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Date:	Fri, 12 Feb 2016 12:32:06 -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 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?

-- 
Josh

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