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From: Matthias Kaehlcke <mka@chromium.org>
Date: Mon, 19 Mar 2018 10:29:02 -0700
Subject: Re: clang fails on linux-next since commit 8bf705d13039
Message-ID: <20180319172902.GB37438@google.com>
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>, Nick Desaulniers <ndesaulniers@google.com>, Greg Hackmann <ghackmann@google.com>, Luis Lozano <llozano@google.com>, Michael Davidson <md@google.com>, Sami Tolvanen <samitolvanen@google.com>, Paul Lawrence <paullawrence@google.com>, Ingo Molnar <mingo@kernel.org>, Linux-MM <linux-mm@kvack.org>, kasan-dev <kasan-dev@googlegroups.com>, llvmlinux@lists.linuxfoundation.org, sil2review@lists.osadl.org

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.