Documentation/trace/uprobetracer.txt - arm/linux - Git at Google


            
                        Uprobe-tracer: Uprobe-based Event Tracing

            
                        =========================================


                       Documentation written by Srikar Dronamraju


            Overview

            
            --------

            
            Uprobe based trace events are similar to kprobe based trace events.

            
            To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.


            Similar to the kprobe-event tracer, this doesn't need to be activated via

            
            current_tracer. Instead of that, add probe points via

            
            /sys/kernel/debug/tracing/uprobe_events, and enable it via

            
            /sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.


            However unlike kprobe-event tracer, the uprobe event interface expects the

            
            user to calculate the offset of the probepoint in the object.


            Synopsis of uprobe_tracer

            
            -------------------------

            
              p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe

            
              r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)

            
              -:[GRP/]EVENT                           : Clear uprobe or uretprobe event


              GRP           : Group name. If omitted, "uprobes" is the default value.

            
              EVENT         : Event name. If omitted, the event name is generated based

            
                              on PATH+OFFSET.

            
              PATH          : Path to an executable or a library.

            
              OFFSET        : Offset where the probe is inserted.


              FETCHARGS     : Arguments. Each probe can have up to 128 args.

            
               %REG         : Fetch register REG

            
               @ADDR	: Fetch memory at ADDR (ADDR should be in userspace)

            
               @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)

            
               $stackN	: Fetch Nth entry of stack (N >= 0)

            
               $stack	: Fetch stack address.

            
               $retval	: Fetch return value.(*)

            
               $comm	: Fetch current task comm.

            
               +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)

            
               NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.

            
               FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types

            
            		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types

            
            		       (x8/x16/x32/x64), "string" and bitfield are supported.


              (*) only for return probe.

            
              (**) this is useful for fetching a field of data structures.


            Types

            
            -----

            
            Several types are supported for fetch-args. Uprobe tracer will access memory

            
            by given type. Prefix 's' and 'u' means those types are signed and unsigned

            
            respectively. 'x' prefix implies it is unsigned. Traced arguments are shown

            
            in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'

            
            or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and

            
            x86-64 uses x64).

            
            String type is a special type, which fetches a "null-terminated" string from

            
            user space.

            
            Bitfield is another special type, which takes 3 parameters, bit-width, bit-

            
            offset, and container-size (usually 32). The syntax is;


             b<bit-width>@<bit-offset>/<container-size>


            For $comm, the default type is "string"; any other type is invalid.


            Event Profiling

            
            ---------------

            
            You can check the total number of probe hits and probe miss-hits via

            
            /sys/kernel/debug/tracing/uprobe_profile.

            
            The first column is event name, the second is the number of probe hits,

            
            the third is the number of probe miss-hits.


            Usage examples

            
            --------------

            
             * Add a probe as a new uprobe event, write a new definition to uprobe_events

            
            as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)


                echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events


             * Add a probe as a new uretprobe event:


                echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events


             * Unset registered event:


                echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events


             * Print out the events that are registered:


                cat /sys/kernel/debug/tracing/uprobe_events


             * Clear all events:


                echo > /sys/kernel/debug/tracing/uprobe_events


            Following example shows how to dump the instruction pointer and %ax register

            
            at the probed text address. Probe zfree function in /bin/zsh:


                # cd /sys/kernel/debug/tracing/

            
                # cat /proc/`pgrep zsh`/maps | grep /bin/zsh | grep r-xp

            
                00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh

            
                # objdump -T /bin/zsh | grep -w zfree

            
                0000000000446420 g    DF .text  0000000000000012  Base        zfree


              0x46420 is the offset of zfree in object /bin/zsh that is loaded at

            
              0x00400000. Hence the command to uprobe would be:


                # echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events


              And the same for the uretprobe would be:


                # echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events


            Please note: User has to explicitly calculate the offset of the probe-point

            
            in the object. We can see the events that are registered by looking at the

            
            uprobe_events file.


