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

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

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.

--D

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