1==========================
2Kprobe-based Event Tracing
3==========================
4
5:Author: Masami Hiramatsu
6
7Overview
8--------
9These events are similar to tracepoint-based events. Instead of tracepoints,
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 event tracer, this doesn't need to be activated via
19current_tracer. Instead of that, add probe points via
20/sys/kernel/tracing/kprobe_events, and enable it via
21/sys/kernel/tracing/events/kprobes/<EVENT>/enable.
22
23You can also use /sys/kernel/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  $comm		: Fetch current task comm.
56  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
57  \IMM		: Store an immediate value to the argument.
58  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
59  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
60		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
61		  (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
62                  and bitfield are supported.
63
64  (\*1) only for the probe on function entry (offs == 0).
65  (\*2) only for return probe.
66  (\*3) this is useful for fetching a field of data structures.
67  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
68
69Types
70-----
71Several types are supported for fetchargs. Kprobe tracer will access memory
72by given type. Prefix 's' and 'u' means those types are signed and unsigned
73respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
74in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
75or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
76x86-64 uses x64).
77
78These value types can be an array. To record array data, you can add '[N]'
79(where N is a fixed number, less than 64) to the base type.
80E.g. 'x16[4]' means an array of x16 (2-byte hex) with 4 elements.
81Note that the array can be applied to memory type fetchargs, you can not
82apply it to registers/stack-entries etc. (for example, '$stack1:x8[8]' is
83wrong, but '+8($stack):x8[8]' is OK.)
84
85Char type can be used to show the character value of traced arguments.
86
87String type is a special type, which fetches a "null-terminated" string from
88kernel space. This means it will fail and store NULL if the string container
89has been paged out. "ustring" type is an alternative of string for user-space.
90See :ref:`user_mem_access` for more info.
91
92The string array type is a bit different from other types. For other base
93types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same
94as +0(%di):x32.) But string[1] is not equal to string. The string type itself
95represents "char array", but string array type represents "char * array".
96So, for example, +0(%di):string[1] is equal to +0(+0(%di)):string.
97Bitfield is another special type, which takes 3 parameters, bit-width, bit-
98offset, and container-size (usually 32). The syntax is::
99
100 b<bit-width>@<bit-offset>/<container-size>
101
102Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG)
103which shows given pointer in "symbol+offset" style.
104On the other hand, symbol-string type ('symstr') converts the given address to
105"symbol+offset/symbolsize" style and stores it as a null-terminated string.
106With 'symstr' type, you can filter the event with wildcard pattern of the
107symbols, and you don't need to solve symbol name by yourself.
108For $comm, the default type is "string"; any other type is invalid.
109
110.. _user_mem_access:
111
112User Memory Access
113------------------
114Kprobe events supports user-space memory access. For that purpose, you can use
115either user-space dereference syntax or 'ustring' type.
116
117The user-space dereference syntax allows you to access a field of a data
118structure in user-space. This is done by adding the "u" prefix to the
119dereference syntax. For example, +u4(%si) means it will read memory from the
120address in the register %si offset by 4, and the memory is expected to be in
121user-space. You can use this for strings too, e.g. +u0(%si):string will read
122a string from the address in the register %si that is expected to be in user-
123space. 'ustring' is a shortcut way of performing the same task. That is,
124+0(%si):ustring is equivalent to +u0(%si):string.
125
126Note that kprobe-event provides the user-memory access syntax but it doesn't
127use it transparently. This means if you use normal dereference or string type
128for user memory, it might fail, and may always fail on some architectures. The
129user has to carefully check if the target data is in kernel or user space.
130
131Per-Probe Event Filtering
132-------------------------
133Per-probe event filtering feature allows you to set different filter on each
134probe and gives you what arguments will be shown in trace buffer. If an event
135name is specified right after 'p:' or 'r:' in kprobe_events, it adds an event
136under tracing/events/kprobes/<EVENT>, at the directory you can see 'id',
137'enable', 'format', 'filter' and 'trigger'.
138
139enable:
140  You can enable/disable the probe by writing 1 or 0 on it.
141
142format:
143  This shows the format of this probe event.
144
145filter:
146  You can write filtering rules of this event.
147
148id:
149  This shows the id of this probe event.
150
151trigger:
152  This allows to install trigger commands which are executed when the event is
153  hit (for details, see Documentation/trace/events.rst, section 6).
