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

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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: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
This commit is contained in:
Masami Hiramatsu (Google) 2023-11-10 19:59:03 +09:00
parent 89cdf9d556
commit b2a866975f
2 changed files with 12 additions and 4 deletions
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8
Documentation/trace/fprobetrace.rst
View File

@ -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.

8
Documentation/trace/kprobetrace.rst
View File

@ -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`.