linux-kernel - Re: clang 15 build error

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Message-ID: <20251015165031.GA1465138@ax162>
Date: Wed, 15 Oct 2025 09:50:31 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	llvm@...ts.linux.dev
Subject: Re: clang 15 build error

Hey Boris,

On Wed, Oct 15, 2025 at 04:33:12PM +0200, Borislav Petkov wrote:
> before I go dig into this, any ideas? The likelyhood that you know what the
> issue is, is high: :-P
> 
> That's 32-bit allnoconfig of latest Linus master with Ubuntu clang version
> 15.0.7:
> 
> mm/maccess.c:41:3: error: invalid output size for constraint '=a'
>                 copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
>                 ^
> mm/maccess.c:22:3: note: expanded from macro 'copy_from_kernel_nofault_loop'
>                 __get_kernel_nofault(dst, src, type, err_label);        \
>                 ^
> ./arch/x86/include/asm/uaccess.h:629:18: note: expanded from macro '__get_kernel_nofault'
>         __get_user_size(*((type *)(dst)), (__force type __user *)(src), \
>                         ^
> mm/maccess.c:41:3: error: invalid output size for constraint '=a'
> mm/maccess.c:22:3: note: expanded from macro 'copy_from_kernel_nofault_loop'
>                 __get_kernel_nofault(dst, src, type, err_label);        \
>                 ^
> ./arch/x86/include/asm/uaccess.h:629:18: note: expanded from macro '__get_kernel_nofault'
>         __get_user_size(*((type *)(dst)), (__force type __user *)(src), \
>                         ^
> 2 errors generated.

Heh, I am aware of it but I have not had a chance to actually dig into
see what is going wrong here :/

https://github.com/ClangBuiltLinux/linux/issues/2121

I guess that LLVM is unhappy about a 64-bit variable being passed into
the constraint for eax on 32-bit?

https://github.com/llvm/llvm-project/blob/ce60a03573a4d9f28dd6de42f61f0a3c4b0f2be9/clang/test/CodeGen/X86/x86_32-inline-asm.c#L50
https://github.com/llvm/llvm-project/commit/974131ea8825041673a8c8dda7c42f9a9376eb17

Cheers,
Nathan