1 ==========================
2 Kprobe-based Event Tracing
3 ==========================
5 :Author: Masami Hiramatsu
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 except those with
12 __kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
13 Unlike the Tracepoint based event, this can be added and removed
14 dynamically, on the fly.
16 To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
18 Similar to the events tracer, this doesn't need to be activated via
19 current_tracer. Instead of that, add probe points via
20 /sys/kernel/debug/tracing/kprobe_events, and enable it via
21 /sys/kernel/debug/tracing/events/kprobes/<EVENT>/enable.
24 Synopsis of kprobe_events
25 -------------------------
28 p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
29 r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
30 -:[GRP/]EVENT : Clear a probe
32 GRP : Group name. If omitted, use "kprobes" for it.
33 EVENT : Event name. If omitted, the event name is generated
34 based on SYM+offs or MEMADDR.
35 MOD : Module name which has given SYM.
36 SYM[+offs] : Symbol+offset where the probe is inserted.
37 MEMADDR : Address where the probe is inserted.
38 MAXACTIVE : Maximum number of instances of the specified function that
39 can be probed simultaneously, or 0 for the default value
40 as defined in Documentation/kprobes.txt section 1.3.1.
42 FETCHARGS : Arguments. Each probe can have up to 128 args.
43 %REG : Fetch register REG
44 @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
45 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
46 $stackN : Fetch Nth entry of stack (N >= 0)
47 $stack : Fetch stack address.
48 $argN : Fetch the Nth function argument. (N >= 1) (\*1)
49 $retval : Fetch return value.(\*2)
50 $comm : Fetch current task comm.
51 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(\*3)
52 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
53 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
54 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
55 (x8/x16/x32/x64), "string" and bitfield are supported.
57 (\*1) only for the probe on function entry (offs == 0).
58 (\*2) only for return probe.
59 (\*3) this is useful for fetching a field of data structures.
63 Several types are supported for fetch-args. Kprobe tracer will access memory
64 by given type. Prefix 's' and 'u' means those types are signed and unsigned
65 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
66 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
67 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
69 These value types can be an array. To record array data, you can add '[N]'
70 (where N is a fixed number, less than 64) to the base type.
71 E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
72 Note that the array can be applied to memory type fetchargs, you can not
73 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
74 wrong, but '+8($stack):x8[8]' is OK.)
75 String type is a special type, which fetches a "null-terminated" string from
76 kernel space. This means it will fail and store NULL if the string container
78 The string array type is a bit different from other types. For other base
79 types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
80 as +0(%di):x32.) But string[1] is not equal to string. The string type itself
81 represents "char array", but string array type represents "char * array".
82 So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
83 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
84 offset, and container-size (usually 32). The syntax is::
86 b<bit-width>@<bit-offset>/<container-size>
88 Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
89 which shows given pointer in "symbol+offset" style.
90 For $comm, the default type is "string"; any other type is invalid.
93 Per-Probe Event Filtering
94 -------------------------
95 Per-probe event filtering feature allows you to set different filter on each
96 probe and gives you what arguments will be shown in trace buffer. If an event
97 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
98 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
99 'enable', 'format', 'filter' and 'trigger'.
102 You can enable/disable the probe by writing 1 or 0 on it.
105 This shows the format of this probe event.
108 You can write filtering rules of this event.
111 This shows the id of this probe event.
114 This allows to install trigger commands which are executed when the event is
115 hit (for details, see Documentation/trace/events.rst, section 6).
119 You can check the total number of probe hits and probe miss-hits via
120 /sys/kernel/debug/tracing/kprobe_profile.
121 The first column is event name, the second is the number of probe hits,
122 the third is the number of probe miss-hits.
127 To add a probe as a new event, write a new definition to kprobe_events
130 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
132 This sets a kprobe on the top of do_sys_open() function with recording
133 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
134 assigned to each function argument depends on arch-specific ABI. If you unsure
135 the ABI, please try to use probe subcommand of perf-tools (you can find it
137 As this example shows, users can choose more familiar names for each arguments.
140 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
142 This sets a kretprobe on the return point of do_sys_open() function with
143 recording return value as "myretprobe" event.
144 You can see the format of these events via
145 /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
148 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
152 field:unsigned short common_type; offset:0; size:2; signed:0;
153 field:unsigned char common_flags; offset:2; size:1; signed:0;
154 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
155 field:int common_pid; offset:4; size:4; signed:1;
157 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
158 field:int __probe_nargs; offset:16; size:4; signed:1;
159 field:unsigned long dfd; offset:20; size:4; signed:0;
160 field:unsigned long filename; offset:24; size:4; signed:0;
161 field:unsigned long flags; offset:28; size:4; signed:0;
162 field:unsigned long mode; offset:32; size:4; signed:0;
165 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
166 REC->dfd, REC->filename, REC->flags, REC->mode
168 You can see that the event has 4 arguments as in the expressions you specified.
171 echo > /sys/kernel/debug/tracing/kprobe_events
173 This clears all probe points.
178 echo -:myprobe >> kprobe_events
180 This clears probe points selectively.
182 Right after definition, each event is disabled by default. For tracing these
183 events, you need to enable it.
186 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
187 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
189 And you can see the traced information via /sys/kernel/debug/tracing/trace.
192 cat /sys/kernel/debug/tracing/trace
195 # TASK-PID CPU# TIMESTAMP FUNCTION
197 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
198 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
199 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
200 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
201 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
202 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
205 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
206 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
207 returns from do_sys_open to sys_open+0x1b).