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- Uprobe-tracer: Uprobe-based Event Tracing
- =========================================
- Documentation written by Srikar Dronamraju
-
-Overview
---------
-Uprobe based trace events are similar to kprobe based trace events.
-To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
-
-Similar to the kprobe-event tracer, this doesn't need to be activated via
-current_tracer. Instead of that, add probe points via
-/sys/kernel/debug/tracing/uprobe_events, and enable it via
-/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
-
-However unlike kprobe-event tracer, the uprobe event interface expects the
-user to calculate the offset of the probepoint in the object
-
-Synopsis of uprobe_tracer
--------------------------
- p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a probe
-
- GRP : Group name. If omitted, use "uprobes" for it.
- EVENT : Event name. If omitted, the event name is generated
- based on SYMBOL+offs.
- PATH : path to an executable or a library.
- SYMBOL[+offs] : Symbol+offset where the probe is inserted.
-
- FETCHARGS : Arguments. Each probe can have up to 128 args.
- %REG : Fetch register REG
-
-Event Profiling
----------------
- You can check the total number of probe hits and probe miss-hits via
-/sys/kernel/debug/tracing/uprobe_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 uprobe_events
-as below.
-
- echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
-
- This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash
-
- echo > /sys/kernel/debug/tracing/uprobe_events
-
- This clears all probe points.
-
-The following example shows how to dump the instruction pointer and %ax
-a register at the probed text address. Here we are trying to probe
-function zfree in /bin/zsh
-
- # cd /sys/kernel/debug/tracing/
- # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
- 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
- # objdump -T /bin/zsh | grep -w zfree
- 0000000000446420 g DF .text 0000000000000012 Base zfree
-
-0x46420 is the offset of zfree in object /bin/zsh that is loaded at
-0x00400000. Hence the command to probe would be :
-
- # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
-
-Please note: User has to explicitly calculate the offset of the probepoint
-in the object. We can see the events that are registered by looking at the
-uprobe_events file.
-
- # cat uprobe_events
- p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax
-
-The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format
-
- # cat events/uprobes/p_zsh_0x46420/format
- name: p_zsh_0x46420
- ID: 922
- 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:int common_padding; offset:8; size:4; signed:1;
-
- field:unsigned long __probe_ip; offset:12; size:4; signed:0;
- field:u32 arg1; offset:16; size:4; signed:0;
- field:u32 arg2; offset:20; size:4; signed:0;
-
- print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
-
-Right after definition, each event is disabled by default. For tracing these
-events, you need to enable it by:
-
- # echo 1 > events/uprobes/enable
-
-Lets disable the event after sleeping for some time.
- # sleep 20
- # echo 0 > events/uprobes/enable
-
-And you can see the traced information via /sys/kernel/debug/tracing/trace.
-
- # cat trace
- # tracer: nop
- #
- # TASK-PID CPU# TIMESTAMP FUNCTION
- # | | | | |
- zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
- zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
- zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
- zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
-
-Each line shows us probes were triggered for a pid 24842 with ip being
-0x446421 and contents of ax register being 79.