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[v4,9/9] Documentation: tracing: Add a note about argument and retval access

Message ID 169280382895.282662.14910495061790007288.stgit@devnote2 (mailing list archive)
State Not Applicable
Delegated to: BPF
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Series bpf: fprobe: rethook: Use ftrace_regs instead of pt_regs | expand

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Commit Message

Masami Hiramatsu (Google) Aug. 23, 2023, 3:17 p.m. UTC 
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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(-)

Comments

Florent Revest Aug. 25, 2023, 4:12 p.m. UTC | #1 
On Wed, Aug 23, 2023 at 5:17 PM Masami Hiramatsu (Google)
<mhiramat@kernel.org> wrote:
> 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.

supports*

Otherwise:
Acked-by: Florent Revest <revest@chromium.org>
diff mbox series

Patch

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`.