linux-kernel - Re: [PATCH] tracing/probes: Fix tracepoint event with $arg* to fetch correct argument

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Date:   Thu, 22 Jun 2023 09:56:19 +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,
        Bagas Sanjaya <bagasdotme@...il.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] tracing/probes: Fix tracepoint event with $arg* to
 fetch correct argument

On Wed, 21 Jun 2023 19:03:37 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 12 Jun 2023 20:58:57 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > To hide the first dummy 'data' argument on the tracepoint probe events,
> > the BTF argument array was modified (skip the first argument for tracepoint),
> > but the '$arg*' meta argument parser missed that.
> > 
> > Fix to increment the argument index if it is tracepoint probe. And decrement
> > the index when searching the type of the argument.
> 
> I'm curious. What if we want a variable that points to that data argument? ;-)

That is a dummy parameter which is passed when registering the callback, so
it is passed from trace_fprobe.c. Currently trace_fprobe.c passed NULL to
the data argument.

> 
> Probably just add a new type I guess.

Yeah, if we get any idea to use it, it will be accessed by a new $-variable.

> 
> Anyway,
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Thank you!

> 
> -- Steve
> 
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> >  kernel/trace/trace_probe.c |   10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> > index 473e1c43bc57..643aa3a51d5a 100644
> > --- a/kernel/trace/trace_probe.c
> > +++ b/kernel/trace/trace_probe.c
> > @@ -456,7 +456,10 @@ static int parse_btf_arg(const char *varname, struct fetch_insn *code,
> >  
> >  		if (name && !strcmp(name, varname)) {
> >  			code->op = FETCH_OP_ARG;
> > -			code->param = i;
> > +			if (ctx->flags & TPARG_FL_TPOINT)
> > +				code->param = i + 1;
> > +			else
> > +				code->param = i;
> >  			return 0;
> >  		}
> >  	}
> > @@ -470,8 +473,11 @@ static const struct fetch_type *parse_btf_arg_type(int arg_idx,
> >  	struct btf *btf = traceprobe_get_btf();
> >  	const char *typestr = NULL;
> >  
> > -	if (btf && ctx->params)
> > +	if (btf && ctx->params) {
> > +		if (ctx->flags & TPARG_FL_TPOINT)
> > +			arg_idx--;
> >  		typestr = type_from_btf_id(btf, ctx->params[arg_idx].type);
> > +	}
> >  
> >  	return find_fetch_type(typestr, ctx->flags);
> >  }
> 


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