Loading...
1==========================
2Kprobe-based Event Tracing
3==========================
4
5:Author: Masami Hiramatsu
6
7Overview
8--------
9These events are similar to tracepoint-based events. Instead of tracepoints,
10this is based on kprobes (kprobe and kretprobe). So it can probe wherever
11kprobes can probe (this means, all functions except those with
12__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
13Unlike the tracepoint-based event, this can be added and removed
14dynamically, on the fly.
15
16To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
17
18Similar to the event tracer, this doesn't need to be activated via
19current_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.
22
23You can also use /sys/kernel/tracing/dynamic_events instead of
24kprobe_events. That interface will provide unified access to other
25dynamic events too.
26
27Synopsis of kprobe_events
28-------------------------
29::
30
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
35
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.
46
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), VFS layer common type(%pd/%pD), "char",
62 "string", "ustring", "symbol", "symstr" and bitfield are
63 supported.
64
65 (\*1) only for the probe on function entry (offs == 0). Note, this argument access
66 is best effort, because depending on the argument type, it may be passed on
67 the stack. But this only support the arguments via registers.
68 (\*2) only for return probe. Note that this is also best effort. Depending on the
69 return value type, it might be passed via a pair of registers. But this only
70 accesses one register.
71 (\*3) this is useful for fetching a field of data structures.
72 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
73
74Function arguments at kretprobe
75-------------------------------
76Function arguments can be accessed at kretprobe using $arg<N> fetcharg. This
77is useful to record the function parameter and return value at once, and
78trace the difference of structure fields (for debugging a function whether it
79correctly updates the given data structure or not).
80See the :ref:`sample<fprobetrace_exit_args_sample>` in fprobe event for how
81it works.
82
83.. _kprobetrace_types:
84
85Types
86-----
87Several types are supported for fetchargs. Kprobe tracer will access memory
88by given type. Prefix 's' and 'u' means those types are signed and unsigned
89respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
90in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
91or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
92x86-64 uses x64).
93
94These value types can be an array. To record array data, you can add '[N]'
95(where N is a fixed number, less than 64) to the base type.
96E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
97Note that the array can be applied to memory type fetchargs, you can not
98apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
99wrong, but '+8($stack):x8[8]' is OK.)
100
101Char type can be used to show the character value of traced arguments.
102
103String type is a special type, which fetches a "null-terminated" string from
104kernel space. This means it will fail and store NULL if the string container
105has been paged out. "ustring" type is an alternative of string for user-space.
106See :ref:`user_mem_access` for more info.
107
108The string array type is a bit different from other types. For other base
109types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
110as +0(%di):x32.) But string[1] is not equal to string. The string type itself
111represents "char array", but string array type represents "char * array".
112So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
113Bitfield is another special type, which takes 3 parameters, bit-width, bit-
114offset, and container-size (usually 32). The syntax is::
115
116 b<bit-width>@<bit-offset>/<container-size>
117
118Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
119which shows given pointer in "symbol+offset" style.
120On the other hand, symbol-string type ('symstr') converts the given address to
121"symbol+offset/symbolsize" style and stores it as a null-terminated string.
122With 'symstr' type, you can filter the event with wildcard pattern of the
123symbols, and you don't need to solve symbol name by yourself.
124For $comm, the default type is "string"; any other type is invalid.
125
126VFS layer common type(%pd/%pD) is a special type, which fetches dentry's or
127file's name from struct dentry's address or struct file's address.
128
129.. _user_mem_access:
130
131User Memory Access
132------------------
133Kprobe events supports user-space memory access. For that purpose, you can use
134either user-space dereference syntax or 'ustring' type.
135
136The user-space dereference syntax allows you to access a field of a data
137structure in user-space. This is done by adding the "u" prefix to the
138dereference syntax. For example, +u4(%si) means it will read memory from the
139address in the register %si offset by 4, and the memory is expected to be in
140user-space. You can use this for strings too, e.g. +u0(%si):string will read
141a string from the address in the register %si that is expected to be in user-
142space. 'ustring' is a shortcut way of performing the same task. That is,
143+0(%si):ustring is equivalent to +u0(%si):string.
