Documentation/trace/uprobetracer.txt

   1             Uprobe-tracer: Uprobe-based Event Tracing
   2             =========================================
   3
   4            Documentation written by Srikar Dronamraju
   5
   6
   7 Overview
   8 --------
   9 Uprobe based trace events are similar to kprobe based trace events.
  10 To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
  11
  12 Similar to the kprobe-event tracer, this doesn't need to be activated via
  13 current_tracer. Instead of that, add probe points via
  14 /sys/kernel/debug/tracing/uprobe_events, and enable it via
  15 /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
  16
  17 However unlike kprobe-event tracer, the uprobe event interface expects the
  18 user to calculate the offset of the probepoint in the object.
  19
  20 Synopsis of uprobe_tracer
  21 -------------------------
  22   p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
  23   r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
  24   -:[GRP/]EVENT                           : Clear uprobe or uretprobe event
  25
  26   GRP           : Group name. If omitted, "uprobes" is the default value.
  27   EVENT         : Event name. If omitted, the event name is generated based
  28                   on PATH+OFFSET.
  29   PATH          : Path to an executable or a library.
  30   OFFSET        : Offset where the probe is inserted.
  31
  32   FETCHARGS     : Arguments. Each probe can have up to 128 args.
  33    %REG         : Fetch register REG
  34    @ADDR        : Fetch memory at ADDR (ADDR should be in userspace)
  35    @+OFFSET     : Fetch memory at OFFSET (OFFSET from same file as PATH)
  36    $stackN      : Fetch Nth entry of stack (N >= 0)
  37    $stack       : Fetch stack address.
  38    $retval      : Fetch return value.(*)
  39    +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
  40    NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
  41    FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
  42                        (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
  43                        are supported.
  44
  45   (*) only for return probe.
  46   (**) this is useful for fetching a field of data structures.
  47
  48 Types
  49 -----
  50 Several types are supported for fetch-args. Uprobe tracer will access memory
  51 by given type. Prefix 's' and 'u' means those types are signed and unsigned
  52 respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
  53 String type is a special type, which fetches a "null-terminated" string from
  54 user space.
  55 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
  56 offset, and container-size (usually 32). The syntax is;
  57
  58  b<bit-width>@<bit-offset>/<container-size>
  59
  60
  61 Event Profiling
  62 ---------------
  63 You can check the total number of probe hits and probe miss-hits via
  64 /sys/kernel/debug/tracing/uprobe_profile.
  65 The first column is event name, the second is the number of probe hits,
  66 the third is the number of probe miss-hits.
  67
  68 Usage examples
  69 --------------
  70  * Add a probe as a new uprobe event, write a new definition to uprobe_events
  71 as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
  72
  73     echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
  74
  75  * Add a probe as a new uretprobe event:
  76
  77     echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
  78
  79  * Unset registered event:
  80
  81     echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
  82
  83  * Print out the events that are registered:
  84
  85     cat /sys/kernel/debug/tracing/uprobe_events
  86
  87  * Clear all events:
  88
  89     echo > /sys/kernel/debug/tracing/uprobe_events
  90
  91 Following example shows how to dump the instruction pointer and %ax register
  92 at the probed text address. Probe zfree function in /bin/zsh:
  93
  94     # cd /sys/kernel/debug/tracing/
  95     # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
  96     00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
  97     # objdump -T /bin/zsh | grep -w zfree
  98     0000000000446420 g    DF .text  0000000000000012  Base        zfree
  99
 100   0x46420 is the offset of zfree in object /bin/zsh that is loaded at
 101   0x00400000. Hence the command to uprobe would be:
 102
 103     # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
 104
 105   And the same for the uretprobe would be:
 106
 107     # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
 108
 109 Please note: User has to explicitly calculate the offset of the probe-point
 110 in the object. We can see the events that are registered by looking at the
 111 uprobe_events file.
 112
 113     # cat uprobe_events
 114     p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
 115     r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
 116
 117 Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
 118
 119     # cat events/uprobes/zfree_entry/format
 120     name: zfree_entry
 121     ID: 922
 122     format:
 123          field:unsigned short common_type;         offset:0;  size:2; signed:0;
 124          field:unsigned char common_flags;         offset:2;  size:1; signed:0;
 125          field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
 126          field:int common_pid;                     offset:4;  size:4; signed:1;
 127          field:int common_padding;                 offset:8;  size:4; signed:1;
 128
 129          field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
 130          field:u32 arg1;                           offset:16; size:4; signed:0;
 131          field:u32 arg2;                           offset:20; size:4; signed:0;
 132
 133     print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
 134
 135 Right after definition, each event is disabled by default. For tracing these
 136 events, you need to enable it by:
 137
 138     # echo 1 > events/uprobes/enable
 139
 140 Lets disable the event after sleeping for some time.
 141
 142     # sleep 20
 143     # echo 0 > events/uprobes/enable
 144
 145 And you can see the traced information via /sys/kernel/debug/tracing/trace.
 146
 147     # cat trace
 148     # tracer: nop
 149     #
 150     #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 151     #              | |       |          |         |
 152                  zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
 153                  zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
 154                  zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
 155                  zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
 156
 157 Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
 158 and contents of ax register being 79. And uretprobe was triggered with ip at
 159 0x446540 with counterpart function entry at 0x446420.