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
On Fri, 15 Dec 2023 17:11:25 +0100 Nina Schoetterl-Glausch nsg@linux.ibm.com 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@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;
...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
On Fri, 2023-12-15 at 18:02 +0100, Claudio Imbrenda wrote:
On Fri, 15 Dec 2023 17:11:25 +0100 Nina Schoetterl-Glausch nsg@linux.ibm.com 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@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;
...why not just:
if (!amount) return val;
?
That works if you move it one statement down (128 would also trigger UB). % 128 does the trick, is branchless and there is a bit of a symmetry going on between right and left. But I can use an early return if you want.
- 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
On Mon, 18 Dec 2023 13:18:14 +0100 Nina Schoetterl-Glausch nsg@linux.ibm.com wrote:
[...]
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;
?
That works if you move it one statement down (128 would also trigger UB).
oops, yes it has to be after
% 128 does the trick, is branchless and there is a bit of a symmetry going on between right and left. But I can use an early return if you want.
I think it's more readable, and furthermore...
- 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);
...this is a more idiomatic syntax for a rotate operation
- return (val << left) | (val >> right);
} const unsigned int max_block = 16;
base-commit: 305230142ae0637213bf6e04f6d9f10bbcb74af8
linux-kselftest-mirror@lists.linaro.org
-
Claudio Imbrenda -
Nina Schoetterl-Glausch