dbus-marshal-byteswap: Byte-swap Unix fd indexes if needed

When a D-Bus message includes attached file descriptors, the body of the
message contains unsigned 32-bit indexes pointing into an out-of-band
array of file descriptors. Some D-Bus APIs like GLib's GDBus refer to
these indexes as "handles" for the associated fds (not to be confused
with a Windows HANDLE, which is a kernel object).

The assertion message removed by this commit is arguably correct up to
a point: fd-passing is only reasonable on a local machine, and no known
operating system allows processes of differing endianness even on a
multi-endian ARM or PowerPC CPU, so it makes little sense for the sender
to specify a byte-order that differs from the byte-order of the recipient.

However, this doesn't account for the fact that a malicious sender
doesn't have to restrict itself to only doing things that make sense.
On a system with untrusted local users, a message sender could crash
the system dbus-daemon (a denial of service) by sending a mes...

dbus/dbus-marshal-byteswap.c

@@ -62,6 +62,7 @@ byteswap_body_helper (DBusTypeReader       *reader,
         case DBUS_TYPE_BOOLEAN:
         case DBUS_TYPE_INT32:
         case DBUS_TYPE_UINT32:
+        case DBUS_TYPE_UNIX_FD:
           {
             p = _DBUS_ALIGN_ADDRESS (p, 4);
             *((dbus_uint32_t *) (void *) p) =
@@ -192,11 +193,6 @@ byteswap_body_helper (DBusTypeReader       *reader,
           }
           break;
 
-        case DBUS_TYPE_UNIX_FD:
-          /* fds can only be passed on a local machine, so byte order must always match */
-          _dbus_assert_not_reached("attempted to byteswap unix fds which makes no sense");
-          break;
-
         default:
           _dbus_assert_not_reached ("invalid typecode in supposedly-validated signature");
           break;