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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	                        Kprobe-based Event Tracing
2	                        ==========================
3	
4	                 Documentation is written by Masami Hiramatsu
5	
6	
7	Overview
8	--------
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.
15	
16	To enable this feature, build your kernel with CONFIG_KPROBE_EVENTS=y.
17	
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>/enabled.
22	
23	
24	Synopsis of kprobe_events
25	-------------------------
26	  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
27	  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]	: Set a return probe
28	  -:[GRP/]EVENT						: Clear a probe
29	
30	 GRP		: Group name. If omitted, use "kprobes" for it.
31	 EVENT		: Event name. If omitted, the event name is generated
32			  based on SYM+offs or MEMADDR.
33	 MOD		: Module name which has given SYM.
34	 SYM[+offs]	: Symbol+offset where the probe is inserted.
35	 MEMADDR	: Address where the probe is inserted.
36	 MAXACTIVE	: Maximum number of instances of the specified function that
37			  can be probed simultaneously, or 0 for the default value
38			  as defined in Documentation/kprobes.txt section 1.3.1.
39	
40	 FETCHARGS	: Arguments. Each probe can have up to 128 args.
41	  %REG		: Fetch register REG
42	  @ADDR		: Fetch memory at ADDR (ADDR should be in kernel)
43	  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
44	  $stackN	: Fetch Nth entry of stack (N >= 0)
45	  $stack	: Fetch stack address.
46	  $retval	: Fetch return value.(*)
47	  $comm		: Fetch current task comm.
48	  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
49	  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
50	  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
51			  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
52			  (x8/x16/x32/x64), "string" and bitfield are supported.
53	
54	  (*) only for return probe.
55	  (**) this is useful for fetching a field of data structures.
56	
57	Types
58	-----
59	Several types are supported for fetch-args. Kprobe tracer will access memory
60	by given type. Prefix 's' and 'u' means those types are signed and unsigned
61	respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
62	in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
63	or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
64	x86-64 uses x64).
65	String type is a special type, which fetches a "null-terminated" string from
66	kernel space. This means it will fail and store NULL if the string container
67	has been paged out.
68	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
69	offset, and container-size (usually 32). The syntax is;
70	
71	 b<bit-width>@<bit-offset>/<container-size>
72	
73	For $comm, the default type is "string"; any other type is invalid.
74	
75	
76	Per-Probe Event Filtering
77	-------------------------
78	 Per-probe event filtering feature allows you to set different filter on each
79	probe and gives you what arguments will be shown in trace buffer. If an event
80	name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
81	under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
82	'enabled', 'format' and 'filter'.
83	
84	enabled:
85	  You can enable/disable the probe by writing 1 or 0 on it.
86	
87	format:
88	  This shows the format of this probe event.
89	
90	filter:
91	  You can write filtering rules of this event.
92	
93	id:
94	  This shows the id of this probe event.
95	
96	
97	Event Profiling
98	---------------
99	 You can check the total number of probe hits and probe miss-hits via
100	/sys/kernel/debug/tracing/kprobe_profile.
101	 The first column is event name, the second is the number of probe hits,
102	the third is the number of probe miss-hits.
103	
104	
105	Usage examples
106	--------------
107	To add a probe as a new event, write a new definition to kprobe_events
108	as below.
109	
110	  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
111	
112	 This sets a kprobe on the top of do_sys_open() function with recording
113	1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
114	assigned to each function argument depends on arch-specific ABI. If you unsure
115	the ABI, please try to use probe subcommand of perf-tools (you can find it
116	under tools/perf/).
117	As this example shows, users can choose more familiar names for each arguments.
118	
119	  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
120	
121	 This sets a kretprobe on the return point of do_sys_open() function with
122	recording return value as "myretprobe" event.
123	 You can see the format of these events via
124	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
125	
126	  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
127	name: myprobe
128	ID: 780
129	format:
130	        field:unsigned short common_type;       offset:0;       size:2; signed:0;
131	        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
132	        field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
133	        field:int common_pid;   offset:4;       size:4; signed:1;
134	
135	        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
136	        field:int __probe_nargs;        offset:16;      size:4; signed:1;
137	        field:unsigned long dfd;        offset:20;      size:4; signed:0;
138	        field:unsigned long filename;   offset:24;      size:4; signed:0;
139	        field:unsigned long flags;      offset:28;      size:4; signed:0;
140	        field:unsigned long mode;       offset:32;      size:4; signed:0;
141	
142	
143	print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
144	REC->dfd, REC->filename, REC->flags, REC->mode
145	
146	 You can see that the event has 4 arguments as in the expressions you specified.
147	
148	  echo > /sys/kernel/debug/tracing/kprobe_events
149	
150	 This clears all probe points.
151	
152	 Or,
153	
154	  echo -:myprobe >> kprobe_events
155	
156	 This clears probe points selectively.
157	
158	 Right after definition, each event is disabled by default. For tracing these
159	events, you need to enable it.
160	
161	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
162	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
163	
164	 And you can see the traced information via /sys/kernel/debug/tracing/trace.
165	
166	  cat /sys/kernel/debug/tracing/trace
167	# tracer: nop
168	#
169	#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
170	#              | |       |          |         |
171	           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
172	           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
173	           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
174	           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
175	           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
176	           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
177	
178	
179	 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
180	returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
181	returns from do_sys_open to sys_open+0x1b).
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