Re: allowing for a completely cached umount(2) pathwalk

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2023-04-14 at 11:41 +0200, Christian Brauner wrote:
> On Fri, Apr 14, 2023 at 04:51:04AM +0100, Al Viro wrote:
> > On Fri, Apr 14, 2023 at 03:28:45AM +0000, Trond Myklebust wrote:
> > 
> > > We already have support for directory file descriptors when mounting with move_mount(). Why not add a umountat() with similar support for the unmount side?
> > > Then add a syscall to allow users with (e.g.) the CAP_DAC_OVERRIDE privilege to convert the mount-id into an O_PATH file descriptor.
> > 
> > You can already do umount -l /proc/self/fd/69 if you have a descriptor.
> 
> Way back when we put together stuff for [2] we had umountat() as an item
> but decided against it because it's mostely useful when used as AT_EMPTY_PATH.
> 
> umount("/proc/self/fd/<nr>", ...) is useful when you don't trust the
> path and you need to resolve it with lookup restrictions. Then path
> resolution restrictions of openat2() can be used to get an fd. Which can
> be passed to umount().
> 
> I need to step outside so this is a halfway-out-the-door thought but
> given your description of the problem Jeff, why doesn't the following
> work (Just sketching this, you can't call openat2() like that.):
> 
>         fd_mnt = openat2("/my/funky/nfs/share/mount", RESOLVE_CACHED)
>         umount("/proc/self/fd/fd_mnt", MNT_DETACH)

Something like that might work. A RESOLVE_CACHED flag is something that
would involve more than just umount(2) though. That said, it could be
useful in other situations.

> 
> > Converting mount-id to O_PATH... might be an interesting idea.
> 
> I think using mount-ids would be nice and fwiw, something we considered
> as an alternative to umountat(). Not just can they be gotten from
> /proc/<pid>/mountinfo but we also do expose the mount id to userspace
> nowadays through:
> 
>         STATX_MNT_ID
>         __u64	stx_mnt_id;
> 
> which also came out of [2]. And it should be safe to do via
> AT_STATX_DONT_SYNC:
> 
>         statx(my_cached_fd, AT_NO_AUTOMOUNT|AT_SYMLINK_NOFOLLOW|AT_STATX_DONT_SYNC)
> 
> using STATX_ATTR_MOUNT_ROOT to identify a potential mountpoint. Then
> pass that mount-id to the new system call.
> 
> [2]: https://github.com/uapi-group/kernel-features

This is generating a lot of good ideas! Maybe we should plan to discuss
this further at LSF/MM?

-- 
Jeff Layton <jlayton@kernel.org>