statx: Reserve the top bit of the mask for future struct expansion

Reserve the top bit of the mask for future expansion of the statx struct
and give an error if statx() sees it set.  All the other bits are ignored
if we see them set but don't support the bit; we just clear the bit in the
returned mask.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/stat.c

@@ -562,6 +562,8 @@ SYSCALL_DEFINE5(statx,
 	struct kstat stat;
 	int error;
 
+	if (mask & STATX__RESERVED)
+		return -EINVAL;
 	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
 		return -EINVAL;
 

include/uapi/linux/stat.h

@@ -152,6 +152,7 @@ struct statx {
 #define STATX_BASIC_STATS	0x000007ffU	/* The stuff in the normal stat struct */
 #define STATX_BTIME		0x00000800U	/* Want/got stx_btime */
 #define STATX_ALL		0x00000fffU	/* All currently supported flags */
+#define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
 /*
  * Attributes to be found in stx_attributes