Re: clang fails on linux-next since commit 8bf705d13039

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Mar 19, 2018 at 6:54 PM, Matthias Kaehlcke <mka@chromium.org> wrote:
> El Mon, Mar 19, 2018 at 06:39:32PM +0100 Dmitry Vyukov ha dit:
>
>> On Mon, Mar 19, 2018 at 6:29 PM, Matthias Kaehlcke <mka@chromium.org> wrote:
>> > El Mon, Mar 19, 2018 at 09:43:25AM +0300 Dmitry Vyukov ha dit:
>> >
>> >> On Sat, Mar 17, 2018 at 2:13 PM, Lukas Bulwahn <lukas.bulwahn@gmail.com> wrote:
>> >> > Hi Dmitry, hi Ingo,
>> >> >
>> >> > since commit 8bf705d13039 ("locking/atomic/x86: Switch atomic.h to use atomic-instrumented.h")
>> >> > on linux-next (tested and bisected from tag next-20180316), compiling the
>> >> > kernel with clang fails with:
>> >> >
>> >> > In file included from arch/x86/entry/vdso/vdso32/vclock_gettime.c:33:
>> >> > In file included from arch/x86/entry/vdso/vdso32/../vclock_gettime.c:15:
>> >> > In file included from ./arch/x86/include/asm/vgtod.h:6:
>> >> > In file included from ./include/linux/clocksource.h:13:
>> >> > In file included from ./include/linux/timex.h:56:
>> >> > In file included from ./include/uapi/linux/timex.h:56:
>> >> > In file included from ./include/linux/time.h:6:
>> >> > In file included from ./include/linux/seqlock.h:36:
>> >> > In file included from ./include/linux/spinlock.h:51:
>> >> > In file included from ./include/linux/preempt.h:81:
>> >> > In file included from ./arch/x86/include/asm/preempt.h:7:
>> >> > In file included from ./include/linux/thread_info.h:38:
>> >> > In file included from ./arch/x86/include/asm/thread_info.h:53:
>> >> > In file included from ./arch/x86/include/asm/cpufeature.h:5:
>> >> > In file included from ./arch/x86/include/asm/processor.h:21:
>> >> > In file included from ./arch/x86/include/asm/msr.h:67:
>> >> > In file included from ./arch/x86/include/asm/atomic.h:279:
>> >> > ./include/asm-generic/atomic-instrumented.h:295:10: error: invalid output size for constraint '=a'
>> >> >                 return arch_cmpxchg((u64 *)ptr, (u64)old, (u64)new);
>> >> >                        ^
>> >> > ./arch/x86/include/asm/cmpxchg.h:149:2: note: expanded from macro 'arch_cmpxchg'
>> >> >         __cmpxchg(ptr, old, new, sizeof(*(ptr)))
>> >> >         ^
>> >> > ./arch/x86/include/asm/cmpxchg.h:134:2: note: expanded from macro '__cmpxchg'
>> >> >         __raw_cmpxchg((ptr), (old), (new), (size), LOCK_PREFIX)
>> >> >         ^
>> >> > ./arch/x86/include/asm/cmpxchg.h:95:17: note: expanded from macro '__raw_cmpxchg'
>> >> >                              : "=a" (__ret), "+m" (*__ptr)              \
>> >> >                                      ^
>> >> >
>> >> > (... and some more similar and closely related errors)
>> >>
>> >>
>> >> Thanks for reporting, Lukas.
>> >>
>> >> +more people who are more aware of the current state of clang for kernel.
>> >>
>> >> Are there are known issues in '=a' constraint handling between gcc and
>> >> clang? Is there a recommended way to resolve them?
>> >>
>> >> Also, Lukas what's your version of clang? Potentially there are some
>> >> fixes for kernel in the very latest versions of clang.
>> >
>> > My impression is that the problem only occurs in code built for
>> > 32-bit (like arch/x86/entry/vdso/vdso32/*), where the use of a 64-bit
>> > address with a '=a' constraint is indeed invalid. I think the 'root
>> > cause' is that clang parses unreachable code before it discards it:
>> >
>> > static __always_inline unsigned long
>> > cmpxchg_local_size(volatile void *ptr, unsigned long old, unsigned long new,
>> >                    int size)
>> > {
>> >         ...
>> >         switch (size) {
>> >         ...
>> >         case 8:
>> >                 BUILD_BUG_ON(sizeof(unsigned long) != 8);
>> >                 return arch_cmpxchg_local((u64 *)ptr, (u64)old, (u64)new);
>> >         }
>> >         ...
>> > }
>> >
>> > For 32-bit builds size is 4 and the code in the 'offending' branch is
>> > unreachable, however clang still parses it.
>> >
>> > d135b8b5060e ("arm64: uaccess: suppress spurious clang warning") fixes
>> > a similar issue.
>>
>>
>> Thanks!
>>
>> Do I understand it correctly that this is being fixed in clang?
>
> Personally I am not aware of any development on that side, however I'm
> not an LLVM dev.


This looks like something that will hit us again and again if we don't
fix this in clang.