linux/Documentation/trace/uprobetracer.txt
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   1                Uprobe-tracer: Uprobe-based Event Tracing
   2                =========================================
   3                 Documentation written by Srikar Dronamraju
   4
   5Overview
   6--------
   7Uprobe based trace events are similar to kprobe based trace events.
   8To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
   9
  10Similar to the kprobe-event tracer, this doesn't need to be activated via
  11current_tracer. Instead of that, add probe points via
  12/sys/kernel/debug/tracing/uprobe_events, and enable it via
  13/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
  14
  15However unlike kprobe-event tracer, the uprobe event interface expects the
  16user to calculate the offset of the probepoint in the object
  17
  18Synopsis of uprobe_tracer
  19-------------------------
  20  p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS]        : Set a probe
  21
  22 GRP            : Group name. If omitted, use "uprobes" for it.
  23 EVENT          : Event name. If omitted, the event name is generated
  24                  based on SYMBOL+offs.
  25 PATH           : path to an executable or a library.
  26 SYMBOL[+offs]  : Symbol+offset where the probe is inserted.
  27
  28 FETCHARGS      : Arguments. Each probe can have up to 128 args.
  29  %REG          : Fetch register REG
  30
  31Event Profiling
  32---------------
  33 You can check the total number of probe hits and probe miss-hits via
  34/sys/kernel/debug/tracing/uprobe_profile.
  35 The first column is event name, the second is the number of probe hits,
  36the third is the number of probe miss-hits.
  37
  38Usage examples
  39--------------
  40To add a probe as a new event, write a new definition to uprobe_events
  41as below.
  42
  43  echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
  44
  45 This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash
  46
  47  echo > /sys/kernel/debug/tracing/uprobe_events
  48
  49 This clears all probe points.
  50
  51The following example shows how to dump the instruction pointer and %ax
  52a register at the probed text address.  Here we are trying to probe
  53function zfree in /bin/zsh
  54
  55    # cd /sys/kernel/debug/tracing/
  56    # cat /proc/`pgrep  zsh`/maps | grep /bin/zsh | grep r-xp
  57    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
  58    # objdump -T /bin/zsh | grep -w zfree
  59    0000000000446420 g    DF .text  0000000000000012  Base        zfree
  60
  610x46420 is the offset of zfree in object /bin/zsh that is loaded at
  620x00400000. Hence the command to probe would be :
  63
  64    # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events
  65
  66Please note: User has to explicitly calculate the offset of the probepoint
  67in the object. We can see the events that are registered by looking at the
  68uprobe_events file.
  69
  70    # cat uprobe_events
  71    p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax
  72
  73The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format
  74
  75    # cat events/uprobes/p_zsh_0x46420/format
  76    name: p_zsh_0x46420
  77    ID: 922
  78    format:
  79        field:unsigned short common_type;       offset:0;       size:2; signed:0;
  80        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
  81        field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
  82        field:int common_pid;   offset:4;       size:4; signed:1;
  83        field:int common_padding;       offset:8;       size:4; signed:1;
  84
  85        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
  86        field:u32 arg1; offset:16;      size:4; signed:0;
  87        field:u32 arg2; offset:20;      size:4; signed:0;
  88
  89    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
  90
  91Right after definition, each event is disabled by default. For tracing these
  92events, you need to enable it by:
  93
  94    # echo 1 > events/uprobes/enable
  95
  96Lets disable the event after sleeping for some time.
  97    # sleep 20
  98    # echo 0 > events/uprobes/enable
  99
 100And you can see the traced information via /sys/kernel/debug/tracing/trace.
 101
 102    # cat trace
 103    # tracer: nop
 104    #
 105    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 106    #              | |       |          |         |
 107                 zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
 108                 zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
 109                 zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
 110                 zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
 111
 112Each line shows us probes were triggered for a pid 24842 with ip being
 1130x446421 and contents of ax register being 79.
 114
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