statx: add mount_root

Determining whether a path or file descriptor refers to a mountpoint (or
more precisely a mount root) is not trivial using current tools.

Add a flag to statx that indicates whether the path or fd refers to the
root of a mount or not.

Cc: [email protected]
Cc: [email protected]
Reported-by: Lennart Poettering <[email protected]>
Reported-by: J. Bruce Fields <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>

Author: Miklos Szeredi

fs/stat.c

@@ -202,6 +202,9 @@ int vfs_statx(int dfd, const char __user *filename, int flags,
 	error = vfs_getattr(&path, stat, request_mask, flags);
 	stat->mnt_id = real_mount(path.mnt)->mnt_id;
 	stat->result_mask |= STATX_MNT_ID;
+	if (path.mnt->mnt_root == path.dentry)
+		stat->attributes |= STATX_ATTR_MOUNT_ROOT;
+	stat->attributes_mask |= STATX_ATTR_MOUNT_ROOT;
 	path_put(&path);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;

include/uapi/linux/stat.h

@@ -181,6 +181,7 @@ struct statx {
 #define STATX_ATTR_NODUMP		0x00000040 /* [I] File is not to be dumped */
 #define STATX_ATTR_ENCRYPTED		0x00000800 /* [I] File requires key to decrypt in fs */
 #define STATX_ATTR_AUTOMOUNT		0x00001000 /* Dir: Automount trigger */
+#define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */
 #define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */