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Date:	Fri, 12 Feb 2016 12:34:00 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	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>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Borislav Petkov <bp@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Alok Kataria <akataria@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Len Brown <len.brown@...el.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	Wim Van Sebroeck <wim@...ana.be>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-watchdog@...r.kernel.org, Waiman Long <Waiman.Long@....com>
Subject: Re: [PATCH 00/33] Compile-time stack metadata validation

On Fri, Feb 12, 2016 at 12:32:06PM -0600, Josh Poimboeuf wrote:
> On Fri, Feb 12, 2016 at 06:10:37PM +0100, Peter Zijlstra wrote:
> > On Fri, Feb 12, 2016 at 08:45:43AM -0600, Josh Poimboeuf wrote:
> > > On Fri, Feb 12, 2016 at 11:36:24AM +0100, Jiri Slaby wrote:
> > > 
> > > This seems like a real frame pointer bug caused by the following line in
> > > arch/x86/include/asm/preempt.h:
> > > 
> > >   # define __preempt_schedule() asm ("call ___preempt_schedule")
> > 
> > The purpose there is that:
> > 
> > 	preempt_enable();
> > 
> > turns into:
> > 
> > 	decl	__percpu_prefix:__preempt_count
> > 	jnz	1f:
> > 	call	___preempt_schedule
> > 1:
> > 
> > See arch/x86/include/asm/preempt.h:__preempt_count_dec_and_test()
> 
> Sorry, I'm kind of confused.  Do you mean that's what preempt_enable()
> would turn into *without* the above define?
> 
> What I actually see in the listing is:
> 
>  	decl	__percpu_prefix:__preempt_count
>  	je	1f:
> 	....
>  1:
>  	call	___preempt_schedule
> 
> So it puts the "call ___preempt_schedule" in the slow path.
> 
> I also don't see how that would be related to the use of the asm
> statement in the __preempt_schedule() macro.  Doesn't the use of
> unlikely() in preempt_enable() put the call in the slow path?
> 
>   #define preempt_enable() \
>   do { \
> 	  barrier(); \
> 	  if (unlikely(preempt_count_dec_and_test())) \
> 		  preempt_schedule(); \
>   } while (0)
> 
> Also, why is the thunk needed?  Any reason why preempt_enable() can't be
> called directly from C?

Sorry, s/preempt_enable/preempt_schedule/ on that last sentence.

-- 
Josh

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