View source for Kprobes

[[File:Osseu-commonality.png|thumb|right|Linux tracing systems]]

Kprobes use the breakpoint mechanism to dynamically instrument Linux kernel code. Two types exist: <i>kprobes</i> can be attached to all but a few blacklisted instruction ranges in a running kernel, while <i>kretprobes</i> are attached to a function and run when it returns. This instrumentation can be packaged as a kernel module (using the <tt>register_probe</tt> and <tt>unregister_probe</tt> kernel API, as done by SystemTap), manipulated via debugfs (as done by ftrace), configured using the [[perf]] tool, or implemented as a <tt>BPF_PROG_TYPE_KPROBE</tt>-type [[eBPF]] program.

uprobes are the userspace equivalent of kprobes. jprobes are no longer a thing. i don't believe dprobes to be a thing anymore, either, but might be mistaken. tracepoints are places to hook the same kind of analysis, explicitly specified by kernel authors using <tt>TRACE_EVENT</tt>; think of them as "opt-in", as opposed to dynamic kprobes, though there is a tracepoint for each system call.

==Kernel configuration==
<pre>CONFIG_KPROBES=y
CONFIG_KPROBES_ON_FTRACE=y
CONFIG_HAVE_KPROBES=y
CONFIG_HAVE_KPROBES_ON_FTRACE=y
CONFIG_KPROBE_EVENTS=y</pre>

==Working with kprobes==
To add, trace, and destroy a kprobe, use the <tt>kprobe</tt> binary (sometimes known as <tt>kprobe-perf</tt>) from the [[perf]] toolkit.

The primary means for working with longterm kprobes from userspace is debugfs (typically mounted at <tt>/sys/kernel/debug</tt>) and the [[perf]] tool. Note that <tt>/sys/kernel/debug/tracing/events/kprobes</tt> will not appear until you have enabled at least one kprobe.
{|class="wikitable"
! Task !! sysfs !! perf
|-
| List functions suitable for probing
|| read <tt>debug/tracing/available_filter_functions</tt>
|| <tt>perf probe -F</tt> (note: in my experience, this always lacks a few available from the sysfs list. i'm unsure why.)
|-
| List registered kprobes
|| read <tt>debug/kprobes/list</tt>
|| ?
|-
| List probe events || read <tt>debug/tracing/kprobe_events</tt> || <tt>perf probe -l</tt>
|-
| Add kprobe || write def to <tt>debug/tracing/kprobe_events</tt>
|| <tt>perf probe -a</tt> def
|-
| Remove kprobe
|| write <tt>-:NAME</tt> to <tt>debug/tracing/kprobe_events</tt>
|| <tt>perf probe -d</tt>
|-
| Enable kprobe
|| write <tt>debug/tracing/events/kprobes/NAME/enable</tt>
|| ?
|-
| Trace kprobe
|| read <tt>debug/tracing/trace_pipe</tt>
|| <tt>perf trace -e kprobes:NAME</tt>
|-
|}

===Kprobe definition===
Taken from the 5.3.4 kernel source at [https://www.kernel.org/doc/Documentation/trace/kprobetrace.txt Documentation/trace/kprobetrace.txt]:
<pre>  p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]  : Set a probe
  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]  : Set a return probe
  -:[GRP/]EVENT                     : Clear a probe

 GRP        : Group name. If omitted, use "kprobes" for it.
 EVENT      : Event name. If omitted, the event name is generated
          based on SYM+offs or MEMADDR.
 MOD        : Module name which has given SYM.
 SYM[+offs] : Symbol+offset where the probe is inserted.
 MEMADDR    : Address where the probe is inserted.
 MAXACTIVE  : Maximum number of instances of the specified function that
          can be probed simultaneously, or 0 for the default value
          as defined in Documentation/kprobes.txt section 1.3.1.

 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)
  $stackN   : Fetch Nth entry of stack (N >= 0)
  $stack    : Fetch stack address.
  $argN     : Fetch the Nth function argument. (N >= 1) (\*1)
  $retval   : Fetch return value.(\*2)
  $comm     : Fetch current task comm.
  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)
  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", "ustring" and bitfield
          are supported.

  (\*1) only for the probe on function entry (offs == 0).
  (\*2) only for return probe.
  (\*3) this is useful for fetching a field of data structures.
  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
</pre>

==Further reading==
* LWN's [https://lwn.net/Articles/132196/ Introduction to Kprobes], 2005-04-18

==See also==
* [[perf]]
* [[eBPF]]