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1 | Uprobe-tracer: Uprobe-based Event Tracing |
2 | ========================================= | |
3 | ||
4 | Documentation written by Srikar Dronamraju | |
5 | ||
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6 | |
7 | Overview | |
8 | -------- | |
9 | Uprobe based trace events are similar to kprobe based trace events. | |
ec83db0f | 10 | To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y. |
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11 | |
12 | Similar to the kprobe-event tracer, this doesn't need to be activated via | |
13 | current_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 | ||
17 | However unlike kprobe-event tracer, the uprobe event interface expects the | |
decc6bfb | 18 | user to calculate the offset of the probepoint in the object. |
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19 | |
20 | Synopsis of uprobe_tracer | |
21 | ------------------------- | |
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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 | |
f3f096cf | 25 | |
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26 | GRP : Group name. If omitted, "uprobes" is the default value. |
27 | EVENT : Event name. If omitted, the event name is generated based | |
306cfe20 | 28 | on PATH+OFFSET. |
decc6bfb | 29 | PATH : Path to an executable or a library. |
306cfe20 | 30 | OFFSET : Offset where the probe is inserted. |
f3f096cf | 31 | |
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32 | FETCHARGS : Arguments. Each probe can have up to 128 args. |
33 | %REG : Fetch register REG | |
b079d374 | 34 | @ADDR : Fetch memory at ADDR (ADDR should be in userspace) |
b7e0bf34 | 35 | @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) |
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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 | ||
48 | Types | |
49 | ----- | |
50 | Several types are supported for fetch-args. Uprobe tracer will access memory | |
51 | by given type. Prefix 's' and 'u' means those types are signed and unsigned | |
52 | respectively. Traced arguments are shown in decimal (signed) or hex (unsigned). | |
53 | String type is a special type, which fetches a "null-terminated" string from | |
54 | user space. | |
55 | Bitfield is another special type, which takes 3 parameters, bit-width, bit- | |
56 | offset, and container-size (usually 32). The syntax is; | |
57 | ||
58 | b<bit-width>@<bit-offset>/<container-size> | |
59 | ||
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60 | |
61 | Event Profiling | |
62 | --------------- | |
decc6bfb | 63 | You can check the total number of probe hits and probe miss-hits via |
f3f096cf | 64 | /sys/kernel/debug/tracing/uprobe_profile. |
decc6bfb | 65 | The first column is event name, the second is the number of probe hits, |
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66 | the third is the number of probe miss-hits. |
67 | ||
68 | Usage examples | |
69 | -------------- | |
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70 | * Add a probe as a new uprobe event, write a new definition to uprobe_events |
71 | as 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: | |
f3f096cf | 80 | |
decc6bfb | 81 | echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 82 | |
decc6bfb | 83 | * Print out the events that are registered: |
f3f096cf | 84 | |
decc6bfb | 85 | cat /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 86 | |
decc6bfb | 87 | * Clear all events: |
f3f096cf | 88 | |
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89 | echo > /sys/kernel/debug/tracing/uprobe_events |
90 | ||
91 | Following example shows how to dump the instruction pointer and %ax register | |
92 | at the probed text address. Probe zfree function in /bin/zsh: | |
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93 | |
94 | # cd /sys/kernel/debug/tracing/ | |
decc6bfb | 95 | # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp |
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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 | ||
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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: | |
f3f096cf | 106 | |
decc6bfb | 107 | # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events |
f3f096cf | 108 | |
decc6bfb | 109 | Please note: User has to explicitly calculate the offset of the probe-point |
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110 | in the object. We can see the events that are registered by looking at the |
111 | uprobe_events file. | |
112 | ||
113 | # cat uprobe_events | |
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114 | p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax |
115 | r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax | |
ec83db0f | 116 | |
decc6bfb | 117 | Format of events can be seen by viewing the file events/uprobes/zfree_entry/format |
ec83db0f | 118 | |
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119 | # cat events/uprobes/zfree_entry/format |
120 | name: zfree_entry | |
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121 | ID: 922 |
122 | format: | |
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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; | |
ec83db0f | 128 | |
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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; | |
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132 | |
133 | print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 | |
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134 | |
135 | Right after definition, each event is disabled by default. For tracing these | |
136 | events, you need to enable it by: | |
137 | ||
138 | # echo 1 > events/uprobes/enable | |
139 | ||
140 | Lets disable the event after sleeping for some time. | |
decc6bfb | 141 | |
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142 | # sleep 20 |
143 | # echo 0 > events/uprobes/enable | |
144 | ||
145 | And 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 | # | | | | | | |
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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 | ||
157 | Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 | |
158 | and contents of ax register being 79. And uretprobe was triggered with ip at | |
159 | 0x446540 with counterpart function entry at 0x446420. |