Re: [PATCH RFC v2] fhandle: expose u64 mount id to name_to_handle_at(2)

Jan Kara
2024-06-03T10:30:30Z

On Sat 01-06-24 01:12:31, Aleksa Sarai wrote:
> On 2024-05-28, Miklos Szeredi <miklos@szeredi.hu> wrote:
> > On Tue, 28 May 2024 at 15:24, Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > > On Tue, May 28, 2024 at 02:04:16PM +0200, Christian Brauner wrote:
> > > > Can you please explain how opening an fd based on a handle returned from
> > > > name_to_handle_at() and not using a mount file descriptor for
> > > > open_by_handle_at() would work?
> > >
> > > Same as NFS file handles:
> > >
> > > name_to_handle_at returns a handle that includes a file system
> > > identifier.
> > >
> > > open_by_handle_at looks up the superblock based on that identifier.
> >
> > The open file needs a specific mount, holding the superblock is not sufficient.
>
> Not to mention that providing a mount fd is what allows for extensions
> like Christian's proposed method of allowing restricted forms of
> open_by_handle_at() to be used by unprivileged users.
>
> If file handles really are going to end up being the "correct" mechanism
> of referencing inodes by userspace, then future API designs really need
> to stop assuming that the user is capable(CAP_DAC_READ_SEARCH). Being
> able to open any file in any superblock the kernel knows about
> (presumably using a kernel-internal mount if we are getting rid of the
> mount fd) is also capable(CAP_SYS_ADMIN) territory.

Well, but this is already handled - name_to_handle_at() with AT_HANDLE_FID
is completely unpriviledged operation. Unpriviledged userspace can use
fhandle for comparisons with other file handles but that's all it is good
for (similarly as inode number you get from statx(2) but does not have the
problem with inode number uniqueness on btrfs, bcachefs, etc.). I don't
expect unpriviledged userspace to be able to more with the fhandle it got.

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