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authorWill Deacon <will.deacon@arm.com>2014-04-23 17:52:52 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2014-04-27 15:20:05 -0700
commitec6931b281797b69e6cf109f9cc94d5a2bf994e0 (patch)
tree9e8ab9ff709939a2ca13a7a5556b436689e2908b
parentac6c9e2bed093c4b60e313674fb7aec4f264c3d4 (diff)
word-at-a-time: avoid undefined behaviour in zero_bytemask macro
The asm-generic, big-endian version of zero_bytemask creates a mask of bytes preceding the first zero-byte by left shifting ~0ul based on the position of the first zero byte. Unfortunately, if the first (top) byte is zero, the output of prep_zero_mask has only the top bit set, resulting in undefined C behaviour as we shift left by an amount equal to the width of the type. As it happens, GCC doesn't manage to spot this through the call to fls(), but the issue remains if architectures choose to implement their shift instructions differently. An example would be arch/arm/ (AArch32), where LSL Rd, Rn, #32 results in Rd == 0x0, whilst on arch/arm64 (AArch64) LSL Xd, Xn, #64 results in Xd == Xn. Rather than check explicitly for the problematic shift, this patch adds an extra shift by 1, replacing fls with __fls. Since zero_bytemask is never called with a zero argument (has_zero() is used to check the data first), we don't need to worry about calling __fls(0), which is undefined. Cc: <stable@vger.kernel.org> Cc: Victor Kamensky <victor.kamensky@linaro.org> Signed-off-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/asm-generic/word-at-a-time.h8
1 files changed, 2 insertions, 6 deletions
diff --git a/include/asm-generic/word-at-a-time.h b/include/asm-generic/word-at-a-time.h
index d3909effd725..d96deb443f18 100644
--- a/include/asm-generic/word-at-a-time.h
+++ b/include/asm-generic/word-at-a-time.h
@@ -50,11 +50,7 @@ static inline bool has_zero(unsigned long val, unsigned long *data, const struct
}
#ifndef zero_bytemask
-#ifdef CONFIG_64BIT
-#define zero_bytemask(mask) (~0ul << fls64(mask))
-#else
-#define zero_bytemask(mask) (~0ul << fls(mask))
-#endif /* CONFIG_64BIT */
-#endif /* zero_bytemask */
+#define zero_bytemask(mask) (~0ul << __fls(mask) << 1)
+#endif
#endif /* _ASM_WORD_AT_A_TIME_H */