Re: [PATCH] x86: bitops: fix build regression

[Brian Gerst]

On Tue, May 5, 2020 at 1:47 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> From: Sedat Dilek <sedat.dilek@xxxxxxxxx>
>
> It turns out that if your config tickles __builtin_constant_p via
> differences in choices to inline or not, this now produces invalid
> assembly:
>
> $ cat foo.c
> long a(long b, long c) {
>   asm("orb\t%1, %0" : "+q"(c): "r"(b));
>   return c;
> }
> $ gcc foo.c
> foo.c: Assembler messages:
> foo.c:2: Error: `%rax' not allowed with `orb'
>
> The "q" constraint only has meanting on -m32 otherwise is treated as
> "r".
>
> This is easily reproducible via Clang+CONFIG_STAGING=y+CONFIG_VT6656=m,
> or Clang+allyesconfig.
>
> Keep the masking operation to appease sparse (`make C=1`), add back the
> cast in order to properly select the proper 8b register alias.
>
>  [Nick: reworded]
>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx>
> Link: https://github.com/ClangBuiltLinux/linux/issues/961
> Link: https://lore.kernel.org/lkml/20200504193524.GA221287@xxxxxxxxxx/
> Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
> Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Reported-by: kernelci.org bot <bot@xxxxxxxxxxxx>
> Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> Suggested-by: Ilie Halip <ilie.halip@xxxxxxxxx>
> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Signed-off-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/bitops.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index b392571c1f1d..139122e5b25b 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
>         if (__builtin_constant_p(nr)) {
>                 asm volatile(LOCK_PREFIX "orb %1,%0"
>                         : CONST_MASK_ADDR(nr, addr)
> -                       : "iq" (CONST_MASK(nr) & 0xff)
> +                       : "iq" ((u8)(CONST_MASK(nr) & 0xff))

I think a better fix would be to make CONST_MASK() return a u8 value
rather than have to cast on every use.

Also I question the need for the "q" constraint.  It was added in
commit 437a0a54 as a workaround for GCC 3.4.4.  Now that the minimum
GCC version is 4.6, is this still necessary?

--
Brian Gerst