Re: [PATCH v6] tracing/eprobes: Allow use of BTF names to dereference pointers

[Google]

On Fri, 22 May 2026 10:45:21 -0400
Steven Rostedt <rostedt@xxxxxxxxxx> wrote:

> On Fri, 22 May 2026 07:23:22 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > > > @@ -653,6 +686,20 @@ static int parse_btf_arg(char *varname,
> > > >  		return -EOPNOTSUPP;
> > > >  	}
> > > >  
> > > > +	if (ctx->flags & TPARG_FL_TEVENT) {
> > > > +		int ret;
> > > > +
> > > > +		ret = parse_trace_event(varname, code, ctx);
> > > > +		if (ret < 0)
> > > > +			return ret;  
> > 
> > > When parse_trace_event() returns a negative error code (such as -EINVAL or
> > > -ENOENT) because a field name is invalid, the error is propagated back up
> > > the stack. Does this path miss calling trace_probe_log_err()? 
> > > If so, users might receive a generic failure without context or a caret
> > > pointing to the specific syntax error.  
> > 
> > Hmm, there's a comment in the parse_trace_event() that sets ctx->offset for
> > backward compatibility. I'll investigate to see if we can fix that now.
> 
> Masami,
> 
> I looked at the code for parse_trace_event() that has:
> 
> 	/* backward compatibility */
> 	ctx->offset = 0;
> 	return -EINVAL;
> 
> And it was originally introduced by commit 1b8b0cd754cd ("tracing/probes:
> Move event parameter fetching code to common parser"), with:
> 
> +               ret = parse_trace_event_arg(arg, code, ctx);
> +               if (!ret)
> +                       return 0;
> +               if (strcmp(arg, "comm") == 0 || strcmp(arg, "COMM") == 0) {
> +                       code->op = FETCH_OP_COMM;
> +                       return 0;
> +               }
> +               /* backward compatibility */
> +               ctx->offset = 0;
> +               goto inval;
> +       }
> +
> 
> 
> What was the reason for the "backward compatibility"? Can we make it a real
> error now?

This is because a wrong eprobe syntax parser error position indicator.

In tools/testing/selftests/ftrace/test.d/dynevent/eprobes_syntax_errors.tc:

check_error 'e:foo/bar syscalls/sys_enter_openat arg=^dfd'      # BAD_FETCH_ARG
check_error 'e:foo/bar syscalls/sys_enter_openat ^arg=$foo'     # BAD_ATTACH_ARG

BAD_FETCH_ARG points the fetcharg name correctly, but the
BAD_ATTACH_ARG points wrong place in the test case.
I think we should fix test case. (Previously, since it was
a cleanup, I didn't changed it)

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>