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>/enabled.
22
23
24Synopsis of kprobe_events
25-------------------------
26::
27
28  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
29  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]	: Set a return probe
30  -:[GRP/]EVENT						: Clear a probe
31
32 GRP		: Group name. If omitted, use "kprobes" for it.
33 EVENT		: Event name. If omitted, the event name is generated
34		  based on SYM+offs or MEMADDR.
35 MOD		: Module name which has given SYM.
36 SYM[+offs]	: Symbol+offset where the probe is inserted.
37 MEMADDR	: Address where the probe is inserted.
38 MAXACTIVE	: Maximum number of instances of the specified function that
39		  can be probed simultaneously, or 0 for the default value
40		  as defined in Documentation/kprobes.txt section 1.3.1.
41
42 FETCHARGS	: Arguments. Each probe can have up to 128 args.
43  %REG		: Fetch register REG
44  @ADDR		: Fetch memory at ADDR (ADDR should be in kernel)
45  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
46  $stackN	: Fetch Nth entry of stack (N >= 0)
47  $stack	: Fetch stack address.
48  $retval	: Fetch return value.(*)
49  $comm		: Fetch current task comm.
50  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
51  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
52  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
53		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
54		  (x8/x16/x32/x64), "string" and bitfield are supported.
55
56  (*) only for return probe.
57  (**) this is useful for fetching a field of data structures.
58
59Types
60-----
61Several types are supported for fetch-args. Kprobe tracer will access memory
62by given type. Prefix 's' and 'u' means those types are signed and unsigned
63respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
64in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
65or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
66x86-64 uses x64).
67String type is a special type, which fetches a "null-terminated" string from
68kernel space. This means it will fail and store NULL if the string container
69has been paged out.
70Bitfield is another special type, which takes 3 parameters, bit-width, bit-
71offset, and container-size (usually 32). The syntax is::
72
73 b<bit-width>@<bit-offset>/<container-size>
74
75For $comm, the default type is "string"; any other type is invalid.
76
77
78Per-Probe Event Filtering
79-------------------------
80Per-probe event filtering feature allows you to set different filter on each
81probe and gives you what arguments will be shown in trace buffer. If an event
82name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
83under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
84'enabled', 'format' and 'filter'.
85
86enabled:
87  You can enable/disable the probe by writing 1 or 0 on it.
88
89format:
90  This shows the format of this probe event.
91
92filter:
93  You can write filtering rules of this event.
94
95id:
96  This shows the id of this probe event.
97
98
99Event Profiling
100---------------
101You can check the total number of probe hits and probe miss-hits via
102/sys/kernel/debug/tracing/kprobe_profile.
103The first column is event name, the second is the number of probe hits,
104the third is the number of probe miss-hits.
105
106
107Usage examples
108--------------
109To add a probe as a new event, write a new definition to kprobe_events
110as below::
111
112  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
113
114This sets a kprobe on the top of do_sys_open() function with recording
1151st to 4th arguments as "myprobe" event. Note, which register/stack entry is
116assigned to each function argument depends on arch-specific ABI. If you unsure
117the ABI, please try to use probe subcommand of perf-tools (you can find it
118under tools/perf/).
119As this example shows, users can choose more familiar names for each arguments.
120::
121
122  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
123
124This sets a kretprobe on the return point of do_sys_open() function with
125recording return value as "myretprobe" event.
126You can see the format of these events via
127/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
128::
129
130  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
131  name: myprobe
132  ID: 780
133  format:
134          field:unsigned short common_type;       offset:0;       size:2; signed:0;
135          field:unsigned char common_flags;       offset:2;       size:1; signed:0;
136          field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
137          field:int common_pid;   offset:4;       size:4; signed:1;
138
139          field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
140          field:int __probe_nargs;        offset:16;      size:4; signed:1;
141          field:unsigned long dfd;        offset:20;      size:4; signed:0;
142          field:unsigned long filename;   offset:24;      size:4; signed:0;
143          field:unsigned long flags;      offset:28;      size:4; signed:0;
144          field:unsigned long mode;       offset:32;      size:4; signed:0;
145
146
147  print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
148  REC->dfd, REC->filename, REC->flags, REC->mode
149
150You can see that the event has 4 arguments as in the expressions you specified.
151::
152
153  echo > /sys/kernel/debug/tracing/kprobe_events
154
155This clears all probe points.
156
157Or,
158::
159
160  echo -:myprobe >> kprobe_events
161
162This clears probe points selectively.
163
164Right after definition, each event is disabled by default. For tracing these
165events, you need to enable it.
166::
167
168  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
169  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
170
171And you can see the traced information via /sys/kernel/debug/tracing/trace.
172::
173
174  cat /sys/kernel/debug/tracing/trace
175  # tracer: nop
176  #
177  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
178  #              | |       |          |         |
179             <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
180             <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
181             <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
182             <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
183             <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
184             <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
185
186
187Each line shows when the kernel hits an event, and <- SYMBOL means kernel
188returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
189returns from do_sys_open to sys_open+0x1b).
190
191