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.
23 You can also use /sys/kernel/debug/tracing/dynamic_events instead of
24 kprobe_events. That interface will provide unified access to other
27 Synopsis of kprobe_events
28 -------------------------
31 p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
32 r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
33 -:[GRP/]EVENT : Clear a probe
35 GRP : Group name. If omitted, use "kprobes" for it.
36 EVENT : Event name. If omitted, the event name is generated
37 based on SYM+offs or MEMADDR.
38 MOD : Module name which has given SYM.
39 SYM[+offs] : Symbol+offset where the probe is inserted.
40 MEMADDR : Address where the probe is inserted.
41 MAXACTIVE : Maximum number of instances of the specified function that
42 can be probed simultaneously, or 0 for the default value
43 as defined in Documentation/kprobes.txt section 1.3.1.
45 FETCHARGS : Arguments. Each probe can have up to 128 args.
46 %REG : Fetch register REG
47 @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
48 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
49 $stackN : Fetch Nth entry of stack (N >= 0)
50 $stack : Fetch stack address.
51 $argN : Fetch the Nth function argument. (N >= 1) (\*1)
52 $retval : Fetch return value.(\*2)
53 $comm : Fetch current task comm.
54 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
55 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
56 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
57 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
58 (x8/x16/x32/x64), "string", "ustring" and bitfield
61 (\*1) only for the probe on function entry (offs == 0).
62 (\*2) only for return probe.
63 (\*3) this is useful for fetching a field of data structures.
64 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
68 Several types are supported for fetch-args. Kprobe tracer will access memory
69 by given type. Prefix 's' and 'u' means those types are signed and unsigned
70 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
71 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
72 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
74 These value types can be an array. To record array data, you can add '[N]'
75 (where N is a fixed number, less than 64) to the base type.
76 E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
77 Note that the array can be applied to memory type fetchargs, you can not
78 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
79 wrong, but '+8($stack):x8[8]' is OK.)
80 String type is a special type, which fetches a "null-terminated" string from
81 kernel space. This means it will fail and store NULL if the string container
82 has been paged out. "ustring" type is an alternative of string for user-space.
83 See :ref:`user_mem_access` for more info..
84 The string array type is a bit different from other types. For other base
85 types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
86 as +0(%di):x32.) But string[1] is not equal to string. The string type itself
87 represents "char array", but string array type represents "char * array".
88 So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
89 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
90 offset, and container-size (usually 32). The syntax is::
92 b<bit-width>@<bit-offset>/<container-size>
94 Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
95 which shows given pointer in "symbol+offset" style.
96 For $comm, the default type is "string"; any other type is invalid.
101 Kprobe events supports user-space memory access. For that purpose, you can use
102 either user-space dereference syntax or 'ustring' type.
104 The user-space dereference syntax allows you to access a field of a data
105 structure in user-space. This is done by adding the "u" prefix to the
106 dereference syntax. For example, +u4(%si) means it will read memory from the
107 address in the register %si offset by 4, and the memory is expected to be in
108 user-space. You can use this for strings too, e.g. +u0(%si):string will read
109 a string from the address in the register %si that is expected to be in user-
110 space. 'ustring' is a shortcut way of performing the same task. That is,
111 +0(%si):ustring is equivalent to +u0(%si):string.
113 Note that kprobe-event provides the user-memory access syntax but it doesn't
114 use it transparently. This means if you use normal dereference or string type
115 for user memory, it might fail, and may always fail on some archs. The user
116 has to carefully check if the target data is in kernel or user space.
118 Per-Probe Event Filtering
119 -------------------------
120 Per-probe event filtering feature allows you to set different filter on each
121 probe and gives you what arguments will be shown in trace buffer. If an event
122 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
123 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
124 'enable', 'format', 'filter' and 'trigger'.
127 You can enable/disable the probe by writing 1 or 0 on it.
130 This shows the format of this probe event.
133 You can write filtering rules of this event.
136 This shows the id of this probe event.
139 This allows to install trigger commands which are executed when the event is
140 hit (for details, see Documentation/trace/events.rst, section 6).
144 You can check the total number of probe hits and probe miss-hits via
145 /sys/kernel/debug/tracing/kprobe_profile.
146 The first column is event name, the second is the number of probe hits,
147 the third is the number of probe miss-hits.
149 Kernel Boot Parameter
150 ---------------------
151 You can add and enable new kprobe events when booting up the kernel by
152 "kprobe_event=" parameter. The parameter accepts a semicolon-delimited
153 kprobe events, which format is similar to the kprobe_events.
154 The difference is that the probe definition parameters are comma-delimited
155 instead of space. For example, adding myprobe event on do_sys_open like below
157 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
159 should be below for kernel boot parameter (just replace spaces with comma)
161 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
166 To add a probe as a new event, write a new definition to kprobe_events
169 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
171 This sets a kprobe on the top of do_sys_open() function with recording
172 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
173 assigned to each function argument depends on arch-specific ABI. If you unsure
174 the ABI, please try to use probe subcommand of perf-tools (you can find it
176 As this example shows, users can choose more familiar names for each arguments.
179 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
181 This sets a kretprobe on the return point of do_sys_open() function with
182 recording return value as "myretprobe" event.
183 You can see the format of these events via
184 /sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
187 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
191 field:unsigned short common_type; offset:0; size:2; signed:0;
192 field:unsigned char common_flags; offset:2; size:1; signed:0;
193 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
194 field:int common_pid; offset:4; size:4; signed:1;
196 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
197 field:int __probe_nargs; offset:16; size:4; signed:1;
198 field:unsigned long dfd; offset:20; size:4; signed:0;
199 field:unsigned long filename; offset:24; size:4; signed:0;
200 field:unsigned long flags; offset:28; size:4; signed:0;
201 field:unsigned long mode; offset:32; size:4; signed:0;
204 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
205 REC->dfd, REC->filename, REC->flags, REC->mode
207 You can see that the event has 4 arguments as in the expressions you specified.
210 echo > /sys/kernel/debug/tracing/kprobe_events
212 This clears all probe points.
217 echo -:myprobe >> kprobe_events
219 This clears probe points selectively.
221 Right after definition, each event is disabled by default. For tracing these
222 events, you need to enable it.
225 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
226 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
228 Use the following command to start tracing in an interval.
231 # echo 1 > tracing_on
233 # echo 0 > tracing_on
235 And you can see the traced information via /sys/kernel/debug/tracing/trace.
238 cat /sys/kernel/debug/tracing/trace
241 # TASK-PID CPU# TIMESTAMP FUNCTION
243 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
244 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
245 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
246 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
247 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
248 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
251 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
252 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
253 returns from do_sys_open to sys_open+0x1b).