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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), 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 

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