Re: [PATCH v1 9/9] uretprobes: Documentation update

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* 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