Documentation/trace/kprobetrace.txt

                         Kprobe-based Event Tracer
                         =========================

                 Documentation is written by Masami Hiramatsu


Overview
--------
This tracer is similar to the events tracer which is based on Tracepoint
infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe
and kretprobe). It probes anywhere where kprobes can probe(this means, all
functions body except for __kprobes functions).

Unlike the function tracer, this tracer can probe instructions inside of
kernel functions. It allows you to check which instruction has been executed.

Unlike the Tracepoint based events tracer, this tracer can add and remove
probe points on the fly.

Similar to the events tracer, this tracer doesn't need to be activated via
current_tracer, instead of that, just set probe points via
/sys/kernel/debug/tracing/kprobe_events. And you can set filters on each
probe events via /sys/kernel/debug/tracing/events/kprobes/<EVENT>/filter.


Synopsis of kprobe_events
-------------------------
  p[:[GRP/]EVENT] SYMBOL[+offs]|MEMADDR [FETCHARGS]	: Set a probe
  r[:[GRP/]EVENT] SYMBOL[+0] [FETCHARGS]		: Set a return probe

 GRP		: Group name. If omitted, use "kprobes" for it.
 EVENT		: Event name. If omitted, the event name is generated
		  based on SYMBOL+offs or MEMADDR.
 SYMBOL[+offs]	: Symbol+offset where the probe is inserted.
 MEMADDR	: Address 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 kernel)
  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
  $sN	: Fetch Nth entry of stack (N >= 0)
  $sa	: Fetch stack address.
  $aN	: Fetch function argument. (N >= 0)(*)
  $rv	: Fetch return value.(**)
  $ra	: Fetch return address.(**)
  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(***)
  NAME=FETCHARG: Set NAME as the argument name of FETCHARG.

  (*) aN may not correct on asmlinkaged functions and at the middle of
      function body.
  (**) only for return probe.
  (***) this is useful for fetching a field of data structures.


Per-Probe Event Filtering
-------------------------
 Per-probe event filtering feature allows you to set different filter on each
probe and gives you what arguments will be shown in trace buffer. If an event
name is specified right after 'p:' or 'r:' in kprobe_events, the tracer adds
an event under tracing/events/kprobes/<EVENT>, at the directory you can see
'id', 'enabled', 'format' and 'filter'.

enabled:
  You can enable/disable the probe by writing 1 or 0 on it.

format:
  This shows the format of this probe event.

filter:
  You can write filtering rules of this event.

id:
  This shows the id of this probe event.

Event Profiling
---------------
 You can check the total number of probe hits and probe miss-hits via
/sys/kernel/debug/tracing/kprobe_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 kprobe_events
as below.

  echo p:myprobe do_sys_open dfd=$a0 filename=$a1 flags=$a2 mode=$a3 > /sys/kernel/debug/tracing/kprobe_events

 This sets a kprobe on the top of do_sys_open() function with recording
1st to 4th arguments as "myprobe" event. As this example shows, users can
choose more familiar names for each arguments.

  echo r:myretprobe do_sys_open $rv $ra >> /sys/kernel/debug/tracing/kprobe_events

 This sets a kretprobe on the return point of do_sys_open() function with
recording return value and return address as "myretprobe" event.
 You can see the format of these events via
/sys/kernel/debug/tracing/events/kprobes/<EVENT>/format.

  cat /sys/kernel/debug/tracing/events/kprobes/myprobe/format
name: myprobe
ID: 75
format:
	field:unsigned short common_type;	offset:0;	size:2;
	field:unsigned char common_flags;	offset:2;	size:1;
	field:unsigned char common_preempt_count;	offset:3;	size:1;
	field:int common_pid;	offset:4;	size:4;
	field:int common_tgid;	offset:8;	size:4;

	field: unsigned long ip;	offset:16;tsize:8;
	field: int nargs;	offset:24;tsize:4;
	field: unsigned long dfd;	offset:32;tsize:8;
	field: unsigned long filename;	offset:40;tsize:8;
	field: unsigned long flags;	offset:48;tsize:8;
	field: unsigned long mode;	offset:56;tsize:8;

print fmt: "(%lx) dfd=%lx filename=%lx flags=%lx mode=%lx", REC->ip, REC->dfd, REC->filename, REC->flags, REC->mode


 You can see that the event has 4 arguments as in the expressions you specified.

  echo > /sys/kernel/debug/tracing/kprobe_events

 This clears all probe points.

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

  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
  echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable

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

  cat /sys/kernel/debug/tracing/trace
# tracer: nop
#
#           TASK-PID    CPU#    TIMESTAMP  FUNCTION
#              | |       |          |         |
           <...>-1447  [001] 1038282.286875: myprobe: (do_sys_open+0x0/0xd6) dfd=3 filename=7fffd1ec4440 flags=8000 mode=0
           <...>-1447  [001] 1038282.286878: myretprobe: (sys_openat+0xc/0xe <- do_sys_open) $rv=fffffffffffffffe $ra=ffffffff81367a3a
           <...>-1447  [001] 1038282.286885: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=40413c flags=8000 mode=1b6
           <...>-1447  [001] 1038282.286915: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $rv=3 $ra=ffffffff81367a3a
           <...>-1447  [001] 1038282.286969: myprobe: (do_sys_open+0x0/0xd6) dfd=ffffff9c filename=4041c6 flags=98800 mode=10
           <...>-1447  [001] 1038282.286976: myretprobe: (sys_open+0x1b/0x1d <- do_sys_open) $rv=3 $ra=ffffffff81367a3a


 Each line shows when the kernel hits an event, and <- SYMBOL means kernel
returns from SYMBOL(e.g. "sys_open+0x1b/0x1d <- do_sys_open" means kernel
returns from do_sys_open to sys_open+0x1b).