1 ==========================
2 Kprobe-based Event Tracing
3 ==========================
5 :Author: Masami Hiramatsu
9 These events are similar to tracepoint-based events. Instead of tracepoints,
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 event tracer, this doesn't need to be activated via
19 current_tracer. Instead of that, add probe points via
20 /sys/kernel/tracing/kprobe_events, and enable it via
21 /sys/kernel/tracing/events/kprobes/<EVENT>/enable.
23 You can also use /sys/kernel/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 p[:[GRP/][EVENT]] [MOD:]SYM[+0]%return [FETCHARGS] : Set a return probe
34 -:[GRP/][EVENT] : Clear a probe
36 GRP : Group name. If omitted, use "kprobes" for it.
37 EVENT : Event name. If omitted, the event name is generated
38 based on SYM+offs or MEMADDR.
39 MOD : Module name which has given SYM.
40 SYM[+offs] : Symbol+offset where the probe is inserted.
41 SYM%return : Return address of the symbol
42 MEMADDR : Address where the probe is inserted.
43 MAXACTIVE : Maximum number of instances of the specified function that
44 can be probed simultaneously, or 0 for the default value
45 as defined in Documentation/trace/kprobes.rst section 1.3.1.
47 FETCHARGS : Arguments. Each probe can have up to 128 args.
48 %REG : Fetch register REG
49 @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
50 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
51 $stackN : Fetch Nth entry of stack (N >= 0)
52 $stack : Fetch stack address.
53 $argN : Fetch the Nth function argument. (N >= 1) (\*1)
54 $retval : Fetch return value.(\*2)
55 $comm : Fetch current task comm.
56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57 \IMM : Store an immediate value to the argument.
58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61 (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
62 and bitfield are supported.
64 (\*1) only for the probe on function entry (offs == 0). Note, this argument access
65 is best effort, because depending on the argument type, it may be passed on
66 the stack. But this only support the arguments via registers.
67 (\*2) only for return probe. Note that this is also best effort. Depending on the
68 return value type, it might be passed via a pair of registers. But this only
69 accesses one register.
70 (\*3) this is useful for fetching a field of data structures.
71 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
73 Function arguments at kretprobe
74 -------------------------------
75 Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This
76 is useful to record the function parameter and return value at once, and
77 trace the difference of structure fields (for debuging a function whether it
78 correctly updates the given data structure or not).
79 See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how
82 .. _kprobetrace_types:
86 Several types are supported for fetchargs. Kprobe tracer will access memory
87 by given type. Prefix 's' and 'u' means those types are signed and unsigned
88 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
89 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
90 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
93 These value types can be an array. To record array data, you can add '[N]'
94 (where N is a fixed number, less than 64) to the base type.
95 E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
96 Note that the array can be applied to memory type fetchargs, you can not
97 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
98 wrong, but '+8($stack):x8[8]' is OK.)
100 Char type can be used to show the character value of traced arguments.
102 String type is a special type, which fetches a "null-terminated" string from
103 kernel space. This means it will fail and store NULL if the string container
104 has been paged out. "ustring" type is an alternative of string for user-space.
105 See :ref:`user_mem_access` for more info.
107 The string array type is a bit different from other types. For other base
108 types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
109 as +0(%di):x32.) But string[1] is not equal to string. The string type itself
110 represents "char array", but string array type represents "char * array".
111 So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
112 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
113 offset, and container-size (usually 32). The syntax is::
115 b<bit-width>@<bit-offset>/<container-size>
117 Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
118 which shows given pointer in "symbol+offset" style.
119 On the other hand, symbol-string type ('symstr') converts the given address to
120 "symbol+offset/symbolsize" style and stores it as a null-terminated string.
121 With 'symstr' type, you can filter the event with wildcard pattern of the
122 symbols, and you don't need to solve symbol name by yourself.
123 For $comm, the default type is "string"; any other type is invalid.
129 Kprobe events supports user-space memory access. For that purpose, you can use
130 either user-space dereference syntax or 'ustring' type.
