Re: [PATCH 00/33] Compile-time stack metadata validation

From	Josh Poimboeuf <jpoimboe@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 00/33] Compile-time stack metadata validation
Date	2016-02-12 19:40 +0100
Message-ID	<r1qNl-2ew-27@gated-at.bofh.it> (permalink)
References	<qTwmS-3nT-7@gated-at.bofh.it> <r1jiO-5IA-19@gated-at.bofh.it> <r1ncL-8iz-17@gated-at.bofh.it> <r1pxW-1vA-49@gated-at.bofh.it>
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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

Thread

Re: [PATCH 00/33] Compile-time stack metadata validation Jiri Slaby <jslaby@suse.cz> - 2016-02-12 11:40 +0100
  Re: [PATCH 00/33] Compile-time stack metadata validation Jiri Slaby <jslaby@suse.cz> - 2016-02-12 11:50 +0100
  Re: [PATCH 00/33] Compile-time stack metadata validation Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-12 15:50 +0100
    Re: [PATCH 00/33] Compile-time stack metadata validation Peter Zijlstra <peterz@infradead.org> - 2016-02-12 18:20 +0100
      Re: [PATCH 00/33] Compile-time stack metadata validation Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-12 19:40 +0100
      Re: [PATCH 00/33] Compile-time stack metadata validation Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-12 19:40 +0100
        Re: [PATCH 00/33] Compile-time stack metadata validation Peter Zijlstra <peterz@infradead.org> - 2016-02-12 21:20 +0100
          Re: [PATCH 00/33] Compile-time stack metadata validation Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-15 17:40 +0100
            Re: [PATCH 00/33] Compile-time stack metadata validation Peter Zijlstra <peterz@infradead.org> - 2016-02-15 18:00 +0100
            Re: [PATCH 00/33] Compile-time stack metadata validation Andi Kleen <andi@firstfloor.org> - 2016-02-15 21:10 +0100
            Re: [PATCH 00/33] Compile-time stack metadata validation Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-15 21:10 +0100
              [PATCH] sched/x86: Add stack frame dependency to  __preempt_schedule[_notrace] Josh Poimboeuf <jpoimboe@redhat.com> - 2016-02-18 18:50 +0100
                Re: [PATCH] sched/x86: Add stack frame dependency to  __preempt_schedule[_notrace] Jiri Slaby <jslaby@suse.cz> - 2016-02-19 13:10 +0100

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