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

[Will Deacon]

On Thu, Dec 12, 2019 at 11:46:10AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 12, 2019 at 10:07:56AM +0000, Will Deacon wrote:
> 
> > > So your proposed change _should_ be fine. Will, I'm assuming you never
> > > saw this on your ARGH64 builds when you did this code ?
> > 
> > I did see it, but (a) looking at the code out-of-line makes it look a lot
> > worse than it actually is (so the ext4 example is really helpful -- thanks
> > Michael!) and (b) I chalked it up to a crappy compiler.
> > 
> > However, see this comment from Arnd on my READ_ONCE series from the other
> > day:
> > 
> > https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@xxxxxxxxxxxxxx
> > 
> > In which case, I'm thinking that we should be doing better in READ_ONCE()
> > for non-buggy compilers which would also keep the KCSAN folks happy for this
> > code (and would help with [1] too).
> 
> So something like this then? Although I suppose that should be moved
> into compiler-gcc.h and then guarded by #ifndef READ_ONCE or so.

Ah wait, I think we've been looking at this wrong. The volatile pointer
argument is actually the problem here, not READ_ONCE()! The use of typeof()
means that the temporary variable to which __READ_ONCE_SIZE writes ends up
being a volatile store, so it can't be optimised away. This is why we get
a stack access and why stack protector then wrecks the codegen for us.

I'll cook a patch getting rid of those volatiles.

Will