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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.(*) | |
35abb67d | 39 | $comm : Fetch current task comm. |
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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 | ||
49 | Types | |
50 | ----- | |
51 | Several types are supported for fetch-args. Uprobe tracer will access memory | |
52 | by given type. Prefix 's' and 'u' means those types are signed and unsigned | |
53 | respectively. Traced arguments are shown in decimal (signed) or hex (unsigned). | |
54 | String type is a special type, which fetches a "null-terminated" string from | |
55 | user space. | |
56 | Bitfield is another special type, which takes 3 parameters, bit-width, bit- | |
57 | offset, and container-size (usually 32). The syntax is; | |
58 | ||
59 | b<bit-width>@<bit-offset>/<container-size> | |
60 | ||
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61 | For $comm, the default type is "string"; any other type is invalid. |
62 | ||
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63 | |
64 | Event Profiling | |
65 | --------------- | |
decc6bfb | 66 | You can check the total number of probe hits and probe miss-hits via |
f3f096cf | 67 | /sys/kernel/debug/tracing/uprobe_profile. |
decc6bfb | 68 | The first column is event name, the second is the number of probe hits, |
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69 | the third is the number of probe miss-hits. |
70 | ||
71 | Usage examples | |
72 | -------------- | |
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73 | * Add a probe as a new uprobe event, write a new definition to uprobe_events |
74 | as 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: | |
f3f096cf | 83 | |
decc6bfb | 84 | echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 85 | |
decc6bfb | 86 | * Print out the events that are registered: |
f3f096cf | 87 | |
decc6bfb | 88 | cat /sys/kernel/debug/tracing/uprobe_events |
f3f096cf | 89 | |
decc6bfb | 90 | * Clear all events: |
f3f096cf | 91 | |
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92 | echo > /sys/kernel/debug/tracing/uprobe_events |
93 | ||
94 | Following example shows how to dump the instruction pointer and %ax register | |
95 | at the probed text address. Probe zfree function in /bin/zsh: | |
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96 | |
97 | # cd /sys/kernel/debug/tracing/ | |
decc6bfb | 98 | # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp |
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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 | ||
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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: | |
f3f096cf | 109 | |
decc6bfb | 110 | # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events |
f3f096cf | 111 | |
decc6bfb | 112 | Please note: User has to explicitly calculate the offset of the probe-point |
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113 | in the object. We can see the events that are registered by looking at the |
114 | uprobe_events file. | |
115 | ||
116 | # cat uprobe_events | |
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117 | p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax |
118 | r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax | |
ec83db0f | 119 | |
decc6bfb | 120 | Format of events can be seen by viewing the file events/uprobes/zfree_entry/format |
ec83db0f | 121 | |
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122 | # cat events/uprobes/zfree_entry/format |
123 | name: zfree_entry | |
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124 | ID: 922 |
125 | format: | |
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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; | |
ec83db0f | 131 | |
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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; | |
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135 | |
136 | print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 | |
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137 | |
138 | Right after definition, each event is disabled by default. For tracing these | |
139 | events, you need to enable it by: | |
140 | ||
141 | # echo 1 > events/uprobes/enable | |
142 | ||
143 | Lets disable the event after sleeping for some time. | |
decc6bfb | 144 | |
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145 | # sleep 20 |
146 | # echo 0 > events/uprobes/enable | |
147 | ||
148 | And 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 | # | | | | | | |
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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 | ||
160 | Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420 | |
161 | and contents of ax register being 79. And uretprobe was triggered with ip at | |
162 | 0x446540 with counterpart function entry at 0x446420. |