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Based on kernel version 4.9. Page generated on 2016-12-21 14:37 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 body except for __kprobes
12	functions). Unlike the Tracepoint based event, this can be added and removed
13	dynamically, on the fly.
14	
15	To enable this feature, build your kernel with CONFIG_KPROBE_EVENT=y.
16	
17	Similar to the events tracer, this doesn't need to be activated via
18	current_tracer. Instead of that, add probe points via
19	/sys/kernel/debug/tracing/kprobe_events, and enable it via
20	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/enabled.
21	
22	
23	Synopsis of kprobe_events
24	-------------------------
25	  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
26	  r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]		: Set a return probe
27	  -:[GRP/]EVENT						: Clear a probe
28	
29	 GRP		: Group name. If omitted, use "kprobes" for it.
30	 EVENT		: Event name. If omitted, the event name is generated
31			  based on SYM+offs or MEMADDR.
32	 MOD		: Module name which has given SYM.
33	 SYM[+offs]	: Symbol+offset where the probe is inserted.
34	 MEMADDR	: Address where the probe is inserted.
35	
36	 FETCHARGS	: Arguments. Each probe can have up to 128 args.
37	  %REG		: Fetch register REG
38	  @ADDR		: Fetch memory at ADDR (ADDR should be in kernel)
39	  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
40	  $stackN	: Fetch Nth entry of stack (N >= 0)
41	  $stack	: Fetch stack address.
42	  $retval	: Fetch return value.(*)
43	  $comm		: Fetch current task comm.
44	  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
45	  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
46	  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
47			  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
48			  (x8/x16/x32/x64), "string" and bitfield are supported.
49	
50	  (*) only for return probe.
51	  (**) this is useful for fetching a field of data structures.
52	
53	Types
54	-----
55	Several types are supported for fetch-args. Kprobe tracer will access memory
56	by given type. Prefix 's' and 'u' means those types are signed and unsigned
57	respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
58	in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
59	or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
60	x86-64 uses x64).
61	String type is a special type, which fetches a "null-terminated" string from
62	kernel space. This means it will fail and store NULL if the string container
63	has been paged out.
64	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
65	offset, and container-size (usually 32). The syntax is;
66	
67	 b<bit-width>@<bit-offset>/<container-size>
68	
69	For $comm, the default type is "string"; any other type is invalid.
70	
71	
72	Per-Probe Event Filtering
73	-------------------------
74	 Per-probe event filtering feature allows you to set different filter on each
75	probe and gives you what arguments will be shown in trace buffer. If an event
76	name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
77	under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
78	'enabled', 'format' and 'filter'.
79	
80	enabled:
81	  You can enable/disable the probe by writing 1 or 0 on it.
82	
83	format:
84	  This shows the format of this probe event.
85	
86	filter:
87	  You can write filtering rules of this event.
88	
89	id:
90	  This shows the id of this probe event.
91	
92	
93	Event Profiling
94	---------------
95	 You can check the total number of probe hits and probe miss-hits via
96	/sys/kernel/debug/tracing/kprobe_profile.
97	 The first column is event name, the second is the number of probe hits,
98	the third is the number of probe miss-hits.
99	
100	
101	Usage examples
102	--------------
103	To add a probe as a new event, write a new definition to kprobe_events
104	as below.
105	
106	  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
107	
108	 This sets a kprobe on the top of do_sys_open() function with recording
109	1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
110	assigned to each function argument depends on arch-specific ABI. If you unsure
111	the ABI, please try to use probe subcommand of perf-tools (you can find it
112	under tools/perf/).
113	As this example shows, users can choose more familiar names for each arguments.
114	
115	  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
116	
117	 This sets a kretprobe on the return point of do_sys_open() function with
118	recording return value as "myretprobe" event.
119	 You can see the format of these events via
120	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
121	
122	  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
123	name: myprobe
124	ID: 780
125	format:
126	        field:unsigned short common_type;       offset:0;       size:2; signed:0;
127	        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
128	        field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
129	        field:int common_pid;   offset:4;       size:4; signed:1;
130	
131	        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
132	        field:int __probe_nargs;        offset:16;      size:4; signed:1;
133	        field:unsigned long dfd;        offset:20;      size:4; signed:0;
134	        field:unsigned long filename;   offset:24;      size:4; signed:0;
135	        field:unsigned long flags;      offset:28;      size:4; signed:0;
136	        field:unsigned long mode;       offset:32;      size:4; signed:0;
137	
138	
139	print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
140	REC->dfd, REC->filename, REC->flags, REC->mode
141	
142	 You can see that the event has 4 arguments as in the expressions you specified.
143	
144	  echo > /sys/kernel/debug/tracing/kprobe_events
145	
146	 This clears all probe points.
147	
148	 Or,
149	
150	  echo -:myprobe >> kprobe_events
151	
152	 This clears probe points selectively.
153	
154	 Right after definition, each event is disabled by default. For tracing these
155	events, you need to enable it.
156	
157	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
158	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
159	
160	 And you can see the traced information via /sys/kernel/debug/tracing/trace.
161	
162	  cat /sys/kernel/debug/tracing/trace
163	# tracer: nop
164	#
165	#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
166	#              | |       |          |         |
167	           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
168	           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
169	           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
170	           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
171	           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
172	           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
173	
174	
175	 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
176	returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
177	returns from do_sys_open to sys_open+0x1b).
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