Re: [PATCH] kprobes: Print an error if probe is rejected

[Masami Hiramatsu]

On Thu, 10 Jun 2021 13:33:46 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Thu, 10 Jun 2021 19:16:43 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> 
> > Hmm, Nack for this way, but I understand that is a problem.
> > If you got the error in perf probe, which uses ftrace dynamic-event interface.
> > In that case, the errors should not be output in the dmesg, but are reported
> > via error_log in tracefs.
> > And kprobes itself should return different error code to the caller, instead
> > of printing error in dmesg. See below.
> 
> We should update perf to use libtracefs that also has an interface to
> read the error_log file.

Hmm, it seems that libtracefs has no parser for the error_log file.

What we need is to parse error_log and find appropriate entry of
the error log. Thus, the interface will be;

int tracefs_add_dynamic_event(const char *command, char **error_log);

And the usage will be;

ret = tracefs_add_dynamic_event(command, &error_buf);
if (ret < 0) {
	pr_error("Failed to add dynamic event: %d\n", ret);
	pr_error("Error log:\n%s\n", error_buf);
	free(error_buf);
	return ret;
}
...
This is because error_log file keeps some previous error logs and
you need to find appropriate one from the actual command.

Maybe a part of perf probe (util/probe-file.c) should be shared with
libtracefs and finally it should moved on the libtracefs.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>