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2 Uprobe-tracer: Uprobe-based Event Tracing
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5 :Author: Srikar Dronamraju
10 Uprobe based trace events are similar to kprobe based trace events.
11 To enable this feature, build your kernel with CONFIG_UPROBE_EVENTS=y.
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
16 /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enable.
18 However unlike kprobe-event tracer, the uprobe event interface expects the
19 user to calculate the offset of the probepoint in the object.
21 You can also use /sys/kernel/debug/tracing/dynamic_events instead of
22 uprobe_events. That interface will provide unified access to other
25 Synopsis of uprobe_tracer
26 -------------------------
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
33 GRP : Group name. If omitted, "uprobes" is the default value.
34 EVENT : Event name. If omitted, the event name is generated based
36 PATH : Path to an executable or a library.
37 OFFSET : Offset where the probe is inserted.
39 FETCHARGS : Arguments. Each probe can have up to 128 args.
40 %REG : Fetch register REG
41 @ADDR : Fetch memory at ADDR (ADDR should be in userspace)
42 @+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH)
43 $stackN : Fetch Nth entry of stack (N >= 0)
44 $stack : Fetch stack address.
45 $retval : Fetch return value.(\*1)
46 $comm : Fetch current task comm.
47 +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)
48 NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
49 FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
50 (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
51 (x8/x16/x32/x64), "string" and bitfield are supported.
53 (\*1) only for return probe.
54 (\*2) this is useful for fetching a field of data structures.
55 (\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe
56 events can access only user-space memory.
60 Several types are supported for fetch-args. Uprobe tracer will access memory
61 by given type. Prefix 's' and 'u' means those types are signed and unsigned
62 respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
63 in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
64 or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
66 String type is a special type, which fetches a "null-terminated" string from
68 Bitfield is another special type, which takes 3 parameters, bit-width, bit-
69 offset, and container-size (usually 32). The syntax is::
71 b<bit-width>@<bit-offset>/<container-size>
73 For $comm, the default type is "string"; any other type is invalid.
78 You can check the total number of probe hits per event via
79 /sys/kernel/debug/tracing/uprobe_profile. The first column is the filename,
80 the second is the event name, the third is the number of probe hits.
84 * Add a probe as a new uprobe event, write a new definition to uprobe_events
85 as below (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)::
87 echo 'p /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
89 * Add a probe as a new uretprobe event::
91 echo 'r /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
93 * Unset registered event::
95 echo '-:p_bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
97 * Print out the events that are registered::
99 cat /sys/kernel/debug/tracing/uprobe_events
103 echo > /sys/kernel/debug/tracing/uprobe_events
105 Following example shows how to dump the instruction pointer and %ax register
106 at the probed text address. Probe zfree function in /bin/zsh::
108 # cd /sys/kernel/debug/tracing/
109 # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
110 00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
111 # objdump -T /bin/zsh | grep -w zfree
112 0000000000446420 g DF .text 0000000000000012 Base zfree
114 0x46420 is the offset of zfree in object /bin/zsh that is loaded at
115 0x00400000. Hence the command to uprobe would be::
117 # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
119 And the same for the uretprobe would be::
121 # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
123 .. note:: User has to explicitly calculate the offset of the probe-point
126 We can see the events that are registered by looking at the uprobe_events file.
130 p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
131 r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
133 Format of events can be seen by viewing the file events/uprobes/zfree_entry/format.
136 # cat events/uprobes/zfree_entry/format
140 field:unsigned short common_type; offset:0; size:2; signed:0;
141 field:unsigned char common_flags; offset:2; size:1; signed:0;
142 field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
143 field:int common_pid; offset:4; size:4; signed:1;
144 field:int common_padding; offset:8; size:4; signed:1;
146 field:unsigned long __probe_ip; offset:12; size:4; signed:0;
147 field:u32 arg1; offset:16; size:4; signed:0;
148 field:u32 arg2; offset:20; size:4; signed:0;
150 print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
152 Right after definition, each event is disabled by default. For tracing these
153 events, you need to enable it by::
155 # echo 1 > events/uprobes/enable
157 Lets start tracing, sleep for some time and stop tracing.
160 # echo 1 > tracing_on
162 # echo 0 > tracing_on
164 Also, you can disable the event by::
166 # echo 0 > events/uprobes/enable
168 And you can see the traced information via /sys/kernel/debug/tracing/trace.
174 # TASK-PID CPU# TIMESTAMP FUNCTION
176 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
177 zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
178 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
179 zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
181 Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
182 and contents of ax register being 79. And uretprobe was triggered with ip at
183 0x446540 with counterpart function entry at 0x446420.