| Message ID | 20231215161125.943551-1-nsg@linux.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: s390: selftest: memop: Fix undefined behavior | expand |
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 > > >
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;
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(-) base-commit: 305230142ae0637213bf6e04f6d9f10bbcb74af8