Commit b2a86697 authored by Masami Hiramatsu (Google)'s avatar Masami Hiramatsu (Google)
Browse files

Documentation: tracing: Add a note about argument and retval access 

Add a note about the argument and return value accecss will be best
effort. Depending on the type, it will be passed via stack or a
pair of the registers, but $argN and $retval only support the
single register access.

Link: https://lore.kernel.org/all/169556269377.146934.14829235476649685954.stgit@devnote2/



Suggested-by: default avatarAlexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: default avatarMasami Hiramatsu (Google) <mhiramat@kernel.org>
parent 89cdf9d5

Documentation/trace/fprobetrace.rst

+6 −2
Original line number Diff line number Diff line
@@ -59,8 +59,12 @@ Synopsis of fprobe-events 
                  and bitfield are supported.



  (\*1) This is available only when BTF is enabled.

  (\*2) only for the probe on function entry (offs == 0).

  (\*3) only for return probe.

  (\*2) only for the probe on function entry (offs == 0). Note, this argument access

        is best effort, because depending on the argument type, it may be passed on

        the stack. But this only support the arguments via registers.

  (\*3) only for return probe. Note that this is also best effort. Depending on the

        return value type, it might be passed via a pair of registers. But this only

        accesses one register.

  (\*4) this is useful for fetching a field of data structures.

  (\*5) "u" means user-space dereference.

Documentation/trace/kprobetrace.rst

+6 −2
Original line number Diff line number Diff line
@@ -61,8 +61,12 @@ Synopsis of kprobe_events 
		  (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"

                  and bitfield are supported.



  (\*1) only for the probe on function entry (offs == 0).

  (\*2) only for return probe.

  (\*1) only for the probe on function entry (offs == 0). Note, this argument access

        is best effort, because depending on the argument type, it may be passed on

        the stack. But this only support the arguments via registers.

  (\*2) only for return probe. Note that this is also best effort. Depending on the

        return value type, it might be passed via a pair of registers. But this only

        accesses one register.

  (\*3) this is useful for fetching a field of data structures.

  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.