linux-kernel - Re: [PATCH v4 4/4] tracing/probes: Fix to record 0-length data_loc in fetch_store

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Date:   Mon, 10 Jul 2023 23:34:00 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Dan Carpenter <dan.carpenter@...aro.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/4] tracing/probes: Fix to record 0-length data_loc
 in fetch_store_string*() if fails

On Tue, 11 Jul 2023 11:11:51 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> --- a/kernel/trace/trace_probe_tmpl.h
> +++ b/kernel/trace/trace_probe_tmpl.h
> @@ -267,9 +267,7 @@ store_trace_args(void *data, struct trace_probe *tp, void *rec,
>  		if (unlikely(arg->dynamic))
>  			*dl = make_data_loc(maxlen, dyndata - base);
>  		ret = process_fetch_insn(arg->code, rec, dl, base);
> -		if (unlikely(ret < 0 && arg->dynamic)) {
> -			*dl = make_data_loc(0, dyndata - base);
> -		} else {
> +		if (unlikely(ret > 0 && arg->dynamic)) {

To match the current code, that should be:

		if (likely(ret >= 0 || !arg->dynamic)) {

But I'm guessing that the original code was buggy, as the else block should
only have been processed if arg->dynamic was set? That is, it should have been:

	if (arg->dynamic) {
		if (unlikely(ret < 0)) {
			*dl = make_data_loc(0, dyndata - base);
		} else {
  			dyndata += ret;
  			maxlen -= ret;
  		}
	}

I guess you only want to update if arg->dynamic is true (even though that
wasn't the case before :-/) But in any case, I think you want likely() and
not unlikely().

		if (arg->dynamic && likely(ret > 0)) {

That is, if we only want to updated this if the arg is dynamic.

And I don't think that the arg->dynamic() should have likely/unlikely
around it, as that's determined by user space, and the kernel should not be
adding assumptions about what user space wants.

-- Steve

>  			dyndata += ret;
>  			maxlen -= ret;
>  		}