linux/Documentation/trace/kprobetrace.txt
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   1                        Kprobe-based Event Tracing
   2                        ==========================
   3
   4                 Documentation is written by Masami Hiramatsu
   5
   6
   7Overview
   8--------
   9These events are similar to tracepoint based events. Instead of Tracepoint,
  10this is based on kprobes (kprobe and kretprobe). So it can probe wherever
  11kprobes can probe (this means, all functions body except for __kprobes
  12functions). Unlike the Tracepoint based event, this can be added and removed
  13dynamically, on the fly.
  14
  15To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y.
  16
  17Similar to the events tracer, this doesn't need to be activated via
  18current_tracer. Instead of that, add probe points via
  19/sys/kernel/debug/tracing/kprobe_events, and enable it via
  20/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
  21
  22
  23Synopsis of kprobe_events
  24-------------------------
  25  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]  : Set a probe
  26  r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]             : Set a return probe
  27  -:[GRP/]EVENT                                         : Clear a probe
  28
  29 GRP            : Group name. If omitted, use "kprobes" for it.
  30 EVENT          : Event name. If omitted, the event name is generated
  31                  based on SYM+offs or MEMADDR.
  32 MOD            : Module name which has given SYM.
  33 SYM[+offs]     : Symbol+offset where the probe is inserted.
  34 MEMADDR        : Address where the probe is inserted.
  35
  36 FETCHARGS      : Arguments. Each probe can have up to 128 args.
  37  %REG          : Fetch register REG
  38  @ADDR         : Fetch memory at ADDR (ADDR should be in kernel)
  39  @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
  40  $stackN       : Fetch Nth entry of stack (N >= 0)
  41  $stack        : Fetch stack address.
  42  $retval       : Fetch return value.(*)
  43  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
  44  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
  45  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
  46                  (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
  47                  are supported.
  48
  49  (*) only for return probe.
  50  (**) this is useful for fetching a field of data structures.
  51
  52Types
  53-----
  54Several types are supported for fetch-args. Kprobe tracer will access memory
  55by given type. Prefix 's' and 'u' means those types are signed and unsigned
  56respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
  57String type is a special type, which fetches a "null-terminated" string from
  58kernel space. This means it will fail and store NULL if the string container
  59has been paged out.
  60Bitfield is another special type, which takes 3 parameters, bit-width, bit-
  61offset, and container-size (usually 32). The syntax is;
  62
  63 b<bit-width>@<bit-offset>/<container-size>
  64
  65
  66Per-Probe Event Filtering
  67-------------------------
  68 Per-probe event filtering feature allows you to set different filter on each
  69probe and gives you what arguments will be shown in trace buffer. If an event
  70name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
  71under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
  72'enabled', 'format' and 'filter'.
  73
  74enabled:
  75  You can enable/disable the probe by writing 1 or 0 on it.
  76
  77format:
  78  This shows the format of this probe event.
  79
  80filter:
  81  You can write filtering rules of this event.
  82
  83id:
  84  This shows the id of this probe event.
  85
  86
  87Event Profiling
  88---------------
  89 You can check the total number of probe hits and probe miss-hits via
  90/sys/kernel/debug/tracing/kprobe_profile.
  91 The first column is event name, the second is the number of probe hits,
  92the third is the number of probe miss-hits.
  93
  94
  95Usage examples
  96--------------
  97To add a probe as a new event, write a new definition to kprobe_events
  98as below.
  99
 100  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
 101
 102 This sets a kprobe on the top of do_sys_open() function with recording
 1031st to 4th arguments as "myprobe" event. Note, which register/stack entry is
 104assigned to each function argument depends on arch-specific ABI. If you unsure
 105the ABI, please try to use probe subcommand of perf-tools (you can find it
 106under tools/perf/).
 107As this example shows, users can choose more familiar names for each arguments.
 108
 109  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
 110
 111 This sets a kretprobe on the return point of do_sys_open() function with
 112recording return value as "myretprobe" event.
 113 You can see the format of these events via
 114/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
 115
 116  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
 117name: myprobe
 118ID: 780
 119format:
 120        field:unsigned short common_type;       offset:0;       size:2; signed:0;
 121        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
 122        field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
 123        field:int common_pid;   offset:4;       size:4; signed:1;
 124
 125        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
 126        field:int __probe_nargs;        offset:16;      size:4; signed:1;
 127        field:unsigned long dfd;        offset:20;      size:4; signed:0;
 128        field:unsigned long filename;   offset:24;      size:4; signed:0;
 129        field:unsigned long flags;      offset:28;      size:4; signed:0;
 130        field:unsigned long mode;       offset:32;      size:4; signed:0;
 131
 132
 133print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
 134REC->dfd, REC->filename, REC->flags, REC->mode
 135
 136 You can see that the event has 4 arguments as in the expressions you specified.
 137
 138  echo > /sys/kernel/debug/tracing/kprobe_events
 139
 140 This clears all probe points.
 141
 142 Or,
 143
 144  echo -:myprobe >> kprobe_events
 145
 146 This clears probe points selectively.
 147
 148 Right after definition, each event is disabled by default. For tracing these
 149events, you need to enable it.
 150
 151  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
 152  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
 153
 154 And you can see the traced information via /sys/kernel/debug/tracing/trace.
 155
 156  cat /sys/kernel/debug/tracing/trace
 157# tracer: nop
 158#
 159#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 160#              | |       |          |         |
 161           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
 162           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
 163           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
 164           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 165           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
 166           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 167
 168
 169 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
 170returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
 171returns from do_sys_open to sys_open+0x1b).
 172
 173
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