Remove volatile from diff; add explanatory comment
@@ -85,9 +85,15 @@ int mbedtls_ct_memcmp(const void *a,
size_t n)
{
size_t i = 0;
+ /*
+ * `A` and `B` are cast to volatile to ensure that the compiler
+ * generates code that always fully reads both buffers.
+ * Otherwise it could generate a test to exit early if `diff` has all
+ * bits set early in the loop.
+ */
volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b;
- volatile uint32_t diff = 0;
+ uint32_t diff = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
for (; (i + 4) <= n; i += 4) {
