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Kprobes: Difference between revisions

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To add, trace, and destroy a kprobe, use the <tt>kprobe</tt> binary (sometimes known as <tt>kprobe-perf</tt>) from the [[perf]] toolkit.
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 [[sysfs]] and the [[perf]] tool.
The primary means for working with longterm kprobes from userspace is [[sysfs]] 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"
{|class="wikitable"
! Task !! sysfs !! perf
! Task !! sysfs !! perf
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|| <tt>perf probe -a</tt>
|| <tt>perf probe -a</tt>
|-
|-
 
| Remove kprobe
|| ?
|| <tt>perf probe -d</tt>
|-
|}
|}



Revision as of 05:26, 6 October 2019

Kprobes use the breakpoint mechanism to dynamically instrument Linux kernel code. Two types exist: kprobes can be attached to all but a few blacklisted instruction ranges in a running kernel, while kretprobes are attached to a function and run when it returns. This instrumentation is typically packaged as a kernel module or eBPF.

Kernel configuration

CONFIG_KPROBES=y
CONFIG_KPROBES_ON_FTRACE=y
CONFIG_HAVE_KPROBES=y
CONFIG_HAVE_KPROBES_ON_FTRACE=y
CONFIG_KPROBE_EVENTS=y

Working with kprobes

To add, trace, and destroy a kprobe, use the kprobe binary (sometimes known as kprobe-perf) from the perf toolkit.

The primary means for working with longterm kprobes from userspace is sysfs and the perf tool. Note that /sys/kernel/debug/tracing/events/kprobes will not appear until you have enabled at least one kprobe.

Task sysfs perf
List functions suitable for probing read /sys/kernel/debug/tracing/available_filter_functions perf probe -F (note: in my experience, this always lacks a few available from the sysfs list. i'm unsure why.)
List enabled kprobes read /sys/kernel/debug/tracing/kprobe_events perf probe -l
Enable kprobe write /sys/kernel/debug/tracing/kprobe_events perf probe -a
Remove kprobe ? perf probe -d

Kprobe definition

Taken from the 5.3.4 kernel source at Documentation/trace/kprobetrace.txt:

  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`.

Further reading