statx: Include a mask for stx_attributes in struct statx

Include a mask in struct stat to indicate which bits of stx_attributes the
filesystem actually supports.

This would also be useful if we add another system call that allows you to
do a 'bulk attribute set' and pass in a statx struct with the masks
appropriately set to say what you want to set.

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/ext4/inode.c

···
 	if (flags & EXT4_NODUMP_FL)
 		stat->attributes |= STATX_ATTR_NODUMP;
 
+	stat->attributes_mask |= (STATX_ATTR_APPEND |
+				  STATX_ATTR_COMPRESSED |
+				  STATX_ATTR_ENCRYPTED |
+				  STATX_ATTR_IMMUTABLE |
+				  STATX_ATTR_NODUMP);
+
 	generic_fillattr(inode, stat);
 	return 0;
 }

fs/stat.c

···
 	tmp.stx_ino = stat->ino;
 	tmp.stx_size = stat->size;
 	tmp.stx_blocks = stat->blocks;
+	tmp.stx_attributes_mask = stat->attributes_mask;
 	tmp.stx_atime.tv_sec = stat->atime.tv_sec;
 	tmp.stx_atime.tv_nsec = stat->atime.tv_nsec;
 	tmp.stx_btime.tv_sec = stat->btime.tv_sec;

include/linux/stat.h

···
 	unsigned int	nlink;
 	uint32_t	blksize;	/* Preferred I/O size */
 	u64		attributes;
+	u64		attributes_mask;
 #define KSTAT_ATTR_FS_IOC_FLAGS				\
 	(STATX_ATTR_COMPRESSED |			\
 	 STATX_ATTR_IMMUTABLE |				\

include/uapi/linux/stat.h

···
 	__u64	stx_ino;	/* Inode number */
 	__u64	stx_size;	/* File size */
 	__u64	stx_blocks;	/* Number of 512-byte blocks allocated */
-	__u64	__spare1[1];
+	__u64	stx_attributes_mask; /* Mask to show what's supported in stx_attributes */
 	/* 0x40 */
 	struct statx_timestamp	stx_atime;	/* Last access time */
 	struct statx_timestamp	stx_btime;	/* File creation time */
···
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
 /*
- * Attributes to be found in stx_attributes
+ * Attributes to be found in stx_attributes and masked in stx_attributes_mask.
  *
  * These give information about the features or the state of a file that might
  * be of use to ordinary userspace programs such as GUIs or ls rather than

samples/statx/test-statx.c

···
 	if (stx->stx_mask & STATX_BTIME)
 		print_time(" Birth: ", &stx->stx_btime);
 
-	if (stx->stx_attributes) {
-		unsigned char bits;
+	if (stx->stx_attributes_mask) {
+		unsigned char bits, mbits;
 		int loop, byte;
 
 		static char attr_representation[64 + 1] =
···
 		printf("Attributes: %016llx (", stx->stx_attributes);
 		for (byte = 64 - 8; byte >= 0; byte -= 8) {
 			bits = stx->stx_attributes >> byte;
+			mbits = stx->stx_attributes_mask >> byte;
 			for (loop = 7; loop >= 0; loop--) {
 				int bit = byte + loop;
 
-				if (bits & 0x80)
+				if (!(mbits & 0x80))
+					putchar('.');	/* Not supported */
+				else if (bits & 0x80)
 					putchar(attr_representation[63 - bit]);
 				else
-					putchar('-');
+					putchar('-');	/* Not set */
 				bits <<= 1;
+				mbits <<= 1;
 			}
 			if (byte)
 				putchar(' ');