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1 | ========================================= |
2 | Uprobe-tracer: Uprobe-based Event Tracing | |
3 | ========================================= | |
decc6bfb | 4 | |
00b27da3 | 5 | :Author: Srikar Dronamraju |
decc6bfb | 6 | |
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7 | |
8 | Overview | |
9 | -------- | |
10 | Uprobe based trace events are similar to kprobe based trace events. | |
6b0b7551 | 11 | To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y. |
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12 | |
13 | Similar to the kprobe-event tracer, this doesn't need to be activated via | |
14 | current_tracer. Instead of that, add probe points via | |
15 | /sys/kernel/debug/tracing/uprobe_events, and enable it via | |
e50891d6 | 16 | /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enable. |
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17 | |
18 | However unlike kprobe-event tracer, the uprobe event interface expects the | |
decc6bfb | 19 | user to calculate the offset of the probepoint in the object. |
f3f096cf | 20 | |
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21 | You can also use /sys/kernel/debug/tracing/dynamic_events instead of |
22 | uprobe_events. That interface will provide unified access to other | |
23 | dynamic events too. | |
24 | ||
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25 | Synopsis of uprobe_tracer |
26 | ------------------------- | |
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27 | :: |
28 | ||
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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 | |
f3f096cf | 32 | |
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33 | GRP : Group name. If omitted, "uprobes" is the default value. |
34 | EVENT : Event name. If omitted, the event name is generated based | |
306cfe20 | 35 | on PATH+OFFSET. |
decc6bfb | 36 | PATH : Path to an executable or a library. |
306cfe20 | 37 | OFFSET : Offset where the probe is inserted. |
f3f096cf | 38 | |
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39 | FETCHARGS : Arguments. Each probe can have up to 128 args. |
40 | %REG : Fetch register REG | |
b079d374 | 41 | @ADDR : Fetch memory at ADDR (ADDR should be in userspace) |
b7e0bf34 | 42 | @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH) |
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43 | $stackN : Fetch Nth entry of stack (N >= 0) |
44 | $stack : Fetch stack address. | |
45 | $retval : Fetch return value.(*) | |
35abb67d | 46 | $comm : Fetch current task comm. |
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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 | |
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50 | (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types |
51 | (x8/x16/x32/x64), "string" and bitfield are supported. | |
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52 | |
53 | (*) only for return probe. | |
54 | (**) this is useful for fetching a field of data structures. | |
55 | ||
56 | Types | |
57 | ----- | |
58 | Several types are supported for fetch-args. Uprobe tracer will access memory | |
59 | by given type. Prefix 's' and 'u' means those types are signed and unsigned | |
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60 | respectively. 'x' prefix implies it is unsigned. Traced arguments are shown |
61 | in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32' | |
62 | or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and | |
63 | x86-64 uses x64). | |
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64 | String type is a special type, which fetches a "null-terminated" string from |
65 | user space. | |
66 | Bitfield is another special type, which takes 3 parameters, bit-width, bit- | |
00b27da3 | 67 | offset, and container-size (usually 32). The syntax is:: |
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68 | |
69 | b<bit-width>@<bit-offset>/<container-size> | |
70 | ||
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71 | For $comm, the default type is "string"; any other type is invalid. |
72 | ||
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73 | |
74 | Event Profiling | |
75 | --------------- | |
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76 | You can check the total number of probe hits per event via |
77 | /sys/kernel/debug/tracing/uprobe_profile. The first column is the filename, | |
78 | the second is the event name, the third is the number of probe hits. | |
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79 | |
80 | Usage examples | |
81 | -------------- | |
decc6bfb | 82 | * Add a probe as a new uprobe event, write a new definition to uprobe_events |
00b27da3 | 83 | as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash):: |
decc6bfb | 84 | |
7058763b | 85 | echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events |
decc6bfb | 86 | |
00b27da3 | 87 | * Add a probe as a new uretprobe event:: |
decc6bfb | 88 | |
7058763b | 89 | echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events |
decc6bfb | 90 | |
00b27da3 | 91 | * Unset registered event:: |
f3f096cf | 92 | |
7058763b | 93 | echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 94 | |
00b27da3 | 95 | * Print out the events that are registered:: |
f3f096cf | 96 | |
decc6bfb | 97 | cat /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 98 | |
00b27da3 | 99 | * Clear all events:: |
f3f096cf | 100 | |
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101 | echo > /sys/kernel/debug/tracing/uprobe_events |
102 | ||
103 | Following example shows how to dump the instruction pointer and %ax register | |
00b27da3 | 104 | at the probed text address. Probe zfree function in /bin/zsh:: |
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105 | |
106 | # cd /sys/kernel/debug/tracing/ | |
decc6bfb | 107 | # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp |
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108 | 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh |
109 | # objdump -T /bin/zsh | grep -w zfree | |
110 | 0000000000446420 g DF .text 0000000000000012 Base zfree | |
111 | ||
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112 | 0x46420 is the offset of zfree in object /bin/zsh that is loaded at |
113 | 0x00400000. Hence the command to uprobe would be:: | |
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114 | |
115 | # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events | |
116 | ||
00b27da3 | 117 | And the same for the uretprobe would be:: |
f3f096cf | 118 | |
decc6bfb | 119 | # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events |
f3f096cf | 120 | |
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121 | .. note:: User has to explicitly calculate the offset of the probe-point |
122 | in the object. | |
123 | ||
124 | We can see the events that are registered by looking at the uprobe_events file. | |
125 | :: | |
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126 | |
127 | # cat uprobe_events | |
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128 | p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax |
129 | r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax | |
ec83db0f | 130 | |
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131 | Format of events can be seen by viewing the file events/uprobes/zfree_entry/format. |
132 | :: | |
ec83db0f | 133 | |
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134 | # cat events/uprobes/zfree_entry/format |
135 | name: zfree_entry | |
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136 | ID: 922 |
137 | format: | |
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138 | field:unsigned short common_type; offset:0; size:2; signed:0; |
139 | field:unsigned char common_flags; offset:2; size:1; signed:0; | |
140 | field:unsigned char common_preempt_count; offset:3; size:1; signed:0; | |
141 | field:int common_pid; offset:4; size:4; signed:1; | |
142 | field:int common_padding; offset:8; size:4; signed:1; | |
ec83db0f | 143 | |
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144 | field:unsigned long __probe_ip; offset:12; size:4; signed:0; |
145 | field:u32 arg1; offset:16; size:4; signed:0; | |
146 | field:u32 arg2; offset:20; size:4; signed:0; | |
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147 | |
148 | print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 | |
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149 | |
150 | Right after definition, each event is disabled by default. For tracing these | |
00b27da3 | 151 | events, you need to enable it by:: |
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152 | |
153 | # echo 1 > events/uprobes/enable | |
154 | ||
155 | Lets disable the event after sleeping for some time. | |
00b27da3 | 156 | :: |
decc6bfb | 157 | |
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158 | # sleep 20 |
159 | # echo 0 > events/uprobes/enable | |
160 | ||
161 | And you can see the traced information via /sys/kernel/debug/tracing/trace. | |
00b27da3 | 162 | :: |
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163 | |
164 | # cat trace | |
165 | # tracer: nop | |
166 | # | |
167 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
168 | # | | | | | | |
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169 | zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79 |
170 | zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 | |
171 | zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79 | |
172 | zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0 | |
173 | ||
174 | Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 | |
175 | and contents of ax register being 79. And uretprobe was triggered with ip at | |
176 | 0x446540 with counterpart function entry at 0x446420. |