Re: [PATCH v3 1/8] Documentation: probes: Add a new ret_ip callback parameter

[Florent Revest]

On Sat, Aug 12, 2023 at 7:36 AM Masami Hiramatsu (Google)
<mhiramat@xxxxxxxxxx> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>
> Add a new ret_ip callback parameter description.
>
> Fixes: cb16330d1274 ("fprobe: Pass return address to the handlers")
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  Documentation/trace/fprobe.rst |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst
> index 40dd2fbce861..a6d682478147 100644
> --- a/Documentation/trace/fprobe.rst
> +++ b/Documentation/trace/fprobe.rst
> @@ -91,9 +91,9 @@ The prototype of the entry/exit callback function are as follows:
>
>  .. code-block:: c
>
> - int entry_callback(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs, void *entry_data);
> + int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
>
> - void exit_callback(struct fprobe *fp, unsigned long entry_ip, struct pt_regs *regs, void *entry_data);
> + void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
>
>  Note that the @entry_ip is saved at function entry and passed to exit handler.
>  If the entry callback function returns !0, the corresponding exit callback will be cancelled.
> @@ -108,6 +108,10 @@ If the entry callback function returns !0, the corresponding exit callback will
>          Note that this may not be the actual entry address of the function but
>          the address where the ftrace is instrumented.
>
> +@ret_ip
> +        This is the return address of the traced function. This can be used
> +        at both entry and exit.

Maybe that's just the lack of coffee but I had to think twice to
understand what this paragraph meant :) On my first pass I thought
this meant "the address of the return instruction", which made little
sense since there can of course be multiple "ret"s in a function. I
like the name in the fprobe code "parent_ip" because I find it conveys
better that this is an address in the caller of the traced function.
I'm also fine with this "ret_ip" but I propose we modify the paragraph
a little bit to something like:

This is the address that the traced function will return to, somewhere
in its caller. This can be used at both entry and exit.