Grainger Stefan Bader & others for reporting bugs