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

[Josh Poimboeuf <jpoimboe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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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