linux-kernel - Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 9 Jan 2015 17:00:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35

On Thu, Jan 08, 2015 at 10:22:37AM -0500, Vince Weaver wrote:
> Hello,
> 
> I was working on improving the manpage by working out how the 
> perf_event_open() PERF_FLAG_FD_OUTPUT flag works.
> 
> It turns out it doesn't.
> 
> In kernel/events/core.c when opening a file this is done:
> 
>         if (output_event) {
>            err = perf_event_set_output(event, output_event);
>            if (err)
>               goto err_context;
>         }
> 
> 	and perf_event_set_output() does:
> 
>                 if (output_event->cpu == -1 && output_event->ctx != event->ctx) {
>                    goto out;
>                 }
> 
> 	But event->ctx is always NULL; it does not get set to ctx until
> 	after the call to perf_event_set_output().
> 
> 	It looks like this was broken back in 2.6.35 days with the change:
> 
> 		commit ac9721f3f54b27a16c7e1afb2481e7ee95a70318
> 		Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 		Date:   Thu May 27 12:54:41 2010 +0200
> 
> 		perf_events: Fix races and clean up perf_event and perf_mmap_data interaction
> 
> So is this worth fixing seeing as apparently no one uses this feature?

I think there's a fair argument for removing it, Ingo, Acme?

> As an aside, added error reporting is *really* needed as I had to sprinkle
> all of events/core.c with printk() calls to even begin to understand why
> I was getting the EINVAL result.

Yes, let me see if I can find someone to work on that :/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/