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[:EVENT] SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS]	: Set a probe
  r[:EVENT] SYMBOL[+0] [FETCHARGS]			: Set a return probe

 EVENT			: Event name.
 SYMBOL[+offs|-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
  sN	: Fetch Nth entry of stack (N >= 0)
  sa	: Fetch stack address.
  @ADDR	: Fetch memory at ADDR (ADDR should be in kernel)
  @SYM[+|-offs]	: Fetch memory at SYM +|- offs (SYM should be a data symbol)
  aN	: Fetch function argument. (N >= 0)(*)
  rv	: Fetch return value.(**)
  ra	: Fetch return address.(**)
  +|-offs(FETCHARG) : fetch memory at FETCHARG +|- offs address.(***)

  (*) 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:
  It shows the format of this probe event. It also shows aliases of arguments
 which you specified to kprobe_events.

filter:
  You can write filtering rules of this event. And you can use both of aliase
 names and field names for describing filters.


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 a0 a1 a2 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.

  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: 23
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 arg0;	offset:32;tsize:8;
	field: unsigned long arg1;	offset:40;tsize:8;
	field: unsigned long arg2;	offset:48;tsize:8;
	field: unsigned long arg3;	offset:56;tsize:8;

	alias: a0;	original: arg0;
	alias: a1;	original: arg1;
	alias: a2;	original: arg2;
	alias: a3;	original: arg3;

print fmt: "%lx: 0x%lx 0x%lx 0x%lx 0x%lx", ip, arg0, arg1, arg2, arg3


 You can see that the event has 4 arguments and alias expressions
corresponding to it.

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

 This clears all probe points. 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: do_sys_open+0x0/0xd6: 0x3 0x7fffd1ec4440 0x8000 0x0
           <...>-1447  [001] 1038282.286878: sys_openat+0xc/0xe <- do_sys_open: 0xfffffffffffffffe 0xffffffff81367a3a
           <...>-1447  [001] 1038282.286885: do_sys_open+0x0/0xd6: 0xffffff9c 0x40413c 0x8000 0x1b6
           <...>-1447  [001] 1038282.286915: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a
           <...>-1447  [001] 1038282.286969: do_sys_open+0x0/0xd6: 0xffffff9c 0x4041c6 0x98800 0x10
           <...>-1447  [001] 1038282.286976: sys_open+0x1b/0x1d <- do_sys_open: 0x3 0xffffffff81367a3a


 Each line shows when the kernel hits a probe, 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).