linux-kernel - Re: [PATCH v3 1/8] Documentation: probes: Add a new ret

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Message-ID: <CABRcYm+ZuuJJGGk1cQso=4ZN+jBp2g5NODFJYq-TDqzu8KL85A@mail.gmail.com>
Date:   Thu, 17 Aug 2023 10:57:18 +0200
From:   Florent Revest <revest@...omium.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/8] Documentation: probes: Add a new ret_ip callback parameter

On Sat, Aug 12, 2023 at 7:36 AM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Add a new ret_ip callback parameter description.
>
> Fixes: cb16330d1274 ("fprobe: Pass return address to the handlers")
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
>  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.
>
> +@..._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.