1 ========================== 1 ==========================
2 Kprobe-based Event Tracing 2 Kprobe-based Event Tracing
3 ========================== 3 ==========================
4 4
5 :Author: Masami Hiramatsu 5 :Author: Masami Hiramatsu
6 6
7 Overview 7 Overview
8 -------- 8 --------
9 These events are similar to tracepoint-based e 9 These events are similar to tracepoint-based events. Instead of tracepoints,
10 this is based on kprobes (kprobe and kretprobe 10 this is based on kprobes (kprobe and kretprobe). So it can probe wherever
11 kprobes can probe (this means, all functions e 11 kprobes can probe (this means, all functions except those with
12 __kprobes/nokprobe_inline annotation and those 12 __kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
13 Unlike the tracepoint-based event, this can be 13 Unlike the tracepoint-based event, this can be added and removed
14 dynamically, on the fly. 14 dynamically, on the fly.
15 15
16 To enable this feature, build your kernel with 16 To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
17 17
18 Similar to the event tracer, this doesn't need 18 Similar to the event tracer, this doesn't need to be activated via
19 current_tracer. Instead of that, add probe poi 19 current_tracer. Instead of that, add probe points via
20 /sys/kernel/tracing/kprobe_events, and enable 20 /sys/kernel/tracing/kprobe_events, and enable it via
21 /sys/kernel/tracing/events/kprobes/<EVENT>/ena 21 /sys/kernel/tracing/events/kprobes/<EVENT>/enable.
22 22
23 You can also use /sys/kernel/tracing/dynamic_e 23 You can also use /sys/kernel/tracing/dynamic_events instead of
24 kprobe_events. That interface will provide uni 24 kprobe_events. That interface will provide unified access to other
25 dynamic events too. 25 dynamic events too.
26 26
27 Synopsis of kprobe_events 27 Synopsis of kprobe_events
28 ------------------------- 28 -------------------------
29 :: 29 ::
30 30
31 p[:[GRP/][EVENT]] [MOD:]SYM[+offs]|MEMADDR [ 31 p[:[GRP/][EVENT]] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS] : Set a probe
32 r[MAXACTIVE][:[GRP/][EVENT]] [MOD:]SYM[+0] [ 32 r[MAXACTIVE][:[GRP/][EVENT]] [MOD:]SYM[+0] [FETCHARGS] : Set a return probe
33 p[:[GRP/][EVENT]] [MOD:]SYM[+0]%return [FETC 33 p[:[GRP/][EVENT]] [MOD:]SYM[+0]%return [FETCHARGS] : Set a return probe
34 -:[GRP/][EVENT] 34 -:[GRP/][EVENT] : Clear a probe
35 35
36 GRP : Group name. If omitted, use 36 GRP : Group name. If omitted, use "kprobes" for it.
37 EVENT : Event name. If omitted, the 37 EVENT : Event name. If omitted, the event name is generated
38 based on SYM+offs or MEMADDR 38 based on SYM+offs or MEMADDR.
39 MOD : Module name which has given 39 MOD : Module name which has given SYM.
40 SYM[+offs] : Symbol+offset where the prob 40 SYM[+offs] : Symbol+offset where the probe is inserted.
41 SYM%return : Return address of the symbol 41 SYM%return : Return address of the symbol
42 MEMADDR : Address where the probe is i 42 MEMADDR : Address where the probe is inserted.
43 MAXACTIVE : Maximum number of instances 43 MAXACTIVE : Maximum number of instances of the specified function that
44 can be probed simultaneously 44 can be probed simultaneously, or 0 for the default value
45 as defined in Documentation/ 45 as defined in Documentation/trace/kprobes.rst section 1.3.1.
46 46
47 FETCHARGS : Arguments. Each probe can ha 47 FETCHARGS : Arguments. Each probe can have up to 128 args.
48 %REG : Fetch register REG 48 %REG : Fetch register REG
49 @ADDR : Fetch memory at ADDR (ADDR s 49 @ADDR : Fetch memory at ADDR (ADDR should be in kernel)
50 @SYM[+|-offs] : Fetch memory at SYM +|- offs 50 @SYM[+|-offs] : Fetch memory at SYM +|- offs (SYM should be a data symbol)
51 $stackN : Fetch Nth entry of stack (N 51 $stackN : Fetch Nth entry of stack (N >= 0)
52 $stack : Fetch stack address. 52 $stack : Fetch stack address.
53 $argN : Fetch the Nth function argum 53 $argN : Fetch the Nth function argument. (N >= 1) (\*1)
54 $retval : Fetch return value.(\*2) 54 $retval : Fetch return value.(\*2)
55 $comm : Fetch current task comm. 55 $comm : Fetch current task comm.
56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCH 56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57 \IMM : Store an immediate value to 57 \IMM : Store an immediate value to the argument.
