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- Kprobe-based Event Tracing
- ==========================
-
- Documentation is written by Masami Hiramatsu
-
-
-Overview
---------
-These events are similar to tracepoint based events. Instead of Tracepoint,
-this is based on kprobes (kprobe and kretprobe). So it can probe wherever
-kprobes can probe (this means, all functions body except for __kprobes
-functions). Unlike the Tracepoint based event, this can be added and removed
-dynamically, on the fly.
-
-To enable this feature, build your kernel with CONFIG_KPROBE_TRACING=y.
-
-Similar to the events tracer, this doesn't need to be activated via
-current_tracer. Instead of that, add probe points via
-/sys/kernel/debug/tracing/kprobe_events, and enable it via
-/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
-
-
-Synopsis of kprobe_events
--------------------------
- p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
- r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
- -:[GRP/]EVENT : Clear a probe
-
- GRP : Group name. If omitted, use "kprobes" for it.
- EVENT : Event name. If omitted, the event name is generated
- based on SYM+offs or MEMADDR.
- MOD : Module name which has given SYM.
- SYM[+offs] : Symbol+offset where the probe is inserted.
- MEMADDR : Address where the probe is inserted.
-
- FETCHARGS : Arguments. Each probe can have up to 128 args.
- %REG : Fetch register REG
- @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
- @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
- $stackN : Fetch Nth entry of stack (N >= 0)
- $stack : Fetch stack address.
- $retval : Fetch return value.(*)
- +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
- NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
- FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
- (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
- are supported.
-
- (*) only for return probe.
- (**) this is useful for fetching a field of data structures.
-
-Types
------
-Several types are supported for fetch-args. Kprobe tracer will access memory
-by given type. Prefix 's' and 'u' means those types are signed and unsigned
-respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
-String type is a special type, which fetches a "null-terminated" string from
-kernel space. This means it will fail and store NULL if the string container
-has been paged out.
-Bitfield is another special type, which takes 3 parameters, bit-width, bit-
-offset, and container-size (usually 32). The syntax is;
-
- b<bit-width>@<bit-offset>/<container-size>
-
-
-Per-Probe Event Filtering
--------------------------
- Per-probe event filtering feature allows you to set different filter on each
-probe and gives you what arguments will be shown in trace buffer. If an event
-name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
-under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
-'enabled', 'format' and 'filter'.
-
-enabled:
- You can enable/disable the probe by writing 1 or 0 on it.
-
-format:
- This shows the format of this probe event.
-
-filter:
- You can write filtering rules of this event.
-
-id:
- This shows the id of this probe event.
-
-
-Event Profiling
----------------
- You can check the total number of probe hits and probe miss-hits via
-/sys/kernel/debug/tracing/kprobe_profile.
- The first column is event name, the second is the number of probe hits,
-the third is the number of probe miss-hits.
-
-
-Usage examples
---------------
-To add a probe as a new event, write a new definition to kprobe_events
-as below.
-
- echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
-
- This sets a kprobe on the top of do_sys_open() function with recording
-1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
-assigned to each function argument depends on arch-specific ABI. If you unsure
-the ABI, please try to use probe subcommand of perf-tools (you can find it
-under tools/perf/).
-As this example shows, users can choose more familiar names for each arguments.
-
- echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
-
- This sets a kretprobe on the return point of do_sys_open() function with
-recording return value as "myretprobe" event.
- You can see the format of these events via
-/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
-
- cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
-name: myprobe
-ID: 780
-format:
- field:unsigned short common_type; offset:0; size:2; signed:0;
- field:unsigned char common_flags; offset:2; size:1; signed:0;
- field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
- field:int common_pid; offset:4; size:4; signed:1;
-
- field:unsigned long __probe_ip; offset:12; size:4; signed:0;
- field:int __probe_nargs; offset:16; size:4; signed:1;
- field:unsigned long dfd; offset:20; size:4; signed:0;
- field:unsigned long filename; offset:24; size:4; signed:0;
- field:unsigned long flags; offset:28; size:4; signed:0;
- field:unsigned long mode; offset:32; size:4; signed:0;
-
-
-print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
-REC->dfd, REC->filename, REC->flags, REC->mode
-
- You can see that the event has 4 arguments as in the expressions you specified.
-
- echo > /sys/kernel/debug/tracing/kprobe_events
-
- This clears all probe points.
-
- Or,
-
- echo -:myprobe >> kprobe_events
-
- This clears probe points selectively.
-
- Right after definition, each event is disabled by default. For tracing these
-events, you need to enable it.
-
- echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
- echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
-
- And you can see the traced information via /sys/kernel/debug/tracing/trace.
-
- cat /sys/kernel/debug/tracing/trace
-# tracer: nop
-#
-# TASK-PID CPU# TIMESTAMP FUNCTION
-# | | | | |
- <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
- <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
- <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
- <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
- <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
- <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
-
-
- Each line shows when the kernel hits an event, and <- SYMBOL means kernel
-returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
-returns from do_sys_open to sys_open+0x1b).
-