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_EVENTS=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), hexadecimal types
  48                   (x8/x16/x32/x64), "string" and bitfield 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. 'x' prefix implies it is unsigned. Traced arguments are shown
  58 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
  59 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
  60 x86-64 uses x64).
  61 String type is a special type, which fetches a "null-terminated" string from
  62 kernel space. This means it will fail and store NULL if the string container
  63 has been paged out.
  64 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
  65 offset, and container-size (usually 32). The syntax is;
  66
  67  b<bit-width>@<bit-offset>/<container-size>
  68
  69 For $comm, the default type is "string"; any other type is invalid.
  70
  71
  72 Per-Probe Event Filtering
  73 -------------------------
  74  Per-probe event filtering feature allows you to set different filter on each
  75 probe and gives you what arguments will be shown in trace buffer. If an event
  76 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
  77 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
  78 'enabled', 'format' and 'filter'.
  79
  80 enabled:
  81   You can enable/disable the probe by writing 1 or 0 on it.
  82
  83 format:
  84   This shows the format of this probe event.
  85
  86 filter:
  87   You can write filtering rules of this event.
  88
  89 id:
  90   This shows the id of this probe event.
  91
  92
  93 Event Profiling
  94 ---------------
  95  You can check the total number of probe hits and probe miss-hits via
  96 /sys/kernel/debug/tracing/kprobe_profile.
  97  The first column is event name, the second is the number of probe hits,
  98 the third is the number of probe miss-hits.
  99
 100
 101 Usage examples
 102 --------------
 103 To add a probe as a new event, write a new definition to kprobe_events
 104 as below.
 105
 106   echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
 107
 108  This sets a kprobe on the top of do_sys_open() function with recording
 109 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
 110 assigned to each function argument depends on arch-specific ABI. If you unsure
 111 the ABI, please try to use probe subcommand of perf-tools (you can find it
 112 under tools/perf/).
 113 As this example shows, users can choose more familiar names for each arguments.
 114
 115   echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
 116
 117  This sets a kretprobe on the return point of do_sys_open() function with
 118 recording return value as "myretprobe" event.
 119  You can see the format of these events via
 120 /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
 121
 122   cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
 123 name: myprobe
 124 ID: 780
 125 format:
 126         field:unsigned short common_type;       offset:0;       size:2; signed:0;
 127         field:unsigned char common_flags;       offset:2;       size:1; signed:0;
 128         field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
 129         field:int common_pid;   offset:4;       size:4; signed:1;
 130
 131         field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
 132         field:int __probe_nargs;        offset:16;      size:4; signed:1;
 133         field:unsigned long dfd;        offset:20;      size:4; signed:0;
 134         field:unsigned long filename;   offset:24;      size:4; signed:0;
 135         field:unsigned long flags;      offset:28;      size:4; signed:0;
 136         field:unsigned long mode;       offset:32;      size:4; signed:0;
 137
 138
 139 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
 140 REC->dfd, REC->filename, REC->flags, REC->mode
 141
 142  You can see that the event has 4 arguments as in the expressions you specified.
 143
 144   echo > /sys/kernel/debug/tracing/kprobe_events
 145
 146  This clears all probe points.
 147
 148  Or,
 149
 150   echo -:myprobe >> kprobe_events
 151
 152  This clears probe points selectively.
 153
 154  Right after definition, each event is disabled by default. For tracing these
 155 events, you need to enable it.
 156
 157   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
 158   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
 159
 160  And you can see the traced information via /sys/kernel/debug/tracing/trace.
 161
 162   cat /sys/kernel/debug/tracing/trace
 163 # tracer: nop
 164 #
 165 #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
 166 #              | |       |          |         |
 167            <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
 168            <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
 169            <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
 170            <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 171            <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
 172            <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
 173
 174
 175  Each line shows when the kernel hits an event, and <- SYMBOL means kernel
 176 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
 177 returns from do_sys_open to sys_open+0x1b).
 178