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SubjectRe: [PATCH] bitops: Fix shift overflow in GENMASK macros
On Mon, Nov 03, 2014 at 06:39:58PM +0100, Maxime COQUELIN wrote:
> On some 32 bits architectures, including x86, GENMASK(31, 0) returns 0
> instead of the expected ~0UL.
>
> This is the same on some 64 bits architectures with GENMASK_ULL(63, 0).
>
> This is due to an overflow in the shift operand, 1 << 32 for GENMASK,
> 1 << 64 for GENMASK_ULL.
>
> Fixes: 10ef6b0dffe404bcc54e94cb2ca1a5b18445a66b
> Cc: <stable@vger.kernel.org> #v3.13+
> Reported-by: Eric Paire <eric.paire@st.com>
> Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
> ---
> include/linux/bitops.h | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index be5fd38..81f9725 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -18,8 +18,12 @@
> * position @h. For example
> * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
> */
> -#define GENMASK(h, l) (((U32_C(1) << ((h) - (l) + 1)) - 1) << (l))
> -#define GENMASK_ULL(h, l) (((U64_C(1) << ((h) - (l) + 1)) - 1) << (l))
> +#define GENMASK(h, l) \
> + ((~0UL >> ((BITS_PER_LONG - 1) - (h))) & ~((1UL << (l)) - 1))
> +
> +#define GENMASK_ULL(h, l) \
> + ((~0ULL >> ((BITS_PER_LONG_LONG - 1) - (h))) & ~((1ULL << (l)) - 1))
> +

I was not expecting the mask there, but instead something like:

((~0UL >> (BITS_PER_LONG - (h-l+1))) << l)

which shifts the bits to the desired length and then back to the desired
place. Would that not be more readable?



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