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uretprobes: Documentation update 

add the uretprobe syntax and update an example

Signed-off-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
This commit is contained in:
Anton Arapov 2013-04-03 18:00:39 +02:00 committed by Oleg Nesterov
parent a0d60aef4b
commit decc6bfb49
1 changed files with 66 additions and 46 deletions
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80
Documentation/trace/uprobetracer.txt
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@ -1,7 +1,9 @@
Uprobe-tracer: Uprobe-based Event Tracing Uprobe-tracer: Uprobe-based Event Tracing
========================================= =========================================
Documentation written by Srikar Dronamraju Documentation written by Srikar Dronamraju
Overview Overview
-------- --------
Uprobe based trace events are similar to kprobe based trace events. Uprobe based trace events are similar to kprobe based trace events.
@ -13,16 +15,18 @@ current_tracer. Instead of that, add probe points via
/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled. /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
However unlike kprobe-event tracer, the uprobe event interface expects the However unlike kprobe-event tracer, the uprobe event interface expects the
user to calculate the offset of the probepoint in the object user to calculate the offset of the probepoint in the object.
Synopsis of uprobe_tracer Synopsis of uprobe_tracer
------------------------- -------------------------
p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a probe p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a uprobe
r[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a return uprobe (uretprobe)
-:[GRP/]EVENT : Clear uprobe or uretprobe event
GRP : Group name. If omitted, use "uprobes" for it. GRP : Group name. If omitted, "uprobes" is the default value.
EVENT : Event name. If omitted, the event name is generated EVENT : Event name. If omitted, the event name is generated based
based on SYMBOL+offs. on SYMBOL+offs.
PATH : path to an executable or a library. PATH : Path to an executable or a library.
SYMBOL[+offs] : Symbol+offset where the probe is inserted. SYMBOL[+offs] : Symbol+offset where the probe is inserted.
FETCHARGS : Arguments. Each probe can have up to 128 args. FETCHARGS : Arguments. Each probe can have up to 128 args.
@ -30,27 +34,36 @@ Synopsis of uprobe_tracer
Event Profiling Event Profiling
--------------- ---------------
You can check the total number of probe hits and probe miss-hits via You can check the total number of probe hits and probe miss-hits via
/sys/kernel/debug/tracing/uprobe_profile. /sys/kernel/debug/tracing/uprobe_profile.
The first column is event name, the second is the number of probe hits, The first column is event name, the second is the number of probe hits,
the third is the number of probe miss-hits. the third is the number of probe miss-hits.
Usage examples Usage examples
-------------- --------------
To add a probe as a new event, write a new definition to uprobe_events * Add a probe as a new uprobe event, write a new definition to uprobe_events
as below. as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events 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 * Add a probe as a new uretprobe event:
echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
* Unset registered event:
echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
* Print out the events that are registered:
cat /sys/kernel/debug/tracing/uprobe_events
* Clear all events:
echo > /sys/kernel/debug/tracing/uprobe_events echo > /sys/kernel/debug/tracing/uprobe_events
This clears all probe points. Following example shows how to dump the instruction pointer and %ax register
at the probed text address. Probe zfree function in /bin/zsh:
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/ # cd /sys/kernel/debug/tracing/
# cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
@ -58,22 +71,27 @@ function zfree in /bin/zsh
# objdump -T /bin/zsh | grep -w zfree # objdump -T /bin/zsh | grep -w zfree
0000000000446420 g DF .text 0000000000000012 Base zfree 0000000000446420 g DF .text 0000000000000012 Base zfree
0x46420 is the offset of zfree in object /bin/zsh that is loaded at 0x46420 is the offset of zfree in object /bin/zsh that is loaded at
0x00400000. Hence the command to probe would be : 0x00400000. Hence the command to uprobe would be:
# echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
Please note: User has to explicitly calculate the offset of the probepoint And the same for the uretprobe would be:
# echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
Please note: User has to explicitly calculate the offset of the probe-point
in the object. We can see the events that are registered by looking at the in the object. We can see the events that are registered by looking at the
uprobe_events file. uprobe_events file.
# cat uprobe_events # cat uprobe_events
p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
# cat events/uprobes/p_zsh_0x46420/format # cat events/uprobes/zfree_entry/format
name: p_zsh_0x46420 name: zfree_entry
ID: 922 ID: 922
format: format:
field:unsigned short common_type; offset:0; size:2; signed:0; field:unsigned short common_type; offset:0; size:2; signed:0;
@ -94,6 +112,7 @@ events, you need to enable it by:
# echo 1 > events/uprobes/enable # echo 1 > events/uprobes/enable
Lets disable the event after sleeping for some time. Lets disable the event after sleeping for some time.
# sleep 20 # sleep 20
# echo 0 > events/uprobes/enable # echo 0 > events/uprobes/enable
@ -104,10 +123,11 @@ And you can see the traced information via /sys/kernel/debug/tracing/trace.
# #
# TASK-PID CPU# TIMESTAMP FUNCTION # TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | | # | | | | |
zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
Each line shows us probes were triggered for a pid 24842 with ip being Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
0x446421 and contents of ax register being 79. and contents of ax register being 79. And uretprobe was triggered with ip at
0x446540 with counterpart function entry at 0x446420.