Avoid invalid gcc 14.3 warning about array bounds in mbedtls_xor

The combination of the multi-byte loop with the single byte loop
confuses GCC 14.3's array bounds checker. When the loop size is
constant, check to see if it is a multiple of the multi-byte size and
bail early. As this will be evaluated at compile time, there should be
no run-time cost.

This change has been submitted upstream:

Mbed-TLS/mbedtls#10318

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

library/common.h

@@ -208,6 +208,10 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && __has_builtin(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 16 == 0)
+	    return;
+#endif
 #elif defined(MBEDTLS_ARCH_IS_X64) || defined(MBEDTLS_ARCH_IS_ARM64)
     /* This codepath probably only makes sense on architectures with 64-bit registers */
     for (; (i + 8) <= n; i += 8) {
@@ -219,6 +223,10 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && __has_builtin(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 8 == 0)
+	    return;
+#endif
 #else
     for (; (i + 4) <= n; i += 4) {
         uint32_t x = mbedtls_get_unaligned_uint32(a + i) ^ mbedtls_get_unaligned_uint32(b + i);
@@ -229,6 +237,10 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && __has_builtin(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 4 == 0)
+	    return;
+#endif
 #endif
 #endif
     for (; i < n; i++) {