58 NAME=FETCHARG : Set NAME as the argument nam 58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59 FETCHARG:TYPE : Set TYPE as the type of FETC 59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60 (u8/u16/u32/u64/s8/s16/s32/s 60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61 (x8/x16/x32/x64), VFS layer !! 61 (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
62 "string", "ustring", "symbol !! 62 and bitfield are supported.
63 supported. !! 63
64 !! 64 (\*1) only for the probe on function entry (offs == 0).
65 (\*1) only for the probe on function entry ( !! 65 (\*2) only for return probe.
66 is best effort, because depending on t <<
67 the stack. But this only support the a <<
68 (\*2) only for return probe. Note that this <<
69 return value type, it might be passed <<
70 accesses one register. <<
71 (\*3) this is useful for fetching a field of 66 (\*3) this is useful for fetching a field of data structures.
72 (\*4) "u" means user-space dereference. See 67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
73 68
74 Function arguments at kretprobe <<
75 ------------------------------- <<
76 Function arguments can be accessed at kretprob <<
77 is useful to record the function parameter and <<
78 trace the difference of structure fields (for <<
79 correctly updates the given data structure or <<
80 See the :ref:`sample<fprobetrace_exit_args_sam <<
81 it works. <<
82 <<
83 .. _kprobetrace_types: <<
84 <<
85 Types 69 Types
86 ----- 70 -----
87 Several types are supported for fetchargs. Kpr 71 Several types are supported for fetchargs. Kprobe tracer will access memory
88 by given type. Prefix 's' and 'u' means those 72 by given type. Prefix 's' and 'u' means those types are signed and unsigned
89 respectively. 'x' prefix implies it is unsigne 73 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
90 in decimal ('s' and 'u') or hexadecimal ('x'). 74 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
91 or 'x64' is used depends on the architecture ( 75 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
92 x86-64 uses x64). 76 x86-64 uses x64).
93 77
94 These value types can be an array. To record a 78 These value types can be an array. To record array data, you can add '[N]'
95 (where N is a fixed number, less than 64) to t 79 (where N is a fixed number, less than 64) to the base type.
96 E.g. 'x16[4]' means an array of x16 (2-byte he 80 E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
97 Note that the array can be applied to memory t 81 Note that the array can be applied to memory type fetchargs, you can not
98 apply it to registers/stack-entries etc. (for 82 apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
99 wrong, but '+8($stack):x8[8]' is OK.) 83 wrong, but '+8($stack):x8[8]' is OK.)
100 84
101 Char type can be used to show the character va 85 Char type can be used to show the character value of traced arguments.
102 86
103 String type is a special type, which fetches a 87 String type is a special type, which fetches a "null-terminated" string from
104 kernel space. This means it will fail and stor 88 kernel space. This means it will fail and store NULL if the string container
105 has been paged out. "ustring" type is an alter 89 has been paged out. "ustring" type is an alternative of string for user-space.
106 See :ref:`user_mem_access` for more info. 90 See :ref:`user_mem_access` for more info.
107 91
108 The string array type is a bit different from 92 The string array type is a bit different from other types. For other base
109 types, <base-type>[1] is equal to <base-type> 93 types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
110 as +0(%di):x32.) But string[1] is not equal to 94 as +0(%di):x32.) But string[1] is not equal to string. The string type itself
111 represents "char array", but string array type 95 represents "char array", but string array type represents "char * array".
112 So, for example, +0(%di):string[1] is equal to 96 So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
113 Bitfield is another special type, which takes 97 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
114 offset, and container-size (usually 32). The s 98 offset, and container-size (usually 32). The syntax is::
115 99
116 b<bit-width>@<bit-offset>/<container-size> 100 b<bit-width>@<bit-offset>/<container-size>
117 101
118 Symbol type('symbol') is an alias of u32 or u6 102 Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
119 which shows given pointer in "symbol+offset" s 103 which shows given pointer in "symbol+offset" style.
120 On the other hand, symbol-string type ('symstr 104 On the other hand, symbol-string type ('symstr') converts the given address to
121 "symbol+offset/symbolsize" style and stores it 105 "symbol+offset/symbolsize" style and stores it as a null-terminated string.
122 With 'symstr' type, you can filter the event w 106 With 'symstr' type, you can filter the event with wildcard pattern of the
123 symbols, and you don't need to solve symbol na 107 symbols, and you don't need to solve symbol name by yourself.
124 For $comm, the default type is "string"; any o 108 For $comm, the default type is "string"; any other type is invalid.
125 <<
126 VFS layer common type(%pd/%pD) is a special ty <<
127 file's name from struct dentry's address or st <<
128 109
129 .. _user_mem_access: 110 .. _user_mem_access:
130 111
131 User Memory Access 112 User Memory Access
132 ------------------ 113 ------------------
133 Kprobe events supports user-space memory acces 114 Kprobe events supports user-space memory access. For that purpose, you can use
134 either user-space dereference syntax or 'ustri 115 either user-space dereference syntax or 'ustring' type.
