uprobetracer.txt

		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:SYMBOL[+offs] [FETCHARGS]	: Set a probe

 GRP		: Group name. If omitted, use "uprobes" for it.
 EVENT		: Event name. If omitted, the event name is generated
		  based on SYMBOL+offs.
 PATH		: path to an executable or a library.
 SYMBOL[+offs]	: Symbol+offset where the probe is inserted.

 FETCHARGS	: Arguments. Each probe can have up to 128 args.
  %REG		: Fetch register REG

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
--------------
To add a probe as a new event, write a new definition to uprobe_events
as below.

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

 This sets a uprobe at an offset of 0x4245c0 in the executable /bin/bash

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

 This clears all probe points.

The following example shows how to dump the instruction pointer and %ax
a register at the probed text address.  Here we are trying to probe
function zfree 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 probe would be :

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

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

    # cat uprobe_events
    p:uprobes/p_zsh_0x46420 /bin/zsh:0x00046420 arg1=%ip arg2=%ax

The format of events can be seen by viewing the file events/uprobes/p_zsh_0x46420/format

    # cat events/uprobes/p_zsh_0x46420/format
    name: p_zsh_0x46420
    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: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
                 zsh-24842 [007] 258545.000270: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
                 zsh-24842 [002] 258545.043929: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79
                 zsh-24842 [004] 258547.046129: p_zsh_0x46420: (0x446420) arg1=446421 arg2=79

Each line shows us probes were triggered for a pid 24842 with ip being
0x446421 and contents of ax register being 79.