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:

quoted

On Fri, Feb 12, 2016 at 08:45:43AM -0600, Josh Poimboeuf wrote:

quoted

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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