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  $comm		: Fetch current task comm.
56  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57  \IMM		: Store an immediate value to the argument.
58  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61		  (x8/x16/x32/x64), "string", "ustring" and bitfield
62		  are supported.
63
64  (\*1) only for the probe on function entry (offs == 0).
65  (\*2) only for return probe.
66  (\*3) this is useful for fetching a field of data structures.
67  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
68
69Types
70-----
71Several types are supported for fetch-args. Kprobe tracer will access memory
72by given type. Prefix 's' and 'u' means those types are signed and unsigned
73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
76x86-64 uses x64).
77These value types can be an array. To record array data, you can add '[N]'
78(where N is a fixed number, less than 64) to the base type.
79E.g. 'x16[4]' means an array of x16 (2bytes hex) with 4 elements.
80Note that the array can be applied to memory type fetchargs, you can not
81apply it to registers/stack-entries etc. (for example, '$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.
99For $comm, the default type is "string"; any other type is invalid.
100
101.. _user_mem_access:
102
103User Memory Access
104------------------
105Kprobe events supports user-space memory access. For that purpose, you can use
106either user-space dereference syntax or 'ustring' type.
107
108The user-space dereference syntax allows you to access a field of a data
109structure in user-space. This is done by adding the "u" prefix to the
110dereference syntax. For example, +u4(%si) means it will read memory from the
111address in the register %si offset by 4, and the memory is expected to be in
112user-space. You can use this for strings too, e.g. +u0(%si):string will read
113a string from the address in the register %si that is expected to be in user-
114space. 'ustring' is a shortcut way of performing the same task. That is,
115+0(%si):ustring is equivalent to +u0(%si):string.
116
117Note that kprobe-event provides the user-memory access syntax but it doesn't
118use it transparently. This means if you use normal dereference or string type
119for user memory, it might fail, and may always fail on some archs. The user
120has to carefully check if the target data is in kernel or user space.
121
122Per-Probe Event Filtering
123-------------------------
124Per-probe event filtering feature allows you to set different filter on each
125probe and gives you what arguments will be shown in trace buffer. If an event
126name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
127under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
128'enable', 'format', 'filter' and 'trigger'.
129
130enable:
131  You can enable/disable the probe by writing 1 or 0 on it.
132
133format:
134  This shows the format of this probe event.
135
136filter:
137  You can write filtering rules of this event.
138
139id:
140  This shows the id of this probe event.
141
142trigger:
143  This allows to install trigger commands which are executed when the event is
144  hit (for details, see Documentation/trace/events.rst, section 6).
145
146Event Profiling
147---------------
148You can check the total number of probe hits and probe miss-hits via
149/sys/kernel/debug/tracing/kprobe_profile.
150The first column is event name, the second is the number of probe hits,
151the third is the number of probe miss-hits.
152
153Kernel Boot Parameter
154---------------------
155You can add and enable new kprobe events when booting up the kernel by
156"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
157kprobe events, which format is similar to the kprobe_events.
158The difference is that the probe definition parameters are comma-delimited
159instead of space. For example, adding myprobe event on do_sys_open like below
160
161  p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
162
163should be below for kernel boot parameter (just replace spaces with comma)
164
165  p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
166
167
168Usage examples
169--------------
170To add a probe as a new event, write a new definition to kprobe_events
171as below::
172
173  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
174
175This sets a kprobe on the top of do_sys_open() function with recording
1761st to 4th arguments as "myprobe" event. Note, which register/stack entry is
177assigned to each function argument depends on arch-specific ABI. If you unsure
178the ABI, please try to use probe subcommand of perf-tools (you can find it
179under tools/perf/).
180As this example shows, users can choose more familiar names for each arguments.
181::
182
183  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/debug/tracing/kprobe_events
184
185This sets a kretprobe on the return point of do_sys_open() function with
186recording return value as "myretprobe" event.
187You can see the format of these events via
188/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.
189::
190
191  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
192  name: myprobe
193  ID: 780
194  format:
195          field:unsigned short common_type;       offset:0;       size:2; signed:0;
196          field:unsigned char common_flags;       offset:2;       size:1; signed:0;
197          field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
198          field:int common_pid;   offset:4;       size:4; signed:1;
199
200          field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
201          field:int __probe_nargs;        offset:16;      size:4; signed:1;
202          field:unsigned long dfd;        offset:20;      size:4; signed:0;
203          field:unsigned long filename;   offset:24;      size:4; signed:0;
204          field:unsigned long flags;      offset:28;      size:4; signed:0;
205          field:unsigned long mode;       offset:32;      size:4; signed:0;
206
207
208  print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
209  REC->dfd, REC->filename, REC->flags, REC->mode
210
211You can see that the event has 4 arguments as in the expressions you specified.
212::
213
214  echo > /sys/kernel/debug/tracing/kprobe_events
215
216This clears all probe points.
217
218Or,
219::
220
221  echo -:myprobe >> kprobe_events
222
223This clears probe points selectively.
224
225Right after definition, each event is disabled by default. For tracing these
226events, you need to enable it.
227::
228
229  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
230  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
231
232Use the following command to start tracing in an interval.
233::
234
235    # echo 1 > tracing_on
236    Open something...
237    # echo 0 > tracing_on
238
239And you can see the traced information via /sys/kernel/debug/tracing/trace.
240::
241
242  cat /sys/kernel/debug/tracing/trace
243  # tracer: nop
244  #
245  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
246  #              | |       |          |         |
247             <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
248             <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
249             <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
250             <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
251             <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
252             <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
253
254
255Each line shows when the kernel hits an event, and <- SYMBOL means kernel
256returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
257returns from do_sys_open to sys_open+0x1b).
258