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Linux/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   +|-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 
 52 Types
 53 -----
 54 Several types are supported for fetch-args. Kprobe tracer will access memory
 55 by given type. Prefix 's' and 'u' means those types are signed and unsigned
 56 respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
 57 String type is a special type, which fetches a "null-terminated" string from
 58 kernel space. This means it will fail and store NULL if the string container
 59 has been paged out.
 60 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
 61 offset, and container-size (usually 32). The syntax is;
 62 
 63  b<bit-width>@<bit-offset>/<container-size>
 64 
 65 
 66 Per-Probe Event Filtering
 67 -------------------------
 68  Per-probe event filtering feature allows you to set different filter on each
 69 probe and gives you what arguments will be shown in trace buffer. If an event
 70 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
 71 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
 72 'enabled', 'format' and 'filter'.
 73 
 74 enabled:
 75   You can enable/disable the probe by writing 1 or 0 on it.
 76 
 77 format:
 78   This shows the format of this probe event.
 79 
 80 filter:
 81   You can write filtering rules of this event.
 82 
 83 id:
 84   This shows the id of this probe event.
 85 
 86 
 87 Event 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,
 92 the third is the number of probe miss-hits.
 93 
 94 
 95 Usage examples
 96 --------------
 97 To add a probe as a new event, write a new definition to kprobe_events
 98 as 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
103 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
104 assigned to each function argument depends on arch-specific ABI. If you unsure
105 the ABI, please try to use probe subcommand of perf-tools (you can find it
106 under tools/perf/).
107 As 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
112 recording 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
117 name: myprobe
118 ID: 780
119 format:
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 
133 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
134 REC->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
149 events, 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
170 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
171 returns from do_sys_open to sys_open+0x1b).
172 

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