Re: [PATCH v4 4/4] tracing/probes: Fix to record 0-length data_loc in fetch_store_string*() if fails

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 10 Jul 2023 23:34:00 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

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

Good point, yes, that's right. Since dyndata and maxlen is only used when
arg->dynamic == true, we don't have to care about that.

> 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.

Indeed.

> 
> 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.

OK.

Let me fix that with a new patch because it is another bug.

Thanks,

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


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