            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.(*)
   +|-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), "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. Traced arguments are shown in decimal (signed) or hex (unsigned).
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>


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.