                # cat uprobe_events

            
                p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax

            
                r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax


            Format of events can be seen by viewing the file events/uprobes/zfree_entry/format


                # cat events/uprobes/zfree_entry/format

            
                name: zfree_entry

            
                ID: 922

            
                format:

            
                     field:unsigned short common_type;         offset:0;  size:2; signed:0;

            
                     field:unsigned char common_flags;         offset:2;  size:1; signed:0;

            
                     field:unsigned char common_preempt_count; offset:3;  size:1; signed:0;

            
                     field:int common_pid;                     offset:4;  size:4; signed:1;

            
                     field:int common_padding;                 offset:8;  size:4; signed:1;


                     field:unsigned long __probe_ip;           offset:12; size:4; signed:0;

            
                     field:u32 arg1;                           offset:16; size:4; signed:0;

            
                     field:u32 arg2;                           offset:20; size:4; signed:0;


                print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2


            Right after definition, each event is disabled by default. For tracing these

            
            events, you need to enable it by:


                # echo 1 > events/uprobes/enable


            Lets disable the event after sleeping for some time.


                # sleep 20

            
                # echo 0 > events/uprobes/enable


            And you can see the traced information via /sys/kernel/debug/tracing/trace.


                # cat trace

            
                # tracer: nop

            
                #

            
                #           TASK-PID    CPU#    TIMESTAMP  FUNCTION

            
                #              | |       |          |         |

            
                             zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79

            
                             zsh-24842 [007] 258545.000270: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0

            
                             zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79

            
                             zsh-24842 [004] 258547.046129: zfree_exit:  (0x446540 <- 0x446420) arg1=446540 arg2=0


            Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420

            
            and contents of ax register being 79. And uretprobe was triggered with ip at

            
            0x446540 with counterpart function entry at 0x446420.
	Uprobe-tracer: Uprobe-based Event Tracing
	=========================================

	Documentation written by Srikar Dronamraju


	Overview
	--------
	Uprobe based trace events are similar to kprobe based trace events.
	To enable this feature, build your kernel with CONFIG_UPROBE_EVENT=y.

	Similar to the kprobe-event tracer, this doesn't need to be activated via
	current_tracer. Instead of that, add probe points via
	/sys/kernel/debug/tracing/uprobe_events, and enable it via
	/sys/kernel/debug/tracing/events/uprobes/<EVENT>/enabled.

	However unlike kprobe-event tracer, the uprobe event interface expects the
	user to calculate the offset of the probepoint in the object.

	Synopsis of uprobe_tracer
	-------------------------
	p[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a uprobe
	r[:[GRP/]EVENT] PATH:OFFSET [FETCHARGS] : Set a return uprobe (uretprobe)
	-:[GRP/]EVENT : Clear uprobe or uretprobe event

	GRP : Group name. If omitted, "uprobes" is the default value.
	EVENT : Event name. If omitted, the event name is generated based
	on PATH+OFFSET.
	PATH : Path to an executable or a library.
	OFFSET : Offset where the probe is inserted.

	FETCHARGS : Arguments. Each probe can have up to 128 args.
	%REG : Fetch register REG
	@ADDR : Fetch memory at ADDR (ADDR should be in userspace)
	@+OFFSET : Fetch memory at OFFSET (OFFSET from same file as PATH)
	$stackN : Fetch Nth entry of stack (N >= 0)
	$stack : Fetch stack address.
	$retval : Fetch return value.(*)
	$comm : Fetch current task comm.
	+\|-offs(FETCHARG) : Fetch memory at FETCHARG +\|- offs address.(**)
	NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
	FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
	(u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
	(x8/x16/x32/x64), "string" and bitfield are supported.

	(*) only for return probe.
	(**) this is useful for fetching a field of data structures.

	Types
	-----
	Several types are supported for fetch-args. Uprobe tracer will access memory
	by given type. Prefix 's' and 'u' means those types are signed and unsigned
	respectively. 'x' prefix implies it is unsigned. Traced arguments are shown
	in decimal ('s' and 'u') or hexadecimal ('x'). Without type casting, 'x32'
	or 'x64' is used depends on the architecture (e.g. x86-32 uses x32, and
	x86-64 uses x64).
	String type is a special type, which fetches a "null-terminated" string from
	user space.
	Bitfield is another special type, which takes 3 parameters, bit-width, bit-
	offset, and container-size (usually 32). The syntax is;

	b<bit-width>@<bit-offset>/<container-size>

	For $comm, the default type is "string"; any other type is invalid.


