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

Andrey Albershteyn
2024-05-22T10:45:15Z

Hi,

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.

--
- Andrey