132 The user-space dereference syntax allows you to access a field of a data
133 structure in user-space. This is done by adding the "u" prefix to the
134 dereference syntax. For example, +u4(%si) means it will read memory from the
135 address in the register %si offset by 4, and the memory is expected to be in
136 user-space. You can use this for strings too, e.g. +u0(%si):string will read
137 a string from the address in the register %si that is expected to be in user-
138 space. 'ustring' is a shortcut way of performing the same task. That is,
139 +0(%si):ustring is equivalent to +u0(%si):string.
141 Note that kprobe-event provides the user-memory access syntax but it doesn't
142 use it transparently. This means if you use normal dereference or string type
143 for user memory, it might fail, and may always fail on some architectures. The
144 user has to carefully check if the target data is in kernel or user space.
146 Per-Probe Event Filtering
147 -------------------------
148 Per-probe event filtering feature allows you to set different filter on each
149 probe and gives you what arguments will be shown in trace buffer. If an event
150 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
151 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
152 'enable', 'format', 'filter' and 'trigger'.
155 You can enable/disable the probe by writing 1 or 0 on it.
158 This shows the format of this probe event.
161 You can write filtering rules of this event.
164 This shows the id of this probe event.
167 This allows to install trigger commands which are executed when the event is
168 hit (for details, see Documentation/trace/events.rst, section 6).
172 You can check the total number of probe hits and probe miss-hits via
173 /sys/kernel/tracing/kprobe_profile.
174 The first column is event name, the second is the number of probe hits,
175 the third is the number of probe miss-hits.
177 Kernel Boot Parameter
178 ---------------------
179 You can add and enable new kprobe events when booting up the kernel by
180 "kprobe_event=" parameter. The parameter accepts a semicolon-delimited
181 kprobe events, which format is similar to the kprobe_events.
182 The difference is that the probe definition parameters are comma-delimited
183 instead of space. For example, adding myprobe event on do_sys_open like below::
185 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
187 should be below for kernel boot parameter (just replace spaces with comma)::
189 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
194 To add a probe as a new event, write a new definition to kprobe_events
197 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
199 This sets a kprobe on the top of do_sys_open() function with recording
200 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
201 assigned to each function argument depends on arch-specific ABI. If you unsure
202 the ABI, please try to use probe subcommand of perf-tools (you can find it
204 As this example shows, users can choose more familiar names for each arguments.
207 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
209 This sets a kretprobe on the return point of do_sys_open() function with
210 recording return value as "myretprobe" event.
211 You can see the format of these events via
212 /sys/kernel/tracing/events/kprobes/<EVENT>/format.
215 cat /sys/kernel/tracing/events/kprobes/myprobe/format
219 field:unsigned short common_type; offset:0; size:2; signed:0;
220 field:unsigned char common_flags; offset:2; size:1; signed:0;
221 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
222 field:int common_pid; offset:4; size:4; signed:1;
224 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
225 field:int __probe_nargs; offset:16; size:4; signed:1;
226 field:unsigned long dfd; offset:20; size:4; signed:0;
227 field:unsigned long filename; offset:24; size:4; signed:0;
228 field:unsigned long flags; offset:28; size:4; signed:0;
229 field:unsigned long mode; offset:32; size:4; signed:0;
232 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
233 REC->dfd, REC->filename, REC->flags, REC->mode
235 You can see that the event has 4 arguments as in the expressions you specified.
238 echo > /sys/kernel/tracing/kprobe_events
240 This clears all probe points.
245 echo -:myprobe >> kprobe_events
247 This clears probe points selectively.
249 Right after definition, each event is disabled by default. For tracing these
250 events, you need to enable it.
253 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
254 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
256 Use the following command to start tracing in an interval.
259 # echo 1 > tracing_on
261 # echo 0 > tracing_on
263 And you can see the traced information via /sys/kernel/tracing/trace.
266 cat /sys/kernel/tracing/trace
269 # TASK-PID CPU# TIMESTAMP FUNCTION
271 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
272 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
273 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
274 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
275 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
276 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
279 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
280 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
281 returns from do_sys_open to sys_open+0x1b).