1            Uprobe-tracer: Uprobe-based Event Tracing
2            =========================================
3
4           Documentation written by Srikar Dronamraju
5
6
7Overview
8--------
9Uprobe based trace events are similar to kprobe based trace events.
10To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
11
12Similar to the kprobe-event tracer, this doesn't need to be activated via
13current_tracer. Instead of that, add probe points via
14/sys/kernel/debug/tracing/uprobe_events, and enable it via
15/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.
16
17However unlike kprobe-event tracer, the uprobe event interface expects the
18user to calculate the offset of the probepoint in the object.
19
20Synopsis of uprobe_tracer
21-------------------------
22  p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
23  r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
24  -:[GRP/]EVENT                           : Clear uprobe or uretprobe event
25
26  GRP           : Group name. If omitted, "uprobes" is the default value.
27  EVENT         : Event name. If omitted, the event name is generated based
28                  on PATH+OFFSET.
29  PATH          : Path to an executable or a library.
30  OFFSET        : Offset where the probe is inserted.
31
32  FETCHARGS     : Arguments. Each probe can have up to 128 args.
33   %REG         : Fetch register REG
34   @ADDR	: Fetch memory at ADDR (ADDR should be in userspace)
35   @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)
36   $stackN	: Fetch Nth entry of stack (N >= 0)
37   $stack	: Fetch stack address.
38   $retval	: Fetch return value.(*)
39   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
40   NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
41   FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
42		       (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
43		       are supported.
44
45  (*) only for return probe.
46  (**) this is useful for fetching a field of data structures.
47
48Types
49-----
50Several types are supported for fetch-args. Uprobe tracer will access memory
51by given type. Prefix 's' and 'u' means those types are signed and unsigned
52respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
53String type is a special type, which fetches a "null-terminated" string from
54user space.
55Bitfield is another special type, which takes 3 parameters, bit-width, bit-
56offset, and container-size (usually 32). The syntax is;
57
58 b<bit-width>@<bit-offset>/<container-size>
59
60
61Event Profiling
62---------------
63You can check the total number of probe hits and probe miss-hits via
64/sys/kernel/debug/tracing/uprobe_profile.
65The first column is event name, the second is the number of probe hits,
66the third is the number of probe miss-hits.
67
68Usage examples
69--------------
70 * Add a probe as a new uprobe event, write a new definition to uprobe_events
71as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
72
73    echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
74
75 * Add a probe as a new uretprobe event:
76
77    echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
78
79 * Unset registered event:
80
81    echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
82
83 * Print out the events that are registered:
84
85    cat /sys/kernel/debug/tracing/uprobe_events
86
87 * Clear all events:
88
89    echo > /sys/kernel/debug/tracing/uprobe_events
90
91Following example shows how to dump the instruction pointer and %ax register
92at the probed text address. Probe zfree function in /bin/zsh:
93
94    # cd /sys/kernel/debug/tracing/
95    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
96    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
97    # objdump -T /bin/zsh | grep -w zfree
98    0000000000446420 g    DF .text  0000000000000012  Base        zfree
99
100  0x46420 is the offset of zfree in object /bin/zsh that is loaded at
101  0x00400000. Hence the command to uprobe would be:
102
103    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
104
105  And the same for the uretprobe would be:
106
107    # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
108
109Please note: User has to explicitly calculate the offset of the probe-point
110in the object. We can see the events that are registered by looking at the
111uprobe_events file.
112
113    # cat uprobe_events
114    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
115    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
116
117Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
118
119    # cat events/uprobes/zfree_entry/format
120    name: zfree_entry
121    ID: 922
122    format:
123         field:unsigned short common_type;         offset:0;  size:2; signed:0;
124         field:unsigned char common_flags;         offset:2;  size:1; signed:0;
125         field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
126         field:int common_pid;                     offset:4;  size:4; signed:1;
127         field:int common_padding;                 offset:8;  size:4; signed:1;
128
129         field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
130         field:u32 arg1;                           offset:16; size:4; signed:0;
131         field:u32 arg2;                           offset:20; size:4; signed:0;
132
133    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
134
135Right after definition, each event is disabled by default. For tracing these
136events, you need to enable it by:
137
138    # echo 1 > events/uprobes/enable
139
140Lets disable the event after sleeping for some time.
141
142    # sleep 20
143    # echo 0 > events/uprobes/enable
144
145And you can see the traced information via /sys/kernel/debug/tracing/trace.
146
147    # cat trace
148    # tracer: nop
149    #
150    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
151    #              | |       |          |         |
152                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
153                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
154                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
155                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
156
157Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
158and contents of ax register being 79. And uretprobe was triggered with ip at
1590x446540 with counterpart function entry at 0x446420.
160