135 116
136 The user-space dereference syntax allows you t 117 The user-space dereference syntax allows you to access a field of a data
137 structure in user-space. This is done by addin 118 structure in user-space. This is done by adding the "u" prefix to the
138 dereference syntax. For example, +u4(%si) mean 119 dereference syntax. For example, +u4(%si) means it will read memory from the
139 address in the register %si offset by 4, and t 120 address in the register %si offset by 4, and the memory is expected to be in
140 user-space. You can use this for strings too, 121 user-space. You can use this for strings too, e.g. +u0(%si):string will read
141 a string from the address in the register %si 122 a string from the address in the register %si that is expected to be in user-
142 space. 'ustring' is a shortcut way of performi 123 space. 'ustring' is a shortcut way of performing the same task. That is,
143 +0(%si):ustring is equivalent to +u0(%si):stri 124 +0(%si):ustring is equivalent to +u0(%si):string.
144 125
145 Note that kprobe-event provides the user-memor 126 Note that kprobe-event provides the user-memory access syntax but it doesn't
146 use it transparently. This means if you use no 127 use it transparently. This means if you use normal dereference or string type
147 for user memory, it might fail, and may always 128 for user memory, it might fail, and may always fail on some architectures. The
148 user has to carefully check if the target data 129 user has to carefully check if the target data is in kernel or user space.
149 130
150 Per-Probe Event Filtering 131 Per-Probe Event Filtering
151 ------------------------- 132 -------------------------
152 Per-probe event filtering feature allows you t 133 Per-probe event filtering feature allows you to set different filter on each
153 probe and gives you what arguments will be sho 134 probe and gives you what arguments will be shown in trace buffer. If an event
154 name is specified right after 'p:' or 'r:' in 135 name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
155 under tracing/events/kprobes/<EVENT>, at the d 136 under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
156 'enable', 'format', 'filter' and 'trigger'. 137 'enable', 'format', 'filter' and 'trigger'.
157 138
158 enable: 139 enable:
159 You can enable/disable the probe by writing 140 You can enable/disable the probe by writing 1 or 0 on it.
160 141
161 format: 142 format:
162 This shows the format of this probe event. 143 This shows the format of this probe event.
163 144
164 filter: 145 filter:
165 You can write filtering rules of this event. 146 You can write filtering rules of this event.
166 147
167 id: 148 id:
168 This shows the id of this probe event. 149 This shows the id of this probe event.
169 150
170 trigger: 151 trigger:
171 This allows to install trigger commands whic 152 This allows to install trigger commands which are executed when the event is
172 hit (for details, see Documentation/trace/ev 153 hit (for details, see Documentation/trace/events.rst, section 6).
173 154
174 Event Profiling 155 Event Profiling
175 --------------- 156 ---------------
176 You can check the total number of probe hits a 157 You can check the total number of probe hits and probe miss-hits via
177 /sys/kernel/tracing/kprobe_profile. 158 /sys/kernel/tracing/kprobe_profile.
178 The first column is event name, the second is 159 The first column is event name, the second is the number of probe hits,
179 the third is the number of probe miss-hits. 160 the third is the number of probe miss-hits.
180 161
181 Kernel Boot Parameter 162 Kernel Boot Parameter
182 --------------------- 163 ---------------------
183 You can add and enable new kprobe events when 164 You can add and enable new kprobe events when booting up the kernel by
184 "kprobe_event=" parameter. The parameter accep 165 "kprobe_event=" parameter. The parameter accepts a semicolon-delimited
185 kprobe events, which format is similar to the 166 kprobe events, which format is similar to the kprobe_events.
186 The difference is that the probe definition pa 167 The difference is that the probe definition parameters are comma-delimited
187 instead of space. For example, adding myprobe 168 instead of space. For example, adding myprobe event on do_sys_open like below::
188 169
189 p:myprobe do_sys_open dfd=%ax filename=%dx f 170 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
190 171
191 should be below for kernel boot parameter (jus 172 should be below for kernel boot parameter (just replace spaces with comma)::
192 173
193 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,f 174 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
194 175
195 176
196 Usage examples 177 Usage examples
197 -------------- 178 --------------
198 To add a probe as a new event, write a new def 179 To add a probe as a new event, write a new definition to kprobe_events
199 as below:: 180 as below::
200 181
201 echo 'p:myprobe do_sys_open dfd=%ax filename 182 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
202 183
203 This sets a kprobe on the top of do_sys_open() 184 This sets a kprobe on the top of do_sys_open() function with recording
204 1st to 4th arguments as "myprobe" event. Note, 185 1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
205 assigned to each function argument depends on 186 assigned to each function argument depends on arch-specific ABI. If you unsure
206 the ABI, please try to use probe subcommand of 187 the ABI, please try to use probe subcommand of perf-tools (you can find it
207 under tools/perf/). 188 under tools/perf/).
