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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 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: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
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 PATH+OFFSET.
29	  PATH          : Path to an executable or a library.
30	  OFFSET        : Offset where the probe is inserted.
31	
32	  FETCHARGS     : Arguments. Each probe can have up to 128 args.
33	   %REG         : Fetch register REG
34	   @ADDR	: Fetch memory at ADDR (ADDR should be in userspace)
35	   @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)
36	   $stackN	: Fetch Nth entry of stack (N >= 0)
37	   $stack	: Fetch stack address.
38	   $retval	: Fetch return value.(*)
39	   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
40	   NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
41	   FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
42			       (u8/u16/u32/u64/s8/s16/s32/s64), "string" and bitfield
43			       are supported.
44	
45	  (*) only for return probe.
46	  (**) this is useful for fetching a field of data structures.
47	
48	Types
49	-----
50	Several types are supported for fetch-args. Uprobe tracer will access memory
51	by given type. Prefix 's' and 'u' means those types are signed and unsigned
52	respectively. Traced arguments are shown in decimal (signed) or hex (unsigned).
53	String type is a special type, which fetches a "null-terminated" string from
54	user space.
55	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
56	offset, and container-size (usually 32). The syntax is;
57	
58	 b<bit-width>@<bit-offset>/<container-size>
59	
60	
61	Event Profiling
62	---------------
63	You can check the total number of probe hits and probe miss-hits via
64	/sys/kernel/debug/tracing/uprobe_profile.
65	The first column is event name, the second is the number of probe hits,
66	the third is the number of probe miss-hits.
67	
68	Usage examples
69	--------------
70	 * Add a probe as a new uprobe event, write a new definition to uprobe_events
71	as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)
72	
73	    echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
74	
75	 * Add a probe as a new uretprobe event:
76	
77	    echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events
78	
79	 * Unset registered event:
80	
81	    echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events
82	
83	 * Print out the events that are registered:
84	
85	    cat /sys/kernel/debug/tracing/uprobe_events
86	
87	 * Clear all events:
88	
89	    echo > /sys/kernel/debug/tracing/uprobe_events
90	
91	Following example shows how to dump the instruction pointer and %ax register
92	at the probed text address. Probe zfree function in /bin/zsh:
93	
94	    # cd /sys/kernel/debug/tracing/
95	    # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp
96	    00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
97	    # objdump -T /bin/zsh | grep -w zfree
98	    0000000000446420 g    DF .text  0000000000000012  Base        zfree
99	
100	  0x46420 is the offset of zfree in object /bin/zsh that is loaded at
101	  0x00400000. Hence the command to uprobe would be:
102	
103	    # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events
104	
105	  And the same for the uretprobe would be:
106	
107	    # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events
108	
109	Please note: User has to explicitly calculate the offset of the probe-point
110	in the object. We can see the events that are registered by looking at the
111	uprobe_events file.
112	
113	    # cat uprobe_events
114	    p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
115	    r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax
116	
117	Format of events can be seen by viewing the file events/uprobes/zfree_entry/format
118	
119	    # cat events/uprobes/zfree_entry/format
120	    name: zfree_entry
121	    ID: 922
122	    format:
123	         field:unsigned short common_type;         offset:0;  size:2; signed:0;
124	         field:unsigned char common_flags;         offset:2;  size:1; signed:0;
125	         field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;
126	         field:int common_pid;                     offset:4;  size:4; signed:1;
127	         field:int common_padding;                 offset:8;  size:4; signed:1;
128	
129	         field:unsigned long __probe_ip;           offset:12; size:4; signed:0;
130	         field:u32 arg1;                           offset:16; size:4; signed:0;
131	         field:u32 arg2;                           offset:20; size:4; signed:0;
132	
133	    print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2
134	
135	Right after definition, each event is disabled by default. For tracing these
136	events, you need to enable it by:
137	
138	    # echo 1 > events/uprobes/enable
139	
140	Lets disable the event after sleeping for some time.
141	
142	    # sleep 20
143	    # echo 0 > events/uprobes/enable
144	
145	And you can see the traced information via /sys/kernel/debug/tracing/trace.
146	
147	    # cat trace
148	    # tracer: nop
149	    #
150	    #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
151	    #              | |       |          |         |
152	                 zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
153	                 zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
154	                 zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
155	                 zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0
156	
157	Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
158	and contents of ax register being 79. And uretprobe was triggered with ip at
159	0x446540 with counterpart function entry at 0x446420.
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