build: disable unaligned access under ubsan

Avoids false positives on e.g. x86 platforms.

Author: jgriffiths

configure.ac

@@ -178,6 +178,9 @@ fi
 if test "x$ub_sanitizer" = "xyes"; then
     AX_CHECK_COMPILE_FLAG([-fsanitize=undefined], [AM_CFLAGS="$AM_CFLAGS -fsanitize=undefined"])
     AX_CHECK_LINK_FLAG([-fsanitize=undefined], [LDFLAGS="$LDFLAGS -fsanitize=undefined"])
+    # ubsan complains about unaligned reads/writes even though they are legal
+    # on x86 archs. Force alignment adjustment to avoid false positives.
+    AX_CHECK_COMPILE_FLAG([-DAVOID_UNALIGNED_ACCESS=1], [AM_CFLAGS="$AM_CFLAGS -DAVOID_UNALIGNED_ACCESS=1"])
 fi
 
 # -flax-vector-conversions is needed for our arm assembly

src/ccan_config.h

@@ -32,11 +32,14 @@
 #define HAVE_BSWAP_64 0
 #endif
 
-#if defined(HAVE_UNALIGNED_ACCESS) && defined(__arm__)
-/* arm unaligned access is incomplete, in that e.g. byte swap instructions
- * can fault on unaligned addresses where a normal load/store would be fine.
- * Since the compiler can optimise some of our accesses into operations like
- * byte swaps, treat this platform as though it doesn't have unaligned access.
+#if defined(AVOID_UNALIGNED_ACCESS) || (defined(HAVE_UNALIGNED_ACCESS) && defined(__arm__))
+/* Disable unaligned access when:
+ * 1) Explicitly asked to via AVOID_UNALIGNED_ACCESS (e.g. for ubsan builds)
+ * 2) For arm builds where unaligned access is incomplete, in that e.g. byte
+ * swap instructions can fault on unaligned addresses where a normal load/store
+ * would be fine. Since the compiler can optimise some of our accesses into
+ * operations like byte swaps, treat this platform as though it doesn't have
+ * unaligned access.
  */
 #undef HAVE_UNALIGNED_ACCESS
 #define HAVE_UNALIGNED_ACCESS 0