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

Jan Kara
2024-05-23T07:48:34Z

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...

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