[PATCH] KVM: s390: selftest: memop: Fix undefined behavior

[Nina Schoetterl-Glausch <nsg@linux.ibm.com>]

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@linux.ibm.com>
---
 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