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Based on kernel version 4.8. Page generated on 2016-10-06 23:19 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), "string" and bitfield
48			  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. Traced arguments are shown in decimal (signed) or hex (unsigned).
58	String type is a special type, which fetches a "null-terminated" string from
59	kernel space. This means it will fail and store NULL if the string container
60	has been paged out.
61	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
62	offset, and container-size (usually 32). The syntax is;
63	
64	 b<bit-width>@<bit-offset>/<container-size>
65	
66	For $comm, the default type is "string"; any other type is invalid.
67	
68	
69	Per-Probe Event Filtering
70	-------------------------
71	 Per-probe event filtering feature allows you to set different filter on each
72	probe and gives you what arguments will be shown in trace buffer. If an event
73	name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
74	under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
75	'enabled', 'format' and 'filter'.
76	
77	enabled:
78	  You can enable/disable the probe by writing 1 or 0 on it.
79	
80	format:
81	  This shows the format of this probe event.
82	
83	filter:
84	  You can write filtering rules of this event.
85	
86	id:
87	  This shows the id of this probe event.
88	
89	
90	Event Profiling
91	---------------
92	 You can check the total number of probe hits and probe miss-hits via
93	/sys/kernel/debug/tracing/kprobe_profile.
94	 The first column is event name, the second is the number of probe hits,
95	the third is the number of probe miss-hits.
96	
97	
98	Usage examples
99	--------------
100	To add a probe as a new event, write a new definition to kprobe_events
101	as below.
102	
103	  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
104	
105	 This sets a kprobe on the top of do_sys_open() function with recording
106	1st to 4th arguments as "myprobe" event. Note, which register/stack entry is
107	assigned to each function argument depends on arch-specific ABI. If you unsure
108	the ABI, please try to use probe subcommand of perf-tools (you can find it
109	under tools/perf/).
110	As this example shows, users can choose more familiar names for each arguments.
111	
112	  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
113	
114	 This sets a kretprobe on the return point of do_sys_open() function with
115	recording return value as "myretprobe" event.
116	 You can see the format of these events via
117	/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
118	
119	  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
120	name: myprobe
121	ID: 780
122	format:
123	        field:unsigned short common_type;       offset:0;       size:2; signed:0;
124	        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
125	        field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
126	        field:int common_pid;   offset:4;       size:4; signed:1;
127	
128	        field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
129	        field:int __probe_nargs;        offset:16;      size:4; signed:1;
130	        field:unsigned long dfd;        offset:20;      size:4; signed:0;
131	        field:unsigned long filename;   offset:24;      size:4; signed:0;
132	        field:unsigned long flags;      offset:28;      size:4; signed:0;
133	        field:unsigned long mode;       offset:32;      size:4; signed:0;
134	
135	
136	print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
137	REC->dfd, REC->filename, REC->flags, REC->mode
138	
139	 You can see that the event has 4 arguments as in the expressions you specified.
140	
141	  echo > /sys/kernel/debug/tracing/kprobe_events
142	
143	 This clears all probe points.
144	
145	 Or,
146	
147	  echo -:myprobe >> kprobe_events
148	
149	 This clears probe points selectively.
150	
151	 Right after definition, each event is disabled by default. For tracing these
152	events, you need to enable it.
153	
154	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
155	  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
156	
157	 And you can see the traced information via /sys/kernel/debug/tracing/trace.
158	
159	  cat /sys/kernel/debug/tracing/trace
160	# tracer: nop
161	#
162	#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
163	#              | |       |          |         |
164	           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
165	           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
166	           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
167	           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
168	           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
169	           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
170	
171	
172	 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
173	returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
174	returns from do_sys_open to sys_open+0x1b).
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