tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
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 $comm : Fetch current task comm.
40 +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
41 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
42 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
43 (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
44 are supported.
45
46 (*) only for return probe.
47 (**) this is useful for fetching a field of data structures.
48
49Types
50-----
51Several types are supported for fetch-args. Uprobe tracer will access memory
52by given type. Prefix 's' and 'u' means those types are signed and unsigned
53respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
54String type is a special type, which fetches a "null-terminated" string from
55user space.
56Bitfield is another special type, which takes 3 parameters, bit-width, bit-
57offset, and container-size (usually 32). The syntax is;
58
59 b<bit-width>@<bit-offset>/<container-size>
60
61For $comm, the default type is "string"; any other type is invalid.
62
63
64Event Profiling
65---------------
66You can check the total number of probe hits and probe miss-hits via
67/sys/kernel/debug/tracing/uprobe_profile.
68The first column is event name, the second is the number of probe hits,
69the third is the number of probe miss-hits.
70
71Usage examples
72--------------
73 * Add a probe as a new uprobe event, write a new definition to uprobe_events
74as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
75
76 echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
77
78 * Add a probe as a new uretprobe event:
79
80 echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
81
82 * Unset registered event:
83
84 echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
85
86 * Print out the events that are registered:
87
88 cat /sys/kernel/debug/tracing/uprobe_events
89
90 * Clear all events:
91
92 echo > /sys/kernel/debug/tracing/uprobe_events
93
94Following example shows how to dump the instruction pointer and %ax register
95at the probed text address. Probe zfree function in /bin/zsh:
96
97 # cd /sys/kernel/debug/tracing/
98 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
99 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
100 # objdump -T /bin/zsh | grep -w zfree
101 0000000000446420 g DF .text 0000000000000012 Base zfree
102
103 0x46420 is the offset of zfree in object /bin/zsh that is loaded at
104 0x00400000. Hence the command to uprobe would be:
105
106 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
107
108 And the same for the uretprobe would be:
109
110 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
111
112Please note: User has to explicitly calculate the offset of the probe-point
113in the object. We can see the events that are registered by looking at the
114uprobe_events file.
115
116 # cat uprobe_events
117 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
118 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
119
120Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
121
122 # cat events/uprobes/zfree_entry/format
123 name: zfree_entry
124 ID: 922
125 format:
126 field:unsigned short common_type; offset:0; size:2; signed:0;
127 field:unsigned char common_flags; offset:2; size:1; signed:0;
128 field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
129 field:int common_pid; offset:4; size:4; signed:1;
130 field:int common_padding; offset:8; size:4; signed:1;
131
132 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
133 field:u32 arg1; offset:16; size:4; signed:0;
134 field:u32 arg2; offset:20; size:4; signed:0;
135
136 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
137
138Right after definition, each event is disabled by default. For tracing these
139events, you need to enable it by:
140
141 # echo 1 > events/uprobes/enable
142
143Lets disable the event after sleeping for some time.
144
145 # sleep 20
146 # echo 0 > events/uprobes/enable
147
148And you can see the traced information via /sys/kernel/debug/tracing/trace.
149
150 # cat trace
151 # tracer: nop
152 #
153 # TASK-PID CPU# TIMESTAMP FUNCTION
154 # | | | | |
155 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
156 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
157 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
158 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
159
160Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
161and contents of ax register being 79. And uretprobe was triggered with ip at
1620x446540 with counterpart function entry at 0x446420.