[PATCH v3 3/5] target/s390x: Fix cc_calc_sla_64() missing overflows

Ilya Leoshkevich posted 5 patches 4 months, 1 week ago
Maintainers: Thomas Huth <thuth@redhat.com>, David Hildenbrand <david@redhat.com>, Richard Henderson <richard.henderson@linaro.org>, Cornelia Huck <cohuck@redhat.com>
[PATCH v3 3/5] target/s390x: Fix cc_calc_sla_64() missing overflows
Posted by Ilya Leoshkevich 4 months, 1 week ago
An overflow occurs for SLAG when at least one shifted bit is not equal
to sign bit. Therefore, we need to check that `shift + 1` bits are
neither all 0s nor all 1s. The current code checks only `shift` bits,
missing some overflows.

Fixes: cbe24bfa91d2 ("target-s390: Convert SHIFT, ROTATE SINGLE")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 target/s390x/tcg/cc_helper.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/target/s390x/tcg/cc_helper.c b/target/s390x/tcg/cc_helper.c
index c2c96c3a3c..b6acffa3e8 100644
--- a/target/s390x/tcg/cc_helper.c
+++ b/target/s390x/tcg/cc_helper.c
@@ -297,7 +297,8 @@ static uint32_t cc_calc_sla_32(uint32_t src, int shift)
 
 static uint32_t cc_calc_sla_64(uint64_t src, int shift)
 {
-    uint64_t mask = ((1ULL << shift) - 1ULL) << (64 - shift);
+    /* Do not use (1ULL << (shift + 1)): it triggers UB when shift is 63.  */
+    uint64_t mask = ((((1ULL << shift) - 1) << 1) + 1) << (64 - (shift + 1));
     uint64_t sign = 1ULL << 63;
     uint64_t match;
     int64_t r;
-- 
2.31.1