xfs: allow cross-linking special files without project quota

There's an issue that if special files is created before quota
project is enabled, then it's not possible to link this file. This
works fine for normal files. This happens because xfs_quota skips
special files (no ioctls to set necessary flags). The check for
having the same project ID for source and destination then fails as
source file doesn't have any ID.

mkfs.xfs -f /dev/sda
mount -o prjquota /dev/sda /mnt/test

mkdir /mnt/test/foo
mkfifo /mnt/test/foo/fifo1

xfs_quota -xc "project -sp /mnt/test/foo 9" /mnt/test
> Setting up project 9 (path /mnt/test/foo)...
> xfs_quota: skipping special file /mnt/test/foo/fifo1
> Processed 1 (/etc/projects and cmdline) paths for project 9 with recursion depth infinite (-1).

ln /mnt/test/foo/fifo1 /mnt/test/foo/fifo1_link
> ln: failed to create hard link '/mnt/test/testdir/fifo1_link' => '/mnt/test/testdir/fifo1': Invalid cross-device link

mkfifo /mnt/test/foo/fifo2
ln /mnt/test/foo/fifo2 /mnt/test/foo/fifo2_link

Fix this by allowing linking of special files to the project quota
if special files doesn't have any ID set (ID = 0).
Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_inode.c

@@ -1301,8 +1301,19 @@ xfs_link(
 	 */
 	if (unlikely((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
 		     tdp->i_projid != sip->i_projid)) {
-		error = -EXDEV;
-		goto error_return;
+		/*
+		 * Project quota setup skips special files which can
+		 * leave inodes in a PROJINHERIT directory without a
+		 * project ID set. We need to allow links to be made
+		 * to these "project-less" inodes because userspace
+		 * expects them to succeed after project ID setup,
+		 * but everything else should be rejected.
+		 */
+		if (!special_file(VFS_I(sip)->i_mode) ||
+		    sip->i_projid != 0) {
+			error = -EXDEV;
+			goto error_return;
+		}
 	}
 
 	if (!resblks) {