linux-kernel - Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp

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]

Message-ID: <20091013223237.GB14430@Krystal>
Date:	Tue, 13 Oct 2009 18:32:37 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced
	ret with jmp

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Tue, 2009-10-13 at 17:21 -0400, Mathieu Desnoyers wrote:
> 
> > > What it was:
> > > 
> > > call function
> > >   function:
> > >   call mcount
> > >      mcount:
> > >      call ftrace_entry
> > 
> > Can we manage to change this call
> 
> Note, that call jumps to C code.
> 
> > 
> > >        ftrace_entry:
> > >        mess up with return code of caller
> > >        ret
> > 
> > .. and this ret for 2 jmp instructions too ?
> 
> The code is all in C, and it too calls functions. Not sure where this
> helps out any. The ret here matches their calls. Thus the prediction
> will work.
> 

Oh, OK. I thought the callback was in assembly. That's a bit more work
than I thought.

> > 
> > Given that we have no choice but to kill call/ret prediction logic, I
> > think it might be good to try to use this logic as little as possible
> > (by favoring jmp jmp over call/ret when the return target is invariant).
> > 
> > That's just an idea, benchmarks could prove me right/wrong.
> 
> I don't see how this would help. And I'm not about to waste time
> experimenting. What's the rational?
> 

The idea is that call/ret are fast when predictions are right. In this
case, I wonder if the fact that we trash the call/ret prediction (even
if this happens after the paired call/ret) would have an impact on
balanced call/ret in the tracing code path. I doubt so, but who knows.

Probably not worth spending much time on this. It would just have been
nice to try if the implementation would have been trivial.

Thanks,

Mathieu

> -- Steve
> 
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
--
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/