144
145Note that kprobe-event provides the user-memory access syntax but it doesn't
146use it transparently. This means if you use normal dereference or string type
147for user memory, it might fail, and may always fail on some architectures. The
148user has to carefully check if the target data is in kernel or user space.
149
150Per-Probe Event Filtering
151-------------------------
152Per-probe event filtering feature allows you to set different filter on each
153probe and gives you what arguments will be shown in trace buffer. If an event
154name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
155under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
156'enable', 'format', 'filter' and 'trigger'.
157
158enable:
159 You can enable/disable the probe by writing 1 or 0 on it.
160
161format:
162 This shows the format of this probe event.
163
164filter:
165 You can write filtering rules of this event.
166
167id:
168 This shows the id of this probe event.
169
170trigger:
171 This allows to install trigger commands which are executed when the event is
172 hit (for details, see Documentation/trace/events.rst, section 6).
173
174Event Profiling
175---------------
176You can check the total number of probe hits and probe miss-hits via
177/sys/kernel/tracing/kprobe_profile.
178The first column is event name, the second is the number of probe hits,
179the third is the number of probe miss-hits.
180
181Kernel Boot Parameter
182---------------------
183You can add and enable new kprobe events when booting up the kernel by
184"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
185kprobe events, which format is similar to the kprobe_events.
186The difference is that the probe definition parameters are comma-delimited
187instead of space. For example, adding myprobe event on do_sys_open like below::
188
189 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
190
191should be below for kernel boot parameter (just replace spaces with comma)::
192
193 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
194
195
196Usage examples
197--------------
198To add a probe as a new event, write a new definition to kprobe_events
199as below::
200
201 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
202
203This sets a kprobe on the top of do_sys_open() function with recording
2041st to 4th arguments as "myprobe" event. Note, which register/stack entry is
205assigned to each function argument depends on arch-specific ABI. If you unsure
206the ABI, please try to use probe subcommand of perf-tools (you can find it
207under tools/perf/).
208As this example shows, users can choose more familiar names for each arguments.
209::
210
211 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
212
213This sets a kretprobe on the return point of do_sys_open() function with
214recording return value as "myretprobe" event.
215You can see the format of these events via
216/sys/kernel/tracing/events/kprobes/<EVENT>/format.
217::
218
219 cat /sys/kernel/tracing/events/kprobes/myprobe/format
220 name: myprobe
221 ID: 780
222 format:
223 field:unsigned short common_type; offset:0; size:2; signed:0;
224 field:unsigned char common_flags; offset:2; size:1; signed:0;
225 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
226 field:int common_pid; offset:4; size:4; signed:1;
227
228 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
229 field:int __probe_nargs; offset:16; size:4; signed:1;
230 field:unsigned long dfd; offset:20; size:4; signed:0;
231 field:unsigned long filename; offset:24; size:4; signed:0;
232 field:unsigned long flags; offset:28; size:4; signed:0;
233 field:unsigned long mode; offset:32; size:4; signed:0;
234
235
236 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
237 REC->dfd, REC->filename, REC->flags, REC->mode
238
239You can see that the event has 4 arguments as in the expressions you specified.
240::
241
242 echo > /sys/kernel/tracing/kprobe_events
243
244This clears all probe points.
245
246Or,
247::
248
249 echo -:myprobe >> kprobe_events
250
251This clears probe points selectively.
252
253Right after definition, each event is disabled by default. For tracing these
254events, you need to enable it.
255::
256
257 echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
258 echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
259
260Use the following command to start tracing in an interval.
261::
262
263 # echo 1 > tracing_on
264 Open something...
265 # echo 0 > tracing_on
266
267And you can see the traced information via /sys/kernel/tracing/trace.
