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.(*)
46   $comm	: Fetch current task comm.
47   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
48   NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
49   FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
50		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
51		       (x8/x16/x32/x64), "string" and bitfield are supported.
52
53  (*) only for return probe.
54  (**) this is useful for fetching a field of data structures.
55
56Types
57-----
58Several types are supported for fetch-args. Uprobe tracer will access memory
59by given type. Prefix 's' and 'u' means those types are signed and unsigned
60respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
61in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
62or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
63x86-64 uses x64).
64String type is a special type, which fetches a "null-terminated" string from
65user space.
66Bitfield is another special type, which takes 3 parameters, bit-width, bit-
67offset, and container-size (usually 32). The syntax is::
68
69 b<bit-width>@<bit-offset>/<container-size>
70
71For $comm, the default type is "string"; any other type is invalid.
72
73
74Event Profiling
75---------------
76You can check the total number of probe hits and probe miss-hits via
77/sys/kernel/debug/tracing/uprobe_profile.
78The first column is event name, the second is the number of probe hits,
79the third is the number of probe miss-hits.
80
81Usage examples
82--------------
83 * Add a probe as a new uprobe event, write a new definition to uprobe_events
84   as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)::
85
86    echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
87
88 * Add a probe as a new uretprobe event::
89
90    echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
91
92 * Unset registered event::
93
94    echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
95
96 * Print out the events that are registered::
97
98    cat /sys/kernel/debug/tracing/uprobe_events
99
100 * Clear all events::
101
102    echo > /sys/kernel/debug/tracing/uprobe_events
103
104Following example shows how to dump the instruction pointer and %ax register
105at the probed text address. Probe zfree function in /bin/zsh::
106
107    # cd /sys/kernel/debug/tracing/
108    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
109    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
110    # objdump -T /bin/zsh | grep -w zfree
111    0000000000446420 g    DF .text  0000000000000012  Base        zfree
112
1130x46420 is the offset of zfree in object /bin/zsh that is loaded at
1140x00400000. Hence the command to uprobe would be::
115
116    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
117
118And the same for the uretprobe would be::
119
120    # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
121
122.. note:: User has to explicitly calculate the offset of the probe-point
123	in the object.
124
125We can see the events that are registered by looking at the uprobe_events file.
126::
127
128    # cat uprobe_events
129    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
130    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
131
132Format of events can be seen by viewing the file events/uprobes/zfree_entry/format.
133::
134
135    # cat events/uprobes/zfree_entry/format
136    name: zfree_entry
137    ID: 922
138    format:
139         field:unsigned short common_type;         offset:0;  size:2; signed:0;
140         field:unsigned char common_flags;         offset:2;  size:1; signed:0;
141         field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
142         field:int common_pid;                     offset:4;  size:4; signed:1;
143         field:int common_padding;                 offset:8;  size:4; signed:1;
144
145         field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
146         field:u32 arg1;                           offset:16; size:4; signed:0;
147         field:u32 arg2;                           offset:20; size:4; signed:0;
148
149    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
150
151Right after definition, each event is disabled by default. For tracing these
152events, you need to enable it by::
153
154    # echo 1 > events/uprobes/enable
155
156Lets disable the event after sleeping for some time.
157::
158
159    # sleep 20
160    # echo 0 > events/uprobes/enable
161
162And you can see the traced information via /sys/kernel/debug/tracing/trace.
163::
164
165    # cat trace
166    # tracer: nop
167    #
168    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
169    #              | |       |          |         |
170                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
171                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
172                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
173                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
174
175Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
176and contents of ax register being 79. And uretprobe was triggered with ip at
1770x446540 with counterpart function entry at 0x446420.
178