linux-kernel - [PATCH] word-at-a-time: use the same return type for has

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Message-ID: <20230801-bitwise-v1-1-799bec468dc4@google.com>
Date:   Tue, 01 Aug 2023 15:22:17 -0700
From:   ndesaulniers@...gle.com
To:     Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH] word-at-a-time: use the same return type for has_zero
 regardless of endianness

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
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@...nel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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@...gle.com>