Re: [PATCH] word-at-a-time: use the same return type for has_zero regardless of endianness

Nick Desaulniers
2023-08-01T22:45:53Z

On Tue, Aug 1, 2023 at 3:22 PM <ndesaulniers@google.com> wrote:
>
> Compiling big-endian targets with Clang produces the diagnostic:
>
> fs/namei.c:2173:13: warning: use of bitwise '|' with boolean operands
> [-Wbitwise-instead-of-logical]
> } while (!(has_zero(a, &adata, &constants) | has_zero(b, &bdata, &constants)));
> ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ||
> fs/namei.c:2173:13: note: cast one or both operands to int to silence
> this warning
>
> It appears that when has_zero was introduced, two definitions were
> produced with different signatures (in particular different return types).
>
> Looking at the usage in hash_name() in fs/namei.c, I suspect that
> has_zero() is meant to be invoked twice per while loop iteration; using
> logical-or would not update `bdata` when `a` did not have zeros. So I
> think it's preferred to always return an unsigned long rather than a
> bool then update the while loop in hash_name() to use a logical-or

s/then/than/

> rather than bitwise-or.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1832
> Fixes: 36126f8f2ed8 ("word-at-a-time: make the interfaces truly generic")
> Debugged-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> include/asm-generic/word-at-a-time.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/word-at-a-time.h b/include/asm-generic/word-at-a-time.h
> index 20c93f08c993..95a1d214108a 100644
> --- a/include/asm-generic/word-at-a-time.h
> +++ b/include/asm-generic/word-at-a-time.h
> @@ -38,7 +38,7 @@ static inline long find_zero(unsigned long mask)
> return (mask >> 8) ? byte : byte + 1;
> }
>
> -static inline bool has_zero(unsigned long val, unsigned long *data, const struct word_at_a_time *c)
> +static inline unsigned long has_zero(unsigned long val, unsigned long *data, const struct word_at_a_time *c)
> {
> unsigned long rhs = val | c->low_bits;
> *data = rhs;
>
> ---
> base-commit: 18b44bc5a67275641fb26f2c54ba7eef80ac5950
> change-id: 20230801-bitwise-7812b11e5fb7
>
> Best regards,
> --
> Nick Desaulniers <ndesaulniers@google.com>
>

--
Thanks,
~Nick Desaulniers