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

[Christian Borntraeger]

On 12.12.19 19:06, Will Deacon wrote:
> On Thu, Dec 12, 2019 at 09:41:32AM -0800, Linus Torvalds wrote:
>> On Thu, Dec 12, 2019 at 2:46 AM Peter Zijlstra <peterz at infradead.org> wrote:
>>>
>>> +#ifdef GCC_VERSION < 40800
>>
>> Where does that 4.8 version check come from, and why?
>>
>> Yeah, I know, but this really wants a comment. Sadly it looks like gcc
>> bugzilla is down, so
>>
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
>>
>> currently gives an "Internal Server Error" for me.
>>
>> [ Delete the horrid code we have because of gcc bugs ]
>>
>>> +#else /* GCC_VERSION < 40800 */
>>> +
>>> +#define READ_ONCE_NOCHECK(x)                                           \
>>> +({                                                                     \
>>> +       typeof(x) __x = *(volatile typeof(x))&(x);                      \
>>
>> I think we can/should just do this unconditionally if it helps th eissue.
> 
> I'm currently trying to solve the issue by removing volatile from the bitop
> function signatures, but it's grotty because there are quite a few callers
> to fix up. I'm still trying to do it, because removing volatile fields from
> structurs is generally a "good thing", but I'd be keen to simplify
> READ_ONCE() as you suggest regardless.

As I am the one who added the foundation of READ_ONCEs uglyness, I am now in
favour of re-simplifying it again. I was first a bit scared about re-introducing
bugs, but the gcc testsuite has this particular case covered, so hopefully we
should not see the issue with volatile and aggregate types again.

Christian