On Fri, 15 Dec 2023 17:11:25 +0100
Nina Schoetterl-Glausch <nsg@xxxxxxxxxxxxx> wrote:
> If an integer's type has x bits, shifting the integer left by x or more
> is undefined behavior.
> This can happen in the rotate function when attempting to do a rotation
> of the whole value by 0.
is 0 the only problematic value? because in that case...
>
> Fixes: 0dd714bfd200 ("KVM: s390: selftest: memop: Add cmpxchg tests")
> Signed-off-by: Nina Schoetterl-Glausch <nsg@xxxxxxxxxxxxx>
> ---
> tools/testing/selftests/kvm/s390x/memop.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/s390x/memop.c b/tools/testing/selftests/kvm/s390x/memop.c
> index bb3ca9a5d731..2eba9575828e 100644
> --- a/tools/testing/selftests/kvm/s390x/memop.c
> +++ b/tools/testing/selftests/kvm/s390x/memop.c
> @@ -485,11 +485,13 @@ static bool popcount_eq(__uint128_t a, __uint128_t b)
>
> static __uint128_t rotate(int size, __uint128_t val, int amount)
> {
> - unsigned int bits = size * 8;
> + unsigned int left, right, bits = size * 8;
>
...why not just:
if (!amount)
return val;
?
> - amount = (amount + bits) % bits;
> + right = (amount + bits) % bits;
> + /* % 128 prevents left shift UB if size == 16 && right == 0 */
> + left = (bits - right) % 128;
> val = cut_to_size(size, val);
> - return (val << (bits - amount)) | (val >> amount);
> + return (val << left) | (val >> right);
> }
>
> const unsigned int max_block = 16;
>
> base-commit: 305230142ae0637213bf6e04f6d9f10bbcb74af8
