[PATCH v5 12/12] Documentation: tracing: Add a note about argument and retval access

Masami Hiramatsu (Google)
2023-09-24T13:38:59Z

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.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Documentation/trace/fprobetrace.rst | 8 ++++++--
Documentation/trace/kprobetrace.rst | 8 ++++++--
2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
index 8e9bebcf0a2e..e35e6b18df40 100644
--- a/Documentation/trace/fprobetrace.rst
+++ b/Documentation/trace/fprobetrace.rst
@@ -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.

diff --git a/Documentation/trace/kprobetrace.rst b/Documentation/trace/kprobetrace.rst
index 8a2dfee38145..bf9cecb69fc9 100644
--- a/Documentation/trace/kprobetrace.rst
+++ b/Documentation/trace/kprobetrace.rst
@@ -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`.