1=========================================
2Uprobe-tracer: Uprobe-based Event Tracing
3=========================================
4
5:Author: Srikar Dronamraju
6
7
8Overview
9--------
10Uprobe based trace events are similar to kprobe based trace events.
11To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y.
12
13Similar to the kprobe-event tracer, this doesn't need to be activated via
14current_tracer. Instead of that, add probe points via
15/sys/kernel/debug/tracing/uprobe_events, and enable it via
16/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enable.
17
18However unlike kprobe-event tracer, the uprobe event interface expects the
19user to calculate the offset of the probepoint in the object.
20
21You can also use /sys/kernel/debug/tracing/dynamic_events instead of
22uprobe_events. That interface will provide unified access to other
23dynamic events too.
24
25Synopsis of uprobe_tracer
26-------------------------
27::
28
29  p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
30  r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
31  -:[GRP/]EVENT                           : Clear uprobe or uretprobe event
32
33  GRP           : Group name. If omitted, "uprobes" is the default value.
34  EVENT         : Event name. If omitted, the event name is generated based
35                  on PATH+OFFSET.
36  PATH          : Path to an executable or a library.
37  OFFSET        : Offset where the probe is inserted.
38
39  FETCHARGS     : Arguments. Each probe can have up to 128 args.
40   %REG         : Fetch register REG
41   @ADDR	: Fetch memory at ADDR (ADDR should be in userspace)
42   @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)
43   $stackN	: Fetch Nth entry of stack (N >= 0)
44   $stack	: Fetch stack address.
45   $retval	: Fetch return value.(\*1)
46   $comm	: Fetch current task comm.
47   +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)
48   \IMM		: Store an immediate value to the argument.
49   NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
50   FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
51		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
52		       (x8/x16/x32/x64), "string" and bitfield are supported.
53
54  (\*1) only for return probe.
55  (\*2) this is useful for fetching a field of data structures.
56  (\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe
57        events can access only user-space memory.
58
59Types
60-----
61Several types are supported for fetch-args. Uprobe 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
68user space.
69Bitfield is another special type, which takes 3 parameters, bit-width, bit-
70offset, and container-size (usually 32). The syntax is::
71
72 b<bit-width>@<bit-offset>/<container-size>
73
74For $comm, the default type is "string"; any other type is invalid.
75
76
77Event Profiling
78---------------
79You can check the total number of probe hits per event via
80/sys/kernel/debug/tracing/uprobe_profile. The first column is the filename,
81the second is the event name, the third is the number of probe hits.
82
83Usage examples
84--------------
85 * Add a probe as a new uprobe event, write a new definition to uprobe_events
86   as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)::
87
88    echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
89
90 * Add a probe as a new uretprobe event::
91
92    echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
93
94 * Unset registered event::
95
96    echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
97
98 * Print out the events that are registered::
99
100    cat /sys/kernel/debug/tracing/uprobe_events
101
102 * Clear all events::
103
104    echo > /sys/kernel/debug/tracing/uprobe_events
105
106Following example shows how to dump the instruction pointer and %ax register
107at the probed text address. Probe zfree function in /bin/zsh::
108
109    # cd /sys/kernel/debug/tracing/
110    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
111    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
112    # objdump -T /bin/zsh | grep -w zfree
113    0000000000446420 g    DF .text  0000000000000012  Base        zfree
114
1150x46420 is the offset of zfree in object /bin/zsh that is loaded at
1160x00400000. Hence the command to uprobe would be::
117
118    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
119
120And the same for the uretprobe would be::
121
122    # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
123
124.. note:: User has to explicitly calculate the offset of the probe-point
125	in the object.
126
127We can see the events that are registered by looking at the uprobe_events file.
128::
129
130    # cat uprobe_events
131    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
132    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
133
134Format of events can be seen by viewing the file events/uprobes/zfree_entry/format.
135::
136
137    # cat events/uprobes/zfree_entry/format
138    name: zfree_entry
139    ID: 922
140    format:
141         field:unsigned short common_type;         offset:0;  size:2; signed:0;
142         field:unsigned char common_flags;         offset:2;  size:1; signed:0;
143         field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
144         field:int common_pid;                     offset:4;  size:4; signed:1;
145         field:int common_padding;                 offset:8;  size:4; signed:1;
146
147         field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
148         field:u32 arg1;                           offset:16; size:4; signed:0;
149         field:u32 arg2;                           offset:20; size:4; signed:0;
150
151    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
152
153Right after definition, each event is disabled by default. For tracing these
154events, you need to enable it by::
155
156    # echo 1 > events/uprobes/enable
157
158Lets start tracing, sleep for some time and stop tracing.
159::
160
161    # echo 1 > tracing_on
162    # sleep 20
163    # echo 0 > tracing_on
164
165Also, you can disable the event by::
166
167    # echo 0 > events/uprobes/enable
168
169And you can see the traced information via /sys/kernel/debug/tracing/trace.
170::
171
172    # cat trace
173    # tracer: nop
174    #
175    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
176    #              | |       |          |         |
177                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
178                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
179                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
180                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
181
182Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
183and contents of ax register being 79. And uretprobe was triggered with ip at
1840x446540 with counterpart function entry at 0x446420.
185