268::
269
270 cat /sys/kernel/tracing/trace
271 # tracer: nop
272 #
273 # TASK-PID CPU# TIMESTAMP FUNCTION
274 # | | | | |
275 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
276 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
277 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
278 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
279 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
280 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
281
282
283Each line shows when the kernel hits an event, and <- SYMBOL means kernel
284returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
285returns from do_sys_open to sys_open+0x1b).
1==========================
2Kprobe-based Event Tracing
3==========================
4
5:Author: Masami Hiramatsu
6
7Overview
8--------
9These events are similar to tracepoint based events. Instead of Tracepoint,
10this is based on kprobes (kprobe and kretprobe). So it can probe wherever
11kprobes can probe (this means, all functions except those with
12__kprobes/nokprobe_inline annotation and those marked NOKPROBE_SYMBOL).
13Unlike the Tracepoint based event, this can be added and removed
14dynamically, on the fly.
15
16To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
17
18Similar to the events tracer, this doesn't need to be activated via
19current_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.
22
23You can also use /sys/kernel/debug/tracing/dynamic_events instead of
24kprobe_events. That interface will provide unified access to other
25dynamic events too.
26
27Synopsis of kprobe_events
28-------------------------
29::
30
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
35
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.
46
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 $comm : Fetch current task comm.
56 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57 \IMM : Store an immediate value to the argument.
58 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61 (x8/x16/x32/x64), "string", "ustring", "symbol", "symstr"
62 and bitfield are supported.
63
64 (\*1) only for the probe on function entry (offs == 0).
65 (\*2) only for return probe.
66 (\*3) this is useful for fetching a field of data structures.
67 (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
68
69Types
70-----
71Several types are supported for fetch-args. Kprobe tracer will access memory
72by given type. Prefix 's' and 'u' means those types are signed and unsigned
73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
76x86-64 uses x64).
77These value types can be an array. To record array data, you can add '[N]'
78(where N is a fixed number, less than 64) to the base type.
79E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
80Note that the array can be applied to memory type fetchargs, you can not
81apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
82wrong, but '+8($stack):x8[8]' is OK.)
83String type is a special type, which fetches a "null-terminated" string from
84kernel space. This means it will fail and store NULL if the string container
85has been paged out. "ustring" type is an alternative of string for user-space.
86See :ref:`user_mem_access` for more info..
87The string array type is a bit different from other types. For other base
88types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
89as +0(%di):x32.) But string[1] is not equal to string. The string type itself
90represents "char array", but string array type represents "char * array".
91So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
92Bitfield is another special type, which takes 3 parameters, bit-width, bit-
93offset, and container-size (usually 32). The syntax is::
94
95 b<bit-width>@<bit-offset>/<container-size>
96
97Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
98which shows given pointer in "symbol+offset" style.
99On the other hand, symbol-string type ('symstr') converts the given address to
100"symbol+offset/symbolsize" style and stores it as a null-terminated string.
101With 'symstr' type, you can filter the event with wildcard pattern of the
102symbols, and you don't need to solve symbol name by yourself.
103For $comm, the default type is "string"; any other type is invalid.
104
105.. _user_mem_access:
106
107User Memory Access
108------------------
109Kprobe events supports user-space memory access. For that purpose, you can use
110either user-space dereference syntax or 'ustring' type.
111
112The user-space dereference syntax allows you to access a field of a data
113structure in user-space. This is done by adding the "u" prefix to the
114dereference syntax. For example, +u4(%si) means it will read memory from the
115address in the register %si offset by 4, and the memory is expected to be in
116user-space. You can use this for strings too, e.g. +u0(%si):string will read
117a string from the address in the register %si that is expected to be in user-
118space. 'ustring' is a shortcut way of performing the same task. That is,
119+0(%si):ustring is equivalent to +u0(%si):string.
120
121Note that kprobe-event provides the user-memory access syntax but it doesn't
122use it transparently. This means if you use normal dereference or string type
123for user memory, it might fail, and may always fail on some archs. The user
124has to carefully check if the target data is in kernel or user space.
