Improve implementation of mbedtls_ct_int_if
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/constant_time_impl.h b/library/constant_time_impl.h
index 9bfb2c5..afec894 100644
--- a/library/constant_time_impl.h
+++ b/library/constant_time_impl.h
@@ -331,6 +331,42 @@
return (unsigned char) (~(low_mask | high_mask)) & to;
}
+static inline int mbedtls_ct_int_if(mbedtls_ct_condition_t condition, int if1, int if0)
+{
+ /*
+ * Use memcpy to avoid unsigned-to-signed conversion UB.
+ *
+ * Although this looks inefficient, in practice (for gcc and clang),
+ * the sizeof test is eliminated at compile-time, and identical code is generated
+ * as for mbedtls_ct_uint_if (for -Os and -O1 or higher).
+ */
+ int result;
+ if (sizeof(int) <= sizeof(unsigned int)) {
+ unsigned int uif1 = 0, uif0 = 0;
+ memcpy(&uif1, &if1, sizeof(if1));
+ memcpy(&uif0, &if0, sizeof(if0));
+ unsigned int uresult = mbedtls_ct_uint_if(condition, uif1, uif0);
+ memcpy(&result, &uresult, sizeof(result));
+ } else {
+ mbedtls_ct_memcpy_if(condition, (unsigned char *) &result, (const unsigned char *) &if1,
+ (const unsigned char *) &if0, sizeof(result));
+ }
+ return result;
+}
+
+static inline int mbedtls_ct_int_if_else_0(mbedtls_ct_condition_t condition, int if1)
+{
+ if (sizeof(int) <= sizeof(unsigned int)) {
+ unsigned int uif1 = 0;
+ memcpy(&uif1, &if1, sizeof(if1));
+ unsigned int uresult = mbedtls_ct_uint_if_else_0(condition, uif1);
+ int result;
+ memcpy(&result, &uresult, sizeof(result));
+ return result;
+ } else {
+ return mbedtls_ct_int_if(condition, if1, 0);
+ }
+}
/* ============================================================================
* Everything below here is trivial wrapper functions
@@ -397,17 +433,6 @@
#endif /* MBEDTLS_BIGNUM_C */
-static inline int mbedtls_ct_int_if(mbedtls_ct_condition_t condition, int if1, int if0)
-{
- int a[2] = {if0, if1};
- return a[mbedtls_ct_uint_if_else_0(condition, 1)];
-}
-
-static inline int mbedtls_ct_int_if_else_0(mbedtls_ct_condition_t condition, int if1)
-{
- return mbedtls_ct_int_if(condition, if1, 0);
-}
-
static inline mbedtls_ct_condition_t mbedtls_ct_uint_eq(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{