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. 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; - 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 -- 2.40.1