Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

From: Segher Boessenkool <[email-masked]>
Date: Fri, 13 Dec 2019 07:53:53 -0600

Hi!

On Fri, Dec 13, 2019 at 11:07:55PM +1100, Michael Ellerman wrote:
> I tried this:
> 
> > @@ -295,6 +296,23 @@ void __write_once_size(volatile void *p, void *res, int size)
> >   */
> >  #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
> >  
> > +#else /* GCC_VERSION < 40800 */
> > +
> > +#define READ_ONCE_NOCHECK(x)						\
> > +({									\
> > +	typeof(x) __x = *(volatile typeof(x))&(x);			\
> 
> Didn't compile, needed:
> 
> 	typeof(x) __x = *(volatile typeof(&x))&(x);			\
> 
> 
> > +	smp_read_barrier_depends();					\
> > +	__x;
> > +})
> 
> 
> And that works for me. No extra stack check stuff.
> 
> I guess the question is does that version of READ_ONCE() implement the
> read once semantics. Do we have a good way to test that?
> 
> The only differences are because of the early return in the generic
> test_and_set_bit_lock():

No, there is another difference:

>   30         ld      r10,560(r9)
>   31         std     r10,104(r1)
>   32         ld      r10,104(r1)
>   33         andi.   r10,r10,1
>   34         bne     <ext4_resize_begin_generic+0xd0>       29         bne     <ext4_resize_begin_ppc+0xd0>

The stack var is volatile, so it is read back immediately after writing
it, here.  This is a bad idea for performance, in general.

Segher