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    Subject[PATCH 5.1 02/85] include/linux/bitops.h: sanitize rotate primitives
    Date
    From: Rasmus Villemoes <linux@rasmusvillemoes.dk>

    commit ef4d6f6b275c498f8e5626c99dbeefdc5027f843 upstream.

    The ror32 implementation (word >> shift) | (word << (32 - shift) has
    undefined behaviour if shift is outside the [1, 31] range. Similarly
    for the 64 bit variants. Most callers pass a compile-time constant
    (naturally in that range), but there's an UBSAN report that these may
    actually be called with a shift count of 0.

    Instead of special-casing that, we can make them DTRT for all values of
    shift while also avoiding UB. For some reason, this was already partly
    done for rol32 (which was well-defined for [0, 31]). gcc 8 recognizes
    these patterns as rotates, so for example

    __u32 rol32(__u32 word, unsigned int shift)
    {
    return (word << (shift & 31)) | (word >> ((-shift) & 31));
    }

    compiles to

    0000000000000020 <rol32>:
    20: 89 f8 mov %edi,%eax
    22: 89 f1 mov %esi,%ecx
    24: d3 c0 rol %cl,%eax
    26: c3 retq

    Older compilers unfortunately do not do as well, but this only affects
    the small minority of users that don't pass constants.

    Due to integer promotions, ro[lr]8 were already well-defined for shifts
    in [0, 8], and ro[lr]16 were mostly well-defined for shifts in [0, 16]
    (only mostly - u16 gets promoted to _signed_ int, so if bit 15 is set,
    word << 16 is undefined). For consistency, update those as well.

    Link: http://lkml.kernel.org/r/20190410211906.2190-1-linux@rasmusvillemoes.dk
    Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
    Reported-by: Ido Schimmel <idosch@mellanox.com>
    Tested-by: Ido Schimmel <idosch@mellanox.com>
    Reviewed-by: Will Deacon <will.deacon@arm.com>
    Cc: Vadim Pasternak <vadimp@mellanox.com>
    Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
    Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
    Cc: Pavel Machek <pavel@ucw.cz>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/bitops.h | 16 ++++++++--------
    1 file changed, 8 insertions(+), 8 deletions(-)

    --- a/include/linux/bitops.h
    +++ b/include/linux/bitops.h
    @@ -60,7 +60,7 @@ static __always_inline unsigned long hwe
    */
    static inline __u64 rol64(__u64 word, unsigned int shift)
    {
    - return (word << shift) | (word >> (64 - shift));
    + return (word << (shift & 63)) | (word >> ((-shift) & 63));
    }

    /**
    @@ -70,7 +70,7 @@ static inline __u64 rol64(__u64 word, un
    */
    static inline __u64 ror64(__u64 word, unsigned int shift)
    {
    - return (word >> shift) | (word << (64 - shift));
    + return (word >> (shift & 63)) | (word << ((-shift) & 63));
    }

    /**
    @@ -80,7 +80,7 @@ static inline __u64 ror64(__u64 word, un
    */
    static inline __u32 rol32(__u32 word, unsigned int shift)
    {
    - return (word << shift) | (word >> ((-shift) & 31));
    + return (word << (shift & 31)) | (word >> ((-shift) & 31));
    }

    /**
    @@ -90,7 +90,7 @@ static inline __u32 rol32(__u32 word, un
    */
    static inline __u32 ror32(__u32 word, unsigned int shift)
    {
    - return (word >> shift) | (word << (32 - shift));
    + return (word >> (shift & 31)) | (word << ((-shift) & 31));
    }

    /**
    @@ -100,7 +100,7 @@ static inline __u32 ror32(__u32 word, un
    */
    static inline __u16 rol16(__u16 word, unsigned int shift)
    {
    - return (word << shift) | (word >> (16 - shift));
    + return (word << (shift & 15)) | (word >> ((-shift) & 15));
    }

    /**
    @@ -110,7 +110,7 @@ static inline __u16 rol16(__u16 word, un
    */
    static inline __u16 ror16(__u16 word, unsigned int shift)
    {
    - return (word >> shift) | (word << (16 - shift));
    + return (word >> (shift & 15)) | (word << ((-shift) & 15));
    }

    /**
    @@ -120,7 +120,7 @@ static inline __u16 ror16(__u16 word, un
    */
    static inline __u8 rol8(__u8 word, unsigned int shift)
    {
    - return (word << shift) | (word >> (8 - shift));
    + return (word << (shift & 7)) | (word >> ((-shift) & 7));
    }

    /**
    @@ -130,7 +130,7 @@ static inline __u8 rol8(__u8 word, unsig
    */
    static inline __u8 ror8(__u8 word, unsigned int shift)
    {
    - return (word >> shift) | (word << (8 - shift));
    + return (word >> (shift & 7)) | (word << ((-shift) & 7));
    }

    /**

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