125
126Per-Probe Event Filtering
127-------------------------
128Per-probe event filtering feature allows you to set different filter on each
129probe and gives you what arguments will be shown in trace buffer. If an event
130name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
131under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
132'enable', 'format', 'filter' and 'trigger'.
133
134enable:
135 You can enable/disable the probe by writing 1 or 0 on it.
136
137format:
138 This shows the format of this probe event.
139
140filter:
141 You can write filtering rules of this event.
142
143id:
144 This shows the id of this probe event.
145
146trigger:
147 This allows to install trigger commands which are executed when the event is
148 hit (for details, see Documentation/trace/events.rst, section 6).
149
150Event Profiling
151---------------
152You can check the total number of probe hits and probe miss-hits via
153/sys/kernel/debug/tracing/kprobe_profile.
154The first column is event name, the second is the number of probe hits,
155the third is the number of probe miss-hits.
156
157Kernel Boot Parameter
158---------------------
159You can add and enable new kprobe events when booting up the kernel by
160"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
161kprobe events, which format is similar to the kprobe_events.
162The difference is that the probe definition parameters are comma-delimited
163instead of space. For example, adding myprobe event on do_sys_open like below
164
165 p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
166
167should be below for kernel boot parameter (just replace spaces with comma)
168
169 p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
170
171
172Usage examples
173--------------
174To add a probe as a new event, write a new definition to kprobe_events
175as below::
176
177 echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
178
179This sets a kprobe on the top of do_sys_open() function with recording
1801st to 4th arguments as "myprobe" event. Note, which register/stack entry is
181assigned to each function argument depends on arch-specific ABI. If you unsure
182the ABI, please try to use probe subcommand of perf-tools (you can find it
183under tools/perf/).
184As this example shows, users can choose more familiar names for each arguments.
185::
186
187 echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
188
189This sets a kretprobe on the return point of do_sys_open() function with
190recording return value as "myretprobe" event.
191You can see the format of these events via
192/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
193::
194
195 cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
196 name: myprobe
197 ID: 780
198 format:
199 field:unsigned short common_type; offset:0; size:2; signed:0;
200 field:unsigned char common_flags; offset:2; size:1; signed:0;
201 field:unsigned char common_preempt_count; offset:3; size:1;signed:0;
202 field:int common_pid; offset:4; size:4; signed:1;
203
204 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
205 field:int __probe_nargs; offset:16; size:4; signed:1;
206 field:unsigned long dfd; offset:20; size:4; signed:0;
207 field:unsigned long filename; offset:24; size:4; signed:0;
208 field:unsigned long flags; offset:28; size:4; signed:0;
209 field:unsigned long mode; offset:32; size:4; signed:0;
210
211
212 print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
213 REC->dfd, REC->filename, REC->flags, REC->mode
214
215You can see that the event has 4 arguments as in the expressions you specified.
216::
217
218 echo > /sys/kernel/debug/tracing/kprobe_events
219
220This clears all probe points.
221
222Or,
223::
224
225 echo -:myprobe >> kprobe_events
226
227This clears probe points selectively.
228
229Right after definition, each event is disabled by default. For tracing these
230events, you need to enable it.
231::
232
233 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
234 echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
235
236Use the following command to start tracing in an interval.
237::
238
239 # echo 1 > tracing_on
240 Open something...
241 # echo 0 > tracing_on
242
243And you can see the traced information via /sys/kernel/debug/tracing/trace.
244::
245
246 cat /sys/kernel/debug/tracing/trace
247 # tracer: nop
248 #
249 # TASK-PID CPU# TIMESTAMP FUNCTION
250 # | | | | |
251 <...>-1447 [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
252 <...>-1447 [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
253 <...>-1447 [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
254 <...>-1447 [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
255 <...>-1447 [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
256 <...>-1447 [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
257
258
259Each line shows when the kernel hits an event, and <- SYMBOL means kernel
260returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
261returns from do_sys_open to sys_open+0x1b).