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

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Date:   Mon, 15 May 2023 07:57:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.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 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?

-- 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(-)
> 
>