	Event Profiling
	---------------
	You can check the total number of probe hits and probe miss-hits via
	/sys/kernel/debug/tracing/uprobe_profile.
	The first column is event name, the second is the number of probe hits,
	the third is the number of probe miss-hits.

	Usage examples
	--------------
	* Add a probe as a new uprobe event, write a new definition to uprobe_events
	as below: (sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash)

	echo 'p: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events

	* Add a probe as a new uretprobe event:

	echo 'r: /bin/bash:0x4245c0' > /sys/kernel/debug/tracing/uprobe_events

	* Unset registered event:

	echo '-:bash_0x4245c0' >> /sys/kernel/debug/tracing/uprobe_events

	* Print out the events that are registered:

	cat /sys/kernel/debug/tracing/uprobe_events

	* Clear all events:

	echo > /sys/kernel/debug/tracing/uprobe_events

	Following example shows how to dump the instruction pointer and %ax register
	at the probed text address. Probe zfree function in /bin/zsh:

	# cd /sys/kernel/debug/tracing/
	# cat /proc/`pgrep zsh`/maps \| grep /bin/zsh \| grep r-xp
	00400000-0048a000 r-xp 00000000 08:03 130904 /bin/zsh
	# objdump -T /bin/zsh \| grep -w zfree
	0000000000446420 g DF .text 0000000000000012 Base zfree

	0x46420 is the offset of zfree in object /bin/zsh that is loaded at
	0x00400000. Hence the command to uprobe would be:

	# echo 'p:zfree_entry /bin/zsh:0x46420 %ip %ax' > uprobe_events

	And the same for the uretprobe would be:

	# echo 'r:zfree_exit /bin/zsh:0x46420 %ip %ax' >> uprobe_events

	Please note: User has to explicitly calculate the offset of the probe-point
	in the object. We can see the events that are registered by looking at the
	uprobe_events file.

	# cat uprobe_events
	p:uprobes/zfree_entry /bin/zsh:0x00046420 arg1=%ip arg2=%ax
	r:uprobes/zfree_exit /bin/zsh:0x00046420 arg1=%ip arg2=%ax

	Format of events can be seen by viewing the file events/uprobes/zfree_entry/format

	# cat events/uprobes/zfree_entry/format
	name: zfree_entry
	ID: 922
	format:
	field:unsigned short common_type; offset:0; size:2; signed:0;
	field:unsigned char common_flags; offset:2; size:1; signed:0;
	field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
	field:int common_pid; offset:4; size:4; signed:1;
	field:int common_padding; offset:8; size:4; signed:1;

	field:unsigned long __probe_ip; offset:12; size:4; signed:0;
	field:u32 arg1; offset:16; size:4; signed:0;
	field:u32 arg2; offset:20; size:4; signed:0;

	print fmt: "(%lx) arg1=%lx arg2=%lx", REC->__probe_ip, REC->arg1, REC->arg2

	Right after definition, each event is disabled by default. For tracing these
	events, you need to enable it by:

	# echo 1 > events/uprobes/enable

	Lets disable the event after sleeping for some time.

	# sleep 20
	# echo 0 > events/uprobes/enable

	And you can see the traced information via /sys/kernel/debug/tracing/trace.

	# cat trace
	# tracer: nop
	#
	# TASK-PID CPU# TIMESTAMP FUNCTION
	# \| \| \| \| \|
	zsh-24842 [006] 258544.995456: zfree_entry: (0x446420) arg1=446420 arg2=79
	zsh-24842 [007] 258545.000270: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0
	zsh-24842 [002] 258545.043929: zfree_entry: (0x446420) arg1=446420 arg2=79
	zsh-24842 [004] 258547.046129: zfree_exit: (0x446540 <- 0x446420) arg1=446540 arg2=0

	Output shows us uprobe was triggered for a pid 24842 with ip being 0x446420
	and contents of ax register being 79. And uretprobe was triggered with ip at
	0x446540 with counterpart function entry at 0x446420.