linux-kernel - Re: [PATCH v7 2/4] x86/stackvalidate: Compile-time stack validation

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Message-ID: <20150714215649.GQ19282@twins.programming.kicks-ass.net>
Date:	Tue, 14 Jul 2015 23:56:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, x86@...nel.org,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v7 2/4] x86/stackvalidate: Compile-time stack validation

On Tue, Jul 14, 2015 at 04:30:34PM -0500, Josh Poimboeuf wrote:

> > How (if it does at all) deal with function-trace / -pg -fprofile-arcs
> > things? Does it silently ignore the __mcount calls and assumes ftrace
> > knows wtf its doing? ;-)
> 
> Adding Steven to CC to keep me honest.
> 
> In the case of "-pg -mfentry", which is what ftrace has relied on for

Ah -mfentry is the magic word, I couldn't find it and a grep led me
astray.

> the past few years for newer versions of gcc, stackvalidate silently
> ignores __fentry__ calls and assumes that ftrace indeed knows wtf it's
> doing.  I don't see a problem there as long as the ftrace handler
> doesn't sleep.

They should not indeed, however it would be very nice if backtraces
would still be 'good'.

> I haven't run stackvalidate on the old "-pg" mcount non-fentry stuff,
> but I think it creates a stack frame before calling mcount, so it should
> be fine.

Ok.

> I don't know much about -fprofile-arcs, but as far as I can tell, it's
> only used for gcov.

Right, nobody much uses that I think.
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