154
155Event Profiling
156---------------
157You can check the total number of probe hits and probe miss-hits via
158/sys/kernel/tracing/kprobe_profile.
159The first column is event name, the second is the number of probe hits,
160the third is the number of probe miss-hits.
161
162Kernel Boot Parameter
163---------------------
164You can add and enable new kprobe events when booting up the kernel by
165"kprobe_event=" parameter. The parameter accepts a semicolon-delimited
166kprobe events, which format is similar to the kprobe_events.
167The difference is that the probe definition parameters are comma-delimited
168instead of space. For example, adding myprobe event on do_sys_open like below::
169
170  p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)
171
172should be below for kernel boot parameter (just replace spaces with comma)::
173
174  p:myprobe,do_sys_open,dfd=%ax,filename=%dx,flags=%cx,mode=+4($stack)
175
176
177Usage examples
178--------------
179To add a probe as a new event, write a new definition to kprobe_events
180as below::
181
182  echo 'p:myprobe do_sys_open dfd=%ax filename=%dx flags=%cx mode=+4($stack)' > /sys/kernel/tracing/kprobe_events
183
184This sets a kprobe on the top of do_sys_open() function with recording
1851st to 4th arguments as "myprobe" event. Note, which register/stack entry is
186assigned to each function argument depends on arch-specific ABI. If you unsure
187the ABI, please try to use probe subcommand of perf-tools (you can find it
188under tools/perf/).
189As this example shows, users can choose more familiar names for each arguments.
190::
191
192  echo 'r:myretprobe do_sys_open $retval' >> /sys/kernel/tracing/kprobe_events
193
194This sets a kretprobe on the return point of do_sys_open() function with
195recording return value as "myretprobe" event.
196You can see the format of these events via
197/sys/kernel/tracing/events/kprobes/<EVENT>/format.
198::
199
200  cat /sys/kernel/tracing/events/kprobes/myprobe/format
201  name: myprobe
202  ID: 780
203  format:
204          field:unsigned short common_type;       offset:0;       size:2; signed:0;
205          field:unsigned char common_flags;       offset:2;       size:1; signed:0;
206          field:unsigned char common_preempt_count;       offset:3; size:1;signed:0;
207          field:int common_pid;   offset:4;       size:4; signed:1;
208
209          field:unsigned long __probe_ip; offset:12;      size:4; signed:0;
210          field:int __probe_nargs;        offset:16;      size:4; signed:1;
211          field:unsigned long dfd;        offset:20;      size:4; signed:0;
212          field:unsigned long filename;   offset:24;      size:4; signed:0;
213          field:unsigned long flags;      offset:28;      size:4; signed:0;
214          field:unsigned long mode;       offset:32;      size:4; signed:0;
215
216
217  print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->__probe_ip,
218  REC->dfd, REC->filename, REC->flags, REC->mode
219
220You can see that the event has 4 arguments as in the expressions you specified.
221::
222
223  echo > /sys/kernel/tracing/kprobe_events
224
225This clears all probe points.
226
227Or,
228::
229
230  echo -:myprobe >> kprobe_events
231
232This clears probe points selectively.
233
234Right after definition, each event is disabled by default. For tracing these
235events, you need to enable it.
236::
237
238  echo 1 > /sys/kernel/tracing/events/kprobes/myprobe/enable
239  echo 1 > /sys/kernel/tracing/events/kprobes/myretprobe/enable
240
241Use the following command to start tracing in an interval.
242::
243
244    # echo 1 > tracing_on
245    Open something...
246    # echo 0 > tracing_on
247
248And you can see the traced information via /sys/kernel/tracing/trace.
249::
250
251  cat /sys/kernel/tracing/trace
252  # tracer: nop
253  #
254  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
255  #              | |       |          |         |
256             <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
257             <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $retval=fffffffffffffffe
258             <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
259             <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
260             <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
261             <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $retval=3
262
263
264Each line shows when the kernel hits an event, and <- SYMBOL means kernel
265returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
266returns from do_sys_open to sys_open+0x1b).
267