About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / trace / uprobetracer.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	            Uprobe-tracer: Uprobe-based Event Tracing
2	            =========================================
3	
4	           Documentation written by Srikar Dronamraju
5	
6	
7	Overview
8	--------
9	Uprobe based trace events are similar to kprobe based trace events.
10	To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.
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
18	user to calculate the offset of the probepoint in the object.
19	
20	Synopsis of uprobe_tracer
21	-------------------------
22	  p[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a uprobe
23	  r[:[GRP/]EVENT] PATH:SYMBOL[+offs] [FETCHARGS] : Set a return uprobe (uretprobe)
24	  -:[GRP/]EVENT                                  : Clear uprobe or uretprobe event
25	
26	  GRP           : Group name. If omitted, "uprobes" is the default value.
27	  EVENT         : Event name. If omitted, the event name is generated based
28	                  on SYMBOL+offs.
29	  PATH          : Path to an executable or a library.
30	  SYMBOL[+offs] : Symbol+offset where the probe is inserted.
31	
32	  FETCHARGS     : Arguments. Each probe can have up to 128 args.
33	   %REG         : Fetch register REG
34	
35	Event Profiling
36	---------------
37	You can check the total number of probe hits and probe miss-hits via
38	/sys/kernel/debug/tracing/uprobe_profile.
39	The first column is event name, the second is the number of probe hits,
40	the third is the number of probe miss-hits.
41	
42	Usage examples
43	--------------
44	 * Add a probe as a new uprobe event, write a new definition to uprobe_events
45	as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
46	
47	    echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
48	
49	 * Add a probe as a new uretprobe event:
50	
51	    echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
52	
53	 * Unset registered event:
54	
55	    echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
56	
57	 * Print out the events that are registered:
58	
59	    cat /sys/kernel/debug/tracing/uprobe_events
60	
61	 * Clear all events:
62	
63	    echo > /sys/kernel/debug/tracing/uprobe_events
64	
65	Following example shows how to dump the instruction pointer and %ax register
66	at the probed text address. Probe zfree function in /bin/zsh:
67	
68	    # cd /sys/kernel/debug/tracing/
69	    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
70	    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
71	    # objdump -T /bin/zsh | grep -w zfree
72	    0000000000446420 g    DF .text  0000000000000012  Base        zfree
73	
74	  0x46420 is the offset of zfree in object /bin/zsh that is loaded at
75	  0x00400000. Hence the command to uprobe would be:
76	
77	    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
78	
79	  And the same for the uretprobe would be:
80	
81	    # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
82	
83	Please note: User has to explicitly calculate the offset of the probe-point
84	in the object. We can see the events that are registered by looking at the
85	uprobe_events file.
86	
87	    # cat uprobe_events
88	    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
89	    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
90	
91	Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
92	
93	    # cat events/uprobes/zfree_entry/format
94	    name: zfree_entry
95	    ID: 922
96	    format:
97	         field:unsigned short common_type;         offset:0;  size:2; signed:0;
98	         field:unsigned char common_flags;         offset:2;  size:1; signed:0;
99	         field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
100	         field:int common_pid;                     offset:4;  size:4; signed:1;
101	         field:int common_padding;                 offset:8;  size:4; signed:1;
102	
103	         field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
104	         field:u32 arg1;                           offset:16; size:4; signed:0;
105	         field:u32 arg2;                           offset:20; size:4; signed:0;
106	
107	    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
108	
109	Right after definition, each event is disabled by default. For tracing these
110	events, you need to enable it by:
111	
112	    # echo 1 > events/uprobes/enable
113	
114	Lets disable the event after sleeping for some time.
115	
116	    # sleep 20
117	    # echo 0 > events/uprobes/enable
118	
119	And you can see the traced information via /sys/kernel/debug/tracing/trace.
120	
121	    # cat trace
122	    # tracer: nop
123	    #
124	    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
125	    #              | |       |          |         |
126	                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
127	                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
128	                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
129	                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
130	
131	Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
132	and contents of ax register being 79. And uretprobe was triggered with ip at
133	0x446540 with counterpart function entry at 0x446420.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.