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4. uprobetracer.txt

            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 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.
   %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
--------------
 * 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 '-: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

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
    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 uprobe would be:

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

  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
uprobe_events file.

    # cat uprobe_events
    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax

Format of events can be seen by viewing the file events/uprobes/zfree_entry/format

    # 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 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: 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.