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SubjectRe: [PATCH v1 9/9] uretprobes: Documentation update
* Anton Arapov <anton@redhat.com> [2013-04-03 18:00:39]:

> 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>

> Documentation/trace/uprobetracer.txt | 114 ++++++++++++++++++++---------------
> 1 file changed, 67 insertions(+), 47 deletions(-)
>
> diff --git a/Documentation/trace/uprobetracer.txt b/Documentation/trace/uprobetracer.txt
> index 24ce682..d9c3e68 100644
> --- a/Documentation/trace/uprobetracer.txt
> +++ b/Documentation/trace/uprobetracer.txt
> @@ -1,6 +1,8 @@
> - Uprobe-tracer: Uprobe-based Event Tracing
> - =========================================
> - Documentation written by Srikar Dronamraju
> + Uprobe-tracer: Uprobe-based Event Tracing
> + =========================================
> +
> + Documentation written by Srikar Dronamraju
> +
>
> Overview
> --------
> @@ -13,78 +15,94 @@ 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
> + 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.
> - 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.
> + 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.
> - %REG : Fetch register REG
> + 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
> +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 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.
> + * 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
> +
> + * Add a probe as a new uretprobe event:
> +
> + echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
> +
> + * Unset registered event:
>
> - echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
> + echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
>
> - This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash
> + * Print out the events that are registered:
>
> - echo > /sys/kernel/debug/tracing/uprobe_events
> + cat /sys/kernel/debug/tracing/uprobe_events
>
> - This clears all probe points.
> + * Clear all 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
> + 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
> + # 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 :
> + 0x46420 is the offset of zfree in object /bin/zsh that is loaded at
> + 0x00400000. Hence the command to uprobe would be:
> +
> + # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
> +
> + And the same for the uretprobe would be:
>
> - # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
> + # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
>
> -Please note: User has to explicitly calculate the offset of the probepoint
> +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;
> - 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 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;
> + 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
>
> @@ -94,6 +112,7 @@ Right after definition, each event is disabled by default. For tracing these
> # 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.
> --
> 1.8.1.4
>

--
Thanks and Regards
Srikar Dronamraju



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