Messages in this thread | | | Date | Tue, 4 Nov 2014 09:40:24 +0100 | From | Maxime Coquelin <> | Subject | Re: [PATCH] bitops: Fix shift overflow in GENMASK macros |
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Hi Peter,
On 11/04/2014 09:29 AM, Peter Zijlstra wrote: > 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? > Yes, this is indeed more readable. I will send a v2 with your implementation.
Thanks, Maxime
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