208 As this example shows, users can choose more f 189 As this example shows, users can choose more familiar names for each arguments.
209 :: 190 ::
210 191
211 echo 'r:myretprobe do_sys_open $retval' >> / 192 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
212 193
213 This sets a kretprobe on the return point of d 194 This sets a kretprobe on the return point of do_sys_open() function with
214 recording return value as "myretprobe" event. 195 recording return value as "myretprobe" event.
215 You can see the format of these events via 196 You can see the format of these events via
216 /sys/kernel/tracing/events/kprobes/<EVENT>/for 197 /sys/kernel/tracing/events/kprobes/<EVENT>/format.
217 :: 198 ::
218 199
219 cat /sys/kernel/tracing/events/kprobes/mypro 200 cat /sys/kernel/tracing/events/kprobes/myprobe/format
220 name: myprobe 201 name: myprobe
221 ID: 780 202 ID: 780
222 format: 203 format:
223 field:unsigned short common_type; 204 field:unsigned short common_type; offset:0; size:2; signed:0;
224 field:unsigned char common_flags; 205 field:unsigned char common_flags; offset:2; size:1; signed:0;
225 field:unsigned char common_preempt_c 206 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
226 field:int common_pid; offset:4; 207 field:int common_pid; offset:4; size:4; signed:1;
227 208
228 field:unsigned long __probe_ip; offs 209 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
229 field:int __probe_nargs; offs 210 field:int __probe_nargs; offset:16; size:4; signed:1;
230 field:unsigned long dfd; offs 211 field:unsigned long dfd; offset:20; size:4; signed:0;
231 field:unsigned long filename; offs 212 field:unsigned long filename; offset:24; size:4; signed:0;
232 field:unsigned long flags; offs 213 field:unsigned long flags; offset:28; size:4; signed:0;
233 field:unsigned long mode; offs 214 field:unsigned long mode; offset:32; size:4; signed:0;
234 215
235 216
236 print fmt: "(%lx) dfd=%lx filename=%lx flags 217 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
237 REC->dfd, REC->filename, REC->flags, REC->mo 218 REC->dfd, REC->filename, REC->flags, REC->mode
238 219
239 You can see that the event has 4 arguments as 220 You can see that the event has 4 arguments as in the expressions you specified.
240 :: 221 ::
241 222
242 echo > /sys/kernel/tracing/kprobe_events 223 echo > /sys/kernel/tracing/kprobe_events
243 224
244 This clears all probe points. 225 This clears all probe points.
245 226
246 Or, 227 Or,
247 :: 228 ::
248 229
249 echo -:myprobe >> kprobe_events 230 echo -:myprobe >> kprobe_events
250 231
251 This clears probe points selectively. 232 This clears probe points selectively.
252 233
253 Right after definition, each event is disabled 234 Right after definition, each event is disabled by default. For tracing these
254 events, you need to enable it. 235 events, you need to enable it.
255 :: 236 ::
256 237
257 echo 1 > /sys/kernel/tracing/events/kprobes/ 238 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
258 echo 1 > /sys/kernel/tracing/events/kprobes/ 239 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
259 240
260 Use the following command to start tracing in 241 Use the following command to start tracing in an interval.
261 :: 242 ::
262 243
263 # echo 1 > tracing_on 244 # echo 1 > tracing_on
264 Open something... 245 Open something...
265 # echo 0 > tracing_on 246 # echo 0 > tracing_on
266 247
267 And you can see the traced information via /sy 248 And you can see the traced information via /sys/kernel/tracing/trace.
268 :: 249 ::
269 250
270 cat /sys/kernel/tracing/trace 251 cat /sys/kernel/tracing/trace
271 # tracer: nop 252 # tracer: nop
272 # 253 #
273 # TASK-PID CPU# TIMESTAMP F 254 # TASK-PID CPU# TIMESTAMP FUNCTION
274 # | | | | 255 # | | | | |
275 <...>-1447 [001] 1038282.286875: 256 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
276 <...>-1447 [001] 1038282.286878: 257 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
277 <...>-1447 [001] 1038282.286885: 258 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
278 <...>-1447 [001] 1038282.286915: 259 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
279 <...>-1447 [001] 1038282.286969: 260 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
280 <...>-1447 [001] 1038282.286976: 261 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
281 262
282 263
283 Each line shows when the kernel hits an event, 264 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
284 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d < 265 returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
285 returns from do_sys_open to sys_open+0x1b). 266 returns from do_sys_open to sys_open+0x1b).
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