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

Josh Poimboeuf
2016-02-12T18:32:20Z

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