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 $comm : Fetch current task comm.
56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57 \IMM : Store an immediate value to the argument.
58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61 (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
62 and bitfield are supported.
64 (\*1) only for the probe on function entry (offs == 0). Note, this argument access
65 is best effort, because depending on the argument type, it may be passed on
66 the stack. But this only support the arguments via registers.
67 (\*2) only for return probe. Note that this is also best effort. Depending on the
68 return value type, it might be passed via a pair of registers. But this only
69 accesses one register.
70 (\*3) this is useful for fetching a field of data structures.
71 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
73 .. _kprobetrace_types:
77 Several types are supported for fetchargs. Kprobe tracer will access memory
78 by given type. Prefix 's' and 'u' means those types are signed and unsigned
79 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
80 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
81 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
84 These value types can be an array. To record array data, you can add '[N]'
85 (where N is a fixed number, less than 64) to the base type.
86 E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
87 Note that the array can be applied to memory type fetchargs, you can not
88 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
89 wrong, but '+8($stack):x8[8]' is OK.)
91 Char type can be used to show the character value of traced arguments.
93 String type is a special type, which fetches a "null-terminated" string from
94 kernel space. This means it will fail and store NULL if the string container
95 has been paged out. "ustring" type is an alternative of string for user-space.
96 See :ref:`user_mem_access` for more info.
98 The string array type is a bit different from other types. For other base
99 types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
100 as +0(%di):x32.) But string[1] is not equal to string. The string type itself
101 represents "char array", but string array type represents "char * array".
102 So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
103 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
104 offset, and container-size (usually 32). The syntax is::
106 b<bit-width>@<bit-offset>/<container-size>
108 Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
109 which shows given pointer in "symbol+offset" style.
110 On the other hand, symbol-string type ('symstr') converts the given address to
111 "symbol+offset/symbolsize" style and stores it as a null-terminated string.
112 With 'symstr' type, you can filter the event with wildcard pattern of the
113 symbols, and you don't need to solve symbol name by yourself.
114 For $comm, the default type is "string"; any other type is invalid.
120 Kprobe events supports user-space memory access. For that purpose, you can use
121 either user-space dereference syntax or 'ustring' type.
123 The user-space dereference syntax allows you to access a field of a data
124 structure in user-space. This is done by adding the "u" prefix to the
125 dereference syntax. For example, +u4(%si) means it will read memory from the
126 address in the register %si offset by 4, and the memory is expected to be in
127 user-space. You can use this for strings too, e.g. +u0(%si):string will read
128 a string from the address in the register %si that is expected to be in user-
129 space. 'ustring' is a shortcut way of performing the same task. That is,
130 +0(%si):ustring is equivalent to +u0(%si):string.
132 Note that kprobe-event provides the user-memory access syntax but it doesn't
133 use it transparently. This means if you use normal dereference or string type
134 for user memory, it might fail, and may always fail on some architectures. The
135 user has to carefully check if the target data is in kernel or user space.
137 Per-Probe Event Filtering
138 -------------------------
139 Per-probe event filtering feature allows you to set different filter on each
140 probe and gives you what arguments will be shown in trace buffer. If an event
141 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
142 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
143 'enable', 'format', 'filter' and 'trigger'.
146 You can enable/disable the probe by writing 1 or 0 on it.
149 This shows the format of this probe event.
152 You can write filtering rules of this event.
155 This shows the id of this probe event.
158 This allows to install trigger commands which are executed when the event is
159 hit (for details, see Documentation/trace/events.rst, section 6).
163 You can check the total number of probe hits and probe miss-hits via
164 /sys/kernel/tracing/kprobe_profile.
165 The first column is event name, the second is the number of probe hits,
166 the third is the number of probe miss-hits.
168 Kernel Boot Parameter
169 ---------------------
170 You can add and enable new kprobe events when booting up the kernel by
171 "kprobe_event=" parameter. The parameter accepts a semicolon-delimited
172 kprobe events, which format is similar to the kprobe_events.
173 The difference is that the probe definition parameters are comma-delimited
174 instead of space. For example, adding myprobe event on do_sys_open like below::
176 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
178 should be below for kernel boot parameter (just replace spaces with comma)::
180 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
185 To add a probe as a new event, write a new definition to kprobe_events
188 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
190 This sets a kprobe on the top of do_sys_open() function with recording
191 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
192 assigned to each function argument depends on arch-specific ABI. If you unsure
193 the ABI, please try to use probe subcommand of perf-tools (you can find it
195 As this example shows, users can choose more familiar names for each arguments.
198 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
200 This sets a kretprobe on the return point of do_sys_open() function with
201 recording return value as "myretprobe" event.
202 You can see the format of these events via
203 /sys/kernel/tracing/events/kprobes/<EVENT>/format.
206 cat /sys/kernel/tracing/events/kprobes/myprobe/format
210 field:unsigned short common_type; offset:0; size:2; signed:0;
211 field:unsigned char common_flags; offset:2; size:1; signed:0;
212 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
213 field:int common_pid; offset:4; size:4; signed:1;
215 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
216 field:int __probe_nargs; offset:16; size:4; signed:1;
217 field:unsigned long dfd; offset:20; size:4; signed:0;
218 field:unsigned long filename; offset:24; size:4; signed:0;
219 field:unsigned long flags; offset:28; size:4; signed:0;
220 field:unsigned long mode; offset:32; size:4; signed:0;
223 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
224 REC->dfd, REC->filename, REC->flags, REC->mode
226 You can see that the event has 4 arguments as in the expressions you specified.
229 echo > /sys/kernel/tracing/kprobe_events
231 This clears all probe points.
236 echo -:myprobe >> kprobe_events
238 This clears probe points selectively.
240 Right after definition, each event is disabled by default. For tracing these
241 events, you need to enable it.
244 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
245 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
247 Use the following command to start tracing in an interval.
250 # echo 1 > tracing_on
252 # echo 0 > tracing_on
254 And you can see the traced information via /sys/kernel/tracing/trace.
257 cat /sys/kernel/tracing/trace
260 # TASK-PID CPU# TIMESTAMP FUNCTION
262 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
263 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
264 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
265 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
266 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
267 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
270 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
271 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
272 returns from do_sys_open to sys_open+0x1b).