linux-kernel - Re: [PATCH v10 07/11] tracing/probes: Add $args meta argument for all function args

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Message-Id: <20230516001137.d5c2f16b89c26bfce31f1c2b@kernel.org>
Date:   Tue, 16 May 2023 00:11:37 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Florent Revest <revest@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH v10 07/11] tracing/probes: Add $args meta argument for
 all function args

On Mon, 15 May 2023 07:57:01 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sun, 14 May 2023 23:12:11 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > Add the '$args' meta fetch argument for function-entry probe events. This
> > will be expanded to the all arguments of the function and the tracepoint
> > using BTF function argument information.
> > 
> > e.g.
> >  #  echo 'p vfs_read $args' >> dynamic_events
> >  #  echo 'f vfs_write $args' >> dynamic_events
> >  #  echo 't sched_overutilized_tp $args' >> dynamic_events
> >  # cat dynamic_events
> > p:kprobes/p_vfs_read_0 vfs_read file=file buf=buf count=count pos=pos
> > f:fprobes/vfs_write__entry vfs_write file=file buf=buf count=count pos=pos
> > t:tracepoints/sched_overutilized_tp sched_overutilized_tp rd=rd overutilized=overutilized
> > 
> > NOTE: This is not like other $-vars, you can not use this $args as a
> > part of fetch args, e.g. specifying name "foo=$args" and using it in
> > dereferences "+0($args)" will lead a parse error.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> > Changes in v10:
> >  - Change $$args to $args so that user can use $$ for current task's pid.
> 
> I hate coming up with new apis, because you never know if what you pick is
> correct ;-) And then you are stuck with whatever you decided on. :-/
> 
> I know I suggested $args, but since it is special, should we call it
>  "$arg*" ?
> 
> That way it follows bash wildcard semantics?
> 
>  #  echo 'p vfs_read $arg*' >> dynamic_events
> 
> I think that is more along the lines of what people would expect.
> 
> What do you think?

Good idea!

BTW, user will expect that $arg* will be expanded to available "$argN".
But this $args does different thing at this point, it is expanded
to something equivalent to "<BTF-name>=$argN:<BTF-type>".
So, to give more consistency, I need one more step: when user gives
only "$argN" on BTF supported kernel, it will be translated to
"<BTF-name>=$argN:<BTF-type>". Then, I think it is natural to use '$arg*'.

Thank you,

> 
> -- Steve
> 
> 
> > Changes in v6:
> >  - update patch description.
> > ---
> >  kernel/trace/trace_fprobe.c |   21 ++++++++-
> >  kernel/trace/trace_kprobe.c |   23 ++++++++--
> >  kernel/trace/trace_probe.c  |   98 +++++++++++++++++++++++++++++++++++++++++++
> >  kernel/trace/trace_probe.h  |   10 ++++
> >  4 files changed, 144 insertions(+), 8 deletions(-)
> > 
> >


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