On Mon, Mar 19, 2018 at 6:29 PM, Matthias Kaehlcke <mka@xxxxxxxxxxxx> 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@xxxxxxxxx> 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?
