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1 | Uprobe-tracer: Uprobe-based Event Tracing |
2 | ========================================= | |
3 | Documentation written by Srikar Dronamraju | |
4 | ||
5 | Overview | |
6 | -------- | |
7 | Uprobe based trace events are similar to kprobe based trace events. | |
ec83db0f | 8 | To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y. |
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9 | |
10 | Similar to the kprobe-event tracer, this doesn't need to be activated via | |
11 | current_tracer. Instead of that, add probe points via | |
12 | /sys/kernel/debug/tracing/uprobe_events, and enable it via | |
13 | /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled. | |
14 | ||
15 | However unlike kprobe-event tracer, the uprobe event interface expects the | |
16 | user to calculate the offset of the probepoint in the object | |
17 | ||
18 | Synopsis of uprobe_tracer | |
19 | ------------------------- | |
20 | p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a probe | |
21 | ||
22 | GRP : Group name. If omitted, use "uprobes" for it. | |
23 | EVENT : Event name. If omitted, the event name is generated | |
24 | based on SYMBOL+offs. | |
25 | PATH : path to an executable or a library. | |
26 | SYMBOL[+offs] : Symbol+offset where the probe is inserted. | |
27 | ||
28 | FETCHARGS : Arguments. Each probe can have up to 128 args. | |
29 | %REG : Fetch register REG | |
30 | ||
31 | Event Profiling | |
32 | --------------- | |
33 | You can check the total number of probe hits and probe miss-hits via | |
34 | /sys/kernel/debug/tracing/uprobe_profile. | |
35 | The first column is event name, the second is the number of probe hits, | |
36 | the third is the number of probe miss-hits. | |
37 | ||
38 | Usage examples | |
39 | -------------- | |
40 | To add a probe as a new event, write a new definition to uprobe_events | |
41 | as below. | |
42 | ||
43 | echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events | |
44 | ||
45 | This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash | |
46 | ||
47 | echo > /sys/kernel/debug/tracing/uprobe_events | |
48 | ||
49 | This clears all probe points. | |
50 | ||
51 | The following example shows how to dump the instruction pointer and %ax | |
52 | a register at the probed text address. Here we are trying to probe | |
53 | function zfree in /bin/zsh | |
54 | ||
55 | # cd /sys/kernel/debug/tracing/ | |
56 | # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp | |
57 | 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh | |
58 | # objdump -T /bin/zsh | grep -w zfree | |
59 | 0000000000446420 g DF .text 0000000000000012 Base zfree | |
60 | ||
61 | 0x46420 is the offset of zfree in object /bin/zsh that is loaded at | |
62 | 0x00400000. Hence the command to probe would be : | |
63 | ||
64 | # echo 'p /bin/zsh:0x46420 %ip %ax' > uprobe_events | |
65 | ||
66 | Please note: User has to explicitly calculate the offset of the probepoint | |
67 | in the object. We can see the events that are registered by looking at the | |
68 | uprobe_events file. | |
69 | ||
70 | # cat uprobe_events | |
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71 | p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax |
72 | ||
73 | The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format | |
74 | ||
75 | # cat events/uprobes/p_zsh_0x46420/format | |
76 | name: p_zsh_0x46420 | |
77 | ID: 922 | |
78 | format: | |
79 | field:unsigned short common_type; offset:0; size:2; signed:0; | |
80 | field:unsigned char common_flags; offset:2; size:1; signed:0; | |
81 | field:unsigned char common_preempt_count; offset:3; size:1; signed:0; | |
82 | field:int common_pid; offset:4; size:4; signed:1; | |
83 | field:int common_padding; offset:8; size:4; signed:1; | |
84 | ||
85 | field:unsigned long __probe_ip; offset:12; size:4; signed:0; | |
86 | field:u32 arg1; offset:16; size:4; signed:0; | |
87 | field:u32 arg2; offset:20; size:4; signed:0; | |
88 | ||
89 | print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2 | |
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90 | |
91 | Right after definition, each event is disabled by default. For tracing these | |
92 | events, you need to enable it by: | |
93 | ||
94 | # echo 1 > events/uprobes/enable | |
95 | ||
96 | Lets disable the event after sleeping for some time. | |
97 | # sleep 20 | |
98 | # echo 0 > events/uprobes/enable | |
99 | ||
100 | And you can see the traced information via /sys/kernel/debug/tracing/trace. | |
101 | ||
102 | # cat trace | |
103 | # tracer: nop | |
104 | # | |
105 | # TASK-PID CPU# TIMESTAMP FUNCTION | |
106 | # | | | | | | |
107 | zsh-24842 [006] 258544.995456: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 | |
108 | zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 | |
109 | zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 | |
110 | zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79 | |
111 | ||
112 | Each line shows us probes were triggered for a pid 24842 with ip being | |
113 | 0x446421 and contents of ax register being 79. |