Documentation/trace/kprobetrace.txt

   1                         Kprobe-based Event Tracing
   2                         ==========================
   3
   4                  Documentation is written by Masami Hiramatsu
   5
   6
   7 Overview
   8 --------
   9 These events are similar to tracepoint based events. Instead of Tracepoint,
  10 this is based on kprobes (kprobe and kretprobe). So it can probe wherever
  11 kprobes can probe (this means, all functions body except for __kprobes
  12 functions). Unlike the Tracepoint based event, this can be added and removed
  13 dynamically, on the fly.
  14
  15 To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y.
  16
  17 Similar to the events tracer, this doesn't need to be activated via
  18 current_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
  23 Synopsis 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   $comm         : Fetch current task comm.
  44   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
  45   NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
  46   FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
  47                   (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
  48                   are supported.
  49
  50   (*) only for return probe.
  51   (**) this is useful for fetching a field of data structures.
  52
  53 Types
  54 -----
  55 Several types are supported for fetch-args. Kprobe tracer will access memory
  56 by given type. Prefix 's' and 'u' means those types are signed and unsigned
  57 respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
  58 String type is a special type, which fetches a "null-terminated" string from
  59 kernel space. This means it will fail and store NULL if the string container
  60 has been paged out.
  61 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
  62 offset, and container-size (usually 32). The syntax is;
  63
  64  b<bit-width>@<bit-offset>/<container-size>
  65
  66 For $comm, the default type is "string"; any other type is invalid.
  67
  68
  69 Per-Probe Event Filtering
  70 -------------------------
  71  Per-probe event filtering feature allows you to set different filter on each
  72 probe and gives you what arguments will be shown in trace buffer. If an event
  73 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
  74 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
  75 'enabled', 'format' and 'filter'.
  76
  77 enabled:
  78   You can enable/disable the probe by writing 1 or 0 on it.
  79
  80 format:
  81   This shows the format of this probe event.
  82
  83 filter:
  84   You can write filtering rules of this event.
  85
  86 id:
  87   This shows the id of this probe event.
  88
  89
  90 Event Profiling
  91 ---------------
  92  You can check the total number of probe hits and probe miss-hits via
  93 /sys/kernel/debug/tracing/kprobe_profile.
  94  The first column is event name, the second is the number of probe hits,
  95 the third is the number of probe miss-hits.
  96
  97
  98 Usage examples
  99 --------------
 100 To add a probe as a new event, write a new definition to kprobe_events
 101 as below.
 102
 103   echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
 104
 105  This sets a kprobe on the top of do_sys_open() function with recording
 106 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
 107 assigned to each function argument depends on arch-specific ABI. If you unsure
 108 the ABI, please try to use probe subcommand of perf-tools (you can find it
 109 under tools/perf/).
 110 As this example shows, users can choose more familiar names for each arguments.
 111
 112   echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
 113
 114  This sets a kretprobe on the return point of do_sys_open() function with
 115 recording return value as "myretprobe" event.
 116  You can see the format of these events via
 117 /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
 118
 119   cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
 120 name: myprobe
 121 ID: 780
 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
 128         field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
 129         field:int __probe_nargs;        offset:16;      size:4; signed:1;
 130         field:unsigned long dfd;        offset:20;      size:4; signed:0;
 131         field:unsigned long filename;   offset:24;      size:4; signed:0;
 132         field:unsigned long flags;      offset:28;      size:4; signed:0;
 133         field:unsigned long mode;       offset:32;      size:4; signed:0;
 134
 135
 136 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
 137 REC->dfd, REC->filename, REC->flags, REC->mode
 138
 139  You can see that the event has 4 arguments as in the expressions you specified.
 140
 141   echo > /sys/kernel/debug/tracing/kprobe_events
 142
 143  This clears all probe points.
 144
 145  Or,
 146
 147   echo -:myprobe >> kprobe_events
 148
 149  This clears probe points selectively.
 150
 151  Right after definition, each event is disabled by default. For tracing these
 152 events, you need to enable it.
 153
 154   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
 155   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
 156
 157  And you can see the traced information via /sys/kernel/debug/tracing/trace.
 158
 159   cat /sys/kernel/debug/tracing/trace
 160 # tracer: nop
 161 #
 162 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 163 #              | |       |          |         |
 164            <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
 165            <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
 166            <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
 167            <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 168            <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
 169            <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 170
 171
 172  Each line shows when the kernel hits an event, and <- SYMBOL means kernel
 173 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
 174 returns from do_sys_open to sys_open+0x1b).
 175