Re: [PATCH v2 2/4] fs: add FS_IOC_FSSETXATTRAT and FS_IOC_FSGETXATTRAT

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jun 04, 2024 at 10:58:43AM +0200, Jan Kara wrote:
> On Mon 03-06-24 10:42:59, Darrick J. Wong wrote:
> > On Mon, Jun 03, 2024 at 06:28:47PM +0200, Andrey Albershteyn wrote:
> > > On 2024-06-03 12:42:59, Jan Kara wrote:
> > > > On Fri 31-05-24 07:52:04, Darrick J. Wong wrote:
> > > > > On Fri, May 24, 2024 at 06:11:01PM +0200, Jan Kara wrote:
> > > > > > On Thu 23-05-24 13:16:48, Andrey Albershteyn wrote:
> > > > > > > On 2024-05-23 09:48:28, Jan Kara wrote:
> > > > > > > > Hi!
> > > > > > > > 
> > > > > > > > On Wed 22-05-24 12:45:09, Andrey Albershteyn wrote:
> > > > > > > > > On 2024-05-22 12:00:07, Jan Kara wrote:
> > > > > > > > > > Hello!
> > > > > > > > > > 
> > > > > > > > > > On Mon 20-05-24 18:46:21, Andrey Albershteyn wrote:
> > > > > > > > > > > XFS has project quotas which could be attached to a directory. All
> > > > > > > > > > > new inodes in these directories inherit project ID set on parent
> > > > > > > > > > > directory.
> > > > > > > > > > > 
> > > > > > > > > > > The project is created from userspace by opening and calling
> > > > > > > > > > > FS_IOC_FSSETXATTR on each inode. This is not possible for special
> > > > > > > > > > > files such as FIFO, SOCK, BLK etc. as opening them returns a special
> > > > > > > > > > > inode from VFS. Therefore, some inodes are left with empty project
> > > > > > > > > > > ID. Those inodes then are not shown in the quota accounting but
> > > > > > > > > > > still exist in the directory.
> > > > > > > > > > > 
> > > > > > > > > > > This patch adds two new ioctls which allows userspace, such as
> > > > > > > > > > > xfs_quota, to set project ID on special files by using parent
> > > > > > > > > > > directory to open FS inode. This will let xfs_quota set ID on all
> > > > > > > > > > > inodes and also reset it when project is removed. Also, as
> > > > > > > > > > > vfs_fileattr_set() is now will called on special files too, let's
> > > > > > > > > > > forbid any other attributes except projid and nextents (symlink can
> > > > > > > > > > > have one).
> > > > > > > > > > > 
> > > > > > > > > > > Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> > > > > > > > > > 
> > > > > > > > > > I'd like to understand one thing. Is it practically useful to set project
> > > > > > > > > > IDs for special inodes? There is no significant disk space usage associated
> > > > > > > > > > with them so wrt quotas we are speaking only about the inode itself. So is
> > > > > > > > > > the concern that user could escape inode project quota accounting and
> > > > > > > > > > perform some DoS? Or why do we bother with two new somewhat hairy ioctls
> > > > > > > > > > for something that seems as a small corner case to me?
> > > > > > > > > 
> > > > > > > > > So there's few things:
> > > > > > > > > - Quota accounting is missing only some special files. Special files
> > > > > > > > >   created after quota project is setup inherit ID from the project
> > > > > > > > >   directory.
> > > > > > > > > - For special files created after the project is setup there's no
> > > > > > > > >   way to make them project-less. Therefore, creating a new project
> > > > > > > > >   over those will fail due to project ID miss match.
> > > > > > > > > - It wasn't possible to hardlink/rename project-less special files
> > > > > > > > >   inside a project due to ID miss match. The linking is fixed, and
> > > > > > > > >   renaming is worked around in first patch.
> > > > > > > > > 
> > > > > > > > > The initial report I got was about second and last point, an
> > > > > > > > > application was failing to create a new project after "restart" and
> > > > > > > > > wasn't able to link special files created beforehand.
> > > > > > > > 
> > > > > > > > I see. OK, but wouldn't it then be an easier fix to make sure we *never*
> > > > > > > > inherit project id for special inodes? And make sure inodes with unset
> > > > > > > > project ID don't fail to be linked, renamed, etc...
> > > > > > > 
> > > > > > > But then, in set up project, you can cross-link between projects and
> > > > > > > escape quota this way. During linking/renaming if source inode has
> > > > > > > ID but target one doesn't, we won't be able to tell that this link
> > > > > > > is within the project.
> > > > > > 
> > > > > > Well, I didn't want to charge these special inodes to project quota at all
> > > > > > so "escaping quota" was pretty much what I suggested to do. But my point
> > > > > > was that since the only thing that's really charged for these inodes is the
> > > > > > inodes itself then does this small inaccuracy really matter in practice?
> > > > > > Are we afraid the user is going to fill the filesystem with symlinks?
> > > > > 
> > > > > I thought the worry here is that you can't fully reassign the project
> > > > > id for a directory tree unless you have an *at() version of the ioctl
> > > > > to handle the special files that you can't open directly?
> > > > > 
> > > > > So you start with a directory tree that's (say) 2% symlinks and project
> > > > > id 5.  Later you want to set project id 7 on that subtree, but after the
> > > > > incomplete change, projid 7 is charged for 98% of the tree, and 2% are
> > > > > still stuck on projid 5.  This is a mess, and if enforcement is enabled
> > > > > you've just broken it in a way that can't be fixed aside from recreating
> > > > > those files.
> > > > 
> > > > So the idea I'm trying to propose (and apparently I'm failing to explain it
> > > > properly) is:
> > > > 
> > > > When creating special inode, set i_projid = 0 regardless of directory
> > > > settings.
> > > > 
> > > > When creating hardlink or doing rename, if i_projid of dentry is 0, we
> > > > allow the operation.
> > > > 
> > > > Teach fsck to set i_projid to 0 when inode is special.
> > > > 
> > > > As a result, AFAICT no problem with hardlinks, renames or similar. No need
> > > > for special new ioctl or syscall. The downside is special inodes escape
> > > > project quota accounting. Do we care?
> > > 
> > > I see. But is it fine to allow fill filesystem with special inodes?
> > > Don't know if it can be used somehow but this is exception from
> > > isoft/ihard limits then.
> > > 
> > > I don't see issues with this approach also, if others don't have
> > > other points or other uses for those new syscalls, I can go with
> > > this approach.
> > 
> > I do -- allowing unpriviledged users to create symlinks that consume
> > icount (and possibly bcount) in the root project breaks the entire
> > enforcement mechanism.  That's not the way that project quota has worked
> > on xfs and it would be quite rude to nullify the PROJINHERIT flag bit
> > only for these special cases.
> 
> OK, fair enough. I though someone will hate this. I'd just like to
> understand one thing: Owner of the inode can change the project ID to 0
> anyway so project quotas are more like a cooperative space tracking scheme
> anyway. If you want to escape it, you can. So what are you exactly worried
> about? Is it the container usecase where from within the user namespace you
> cannot change project IDs?

Yep.

> Anyway I just wanted to have an explicit decision that the simple solution
> is not good enough before we go the more complex route ;).

Also, every now and then someone comes along and half-proposes making it
so that non-root cannot change project ids anymore.  Maybe some day that
will succeed.

--D

> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
>