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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   $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 

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