On Wed, Jun 27, 2018 at 11:10 AM, Jason Gunthorpe <jgg@xxxxxxxxxxxx> 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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