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

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Message-ID: <20230621190337.4635ead7@gandalf.local.home>
Date:   Wed, 21 Jun 2023 19:03:37 -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,
        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 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? ;-)

Probably just add a new type I guess.

Anyway,

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

-- 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);
>  }