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Commit decc6bfb authored by Anton Arapov's avatar Anton Arapov Committed by Oleg Nesterov
Browse files

uretprobes: Documentation update 



add the uretprobe syntax and update an example

Signed-off-by: default avatarAnton Arapov <anton@redhat.com>
Acked-by: default avatarSrikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
parent a0d60aef

Documentation/trace/uprobetracer.txt

+67 −47
Original line number Diff line number Diff line 
            Uprobe-tracer: Uprobe-based Event Tracing

            =========================================



           Documentation written by Srikar Dronamraju





Overview

--------

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.



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

-------------------------

  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.

  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, "uprobes" is the default value.

  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.
@@ -37,20 +41,29 @@ 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.

 * Add a probe as a new uprobe event, write a new definition to uprobe_events

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



 This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash

 * Add a probe as a new uretprobe event:



  echo > /sys/kernel/debug/tracing/uprobe_events

    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:



 This clears all probe points.

    cat /sys/kernel/debug/tracing/uprobe_events



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

 * Clear all events:



    echo > /sys/kernel/debug/tracing/uprobe_events



Following example shows how to dump the instruction pointer and %ax register

at the probed text address. Probe zfree function in /bin/zsh:



    # cd /sys/kernel/debug/tracing/

    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
@@ -59,21 +72,26 @@ function zfree in /bin/zsh 
    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 :

  0x00400000. Hence the command to uprobe would be:



    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events



    # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events

  And the same for the uretprobe would be:



Please note: User has to explicitly calculate the offset of the probepoint

    # 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

uprobe_events file.



    # 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

    name: p_zsh_0x46420

    # cat events/uprobes/zfree_entry/format

    name: zfree_entry

    ID: 922

    format:

         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



Lets disable the event after sleeping for some time.



    # sleep 20

    # 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

    #              | |       |          |         |

                 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.
 
                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79

                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0

                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79

                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0



Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420

and contents of ax register being 79. And uretprobe was triggered with ip at

0x446540 with counterpart function entry at 0x446420.