Messages in this thread | | | From | Kees Cook <> | Date | Wed, 27 Jun 2018 11:44:23 -0700 | Subject | Re: [PATCH rdma-next 08/12] overflow.h: Add arithmetic shift helper |
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On Wed, Jun 27, 2018 at 11:10 AM, Jason Gunthorpe <jgg@mellanox.com> wrote: > Leon? Seems like agreement, Can you work with this version? > > #include <stdint.h> > #include <stdbool.h> > #include <assert.h> > > #define u64 uint64_t > > /* > * Compute *d = (a << s) > * > * Returns true if '*d' cannot hold the result or 'a << s' doesn't make sense. > * - 'a << s' causes bits to be lost when stored in d > * - 's' is garbage (eg negative) or so large that a << s is guarenteed to be 0 > * - 'a' is negative > * - 'a << s' sets the sign bit, if any, in '*d' > * *d is not defined if false is returned. > */ > #define check_shift_overflow(a, s, d) \ > ({ \ > typeof(a) _a = a; \ > typeof(s) _s = s; \ > typeof(d) _d = d; \ > u64 _a_full = _a; \ > unsigned int _to_shift = \ > _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ > \ > *_d = (_a_full << _to_shift); \ > \ > (_to_shift != _s || *_d < 0 || _a < 0 || \ > (*_d >> _to_shift) != a); \ > }) > > int main(int argc, const char *argv[]) > { > int32_t s32; > uint32_t u32; > > assert(check_shift_overflow(1, 0, &s32) == false && s32 == (1 << 0)); > assert(check_shift_overflow(1, 1, &s32) == false && s32 == (1 << 1)); > assert(check_shift_overflow(1, 30, &s32) == false && s32 == (1 << 30)); > assert(check_shift_overflow(1, 31, &s32) == true); > assert(check_shift_overflow(1, 32, &s32) == true); > assert(check_shift_overflow(-1, 1, &s32) == true); > assert(check_shift_overflow(-1, 0, &s32) == true); > > assert(check_shift_overflow(1, 0, &u32) == false && u32 == (1 << 0)); > assert(check_shift_overflow(1, 1, &u32) == false && u32 == (1 << 1)); > assert(check_shift_overflow(1, 30, &u32) == false && u32 == (1 << 30)); > assert(check_shift_overflow(1, 31, &u32) == false && u32 == (1UL << 31)); > assert(check_shift_overflow(1, 32, &u32) == true); > assert(check_shift_overflow(-1, 1, &u32) == true); > assert(check_shift_overflow(-1, 0, &u32) == true); > > assert(check_shift_overflow(0xFFFFFFFF, 0, &u32) == false && u32 == (0xFFFFFFFFUL << 0)); > assert(check_shift_overflow(0xFFFFFFFF, 1, &u32) == true); > assert(check_shift_overflow(0xFFFFFFFF, 0, &s32) == true); > assert(check_shift_overflow(0xFFFFFFFF, 1, &s32) == true); > }
Oh yes, please include these tests in lib/test_overflow.c too! Nice. :)
-Kees
-- Kees Cook Pixel Security
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