[PATCH] xfs: allow cross-linking special files without project quota

[PATCH] xfs: allow cross-linking special files without project quota

[Andrey Albershteyn]

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>
---
 fs/xfs/xfs_inode.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 5ca561634164..641270f4d794 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -1232,11 +1232,24 @@ xfs_link(
 	 * the tree quota mechanism could be circumvented.
 	 */
 	if (unlikely((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
+		     !special_file(VFS_I(sip)->i_mode) &&
 		     tdp->i_projid != sip->i_projid)) {
 		error = -EXDEV;
 		goto error_return;
 	}
 
+	/*
+	 * Don't allow cross-linking of special files. However, allow
+	 * cross-linking if original file doesn't have any project.
+	 */
+	if (unlikely((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
+				special_file(VFS_I(sip)->i_mode) &&
+				sip->i_projid != 0 &&
+				tdp->i_projid != sip->i_projid)) {
+		error = -EXDEV;
+		goto error_return;
+	}
+
 	if (!resblks) {
 		error = xfs_dir_canenter(tp, tdp, target_name);
 		if (error)
-- 
2.42.0

Re: [PATCH] xfs: allow cross-linking special files without project quota

[Dave Chinner]

On Mon, Mar 04, 2024 at 04:50:14PM +0100, Andrey Albershteyn wrote:
> 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).

Reasonable.

> Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> ---
>  fs/xfs/xfs_inode.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 5ca561634164..641270f4d794 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1232,11 +1232,24 @@ xfs_link(
>  	 * the tree quota mechanism could be circumvented.
>  	 */
>  	if (unlikely((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
> +		     !special_file(VFS_I(sip)->i_mode) &&
>  		     tdp->i_projid != sip->i_projid)) {
>  		error = -EXDEV;
>  		goto error_return;
>  	}
>  
> +	/*
> +	 * Don't allow cross-linking of special files. However, allow
> +	 * cross-linking if original file doesn't have any project.
> +	 */

The comment should explain why the code exists, not document what
the code does.

> +	if (unlikely((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
> +				special_file(VFS_I(sip)->i_mode) &&
> +				sip->i_projid != 0 &&
> +				tdp->i_projid != sip->i_projid)) {
> +		error = -EXDEV;
> +		goto error_return;
> +	}

I think this would be better written as:

	/*
         * If we are using project inheritance, we only allow hard link
         * creation in our tree when the project IDs are the same; else
         * the tree quota mechanism could be circumvented.
         */
	if ((tdp->i_diflags & XFS_DIFLAG_PROJINHERIT) &&
	    tdp->i_projid != sip->i_projid) {
		/*
		 * 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;
		}
	}

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com