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

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Message-Id: <20230818195918.d0372f968ded553a8ef99932@kernel.org>
Date:   Fri, 18 Aug 2023 19:59:18 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Florent Revest <revest@...omium.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 Thu, 17 Aug 2023 10:57:18 +0200
Florent Revest <revest@...omium.org> wrote:

> 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.

Thanks, that makes it more clear. I'll update it.


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>