Re: [RFC PATCH 0/6] shiftfs fixes and enhancements

[Seth Forshee <seth.forshee@canonical.com>]

On Fri, Nov 02, 2018 at 10:59:38AM +0200, Amir Goldstein wrote:
> [cc: linux-unionfs
> It should the mailing list for *all* "stacking fs".
> We have a lot of common problems I think ;-) ]
> 
> On Thu, Nov 1, 2018 at 11:49 PM Seth Forshee <seth.forshee@canonical.com> wrote:
> >
> > I've done some work to fix and enhance shiftfs for a number of use
> > cases, so that we would have an idea what a more full-featured shiftfs
> > would look like. I'm intending for these to serve as a point of
> > reference for discussing id shifting mounts/filesystems at plumbers in a
> > couple of weeks [1].
> >
> > Note that these are based on 4.18, and I've added a small fix to James'
> > most recent patch to fix a build issue there. To work with 4.19 they
> > will need a number of updates due to changes in the vfs.
> >
> 
> Seth,
> 
> I like the way you addressed my concerns about nesting and stacking depth.
> Will provide specific nits on patch.
> 
> In preparation to the Plumbers talk (which I will not be attending), I wanted to
> get your opinion on the matters I brought up last time:
> https://marc.info/?l=linux-fsdevel&m=153013920904844&w=2

I want the session at plumbers to not be a "talk" but more of a
discussion of the sorts of things you raise below. But I'm also happy to
talk about them here.

> 1) Having seen what it takes to catch up with overlayfs w.r.t inotify bugs
> and having peeked into 4.19 to see what work you still have lined up for you
> to bring shitfs up to speed with vfs, did you have time to look into my proposal
> for sharing code with overlayfs in the manner that I have implemented the
> snapshotfs POC?
> https://github.com/amir73il/linux/commit/25416757f2ca47759f59b115e6461b11898c4f06
> 
> Even if you end up not saving a single line of code for shiftfs v1
> meaning that all shiftfs_inode_ops are completely separate implementation
> from overlayfs inode ops, this may still be beneficial to shitfs in
> the long run.
> For example, you may, in fact, won't need to change anything to work with v4.19.
> shittfs (as an overlayfs alias) would use ovl_file_operations and
> shiftfs_inode_ops.

I don't recall seeing the shapshotfs patches before. If id shifting
remains an overlay-style fs and not a feature of the vfs, then I
absolutely think something like this will make life much easier.

> Another example, from the top of my head, see what it took to add NFS export
> support to snapshotfs, because of the code reuse with overlayfs:
> https://github.com/amir73il/linux/commit/d082eb615133490ec26fa2efaa80ed4723860893
> Its practically the exact same implementation shiftfs would need,
> so in the far future, shitfs and snapshotfs can share the same
> export_operations.
> 
> 2) Regarding this part:
> +               /*
> +                * this part is visible unshifted, so make sure no
> +                * executables that could be used to give suid
> +                * privileges
> +                */
> +               sb->s_iflags = SB_I_NOEXEC;
> 
> Why would you want to make the unshifted fs visible at all?
> Is there a requirement for container users to access the unshifted fs
> content? Is there a requirement for container admin to mount shitfted fs
> NOT from the root of the marked mount?
> 
> If those are not required, then I propose NOOP inode operations for
> the unshifted fs, specifically, empty readdir, just enough ops to be able
> to use the mark mount point as the shitfed mount source - no more.

This is part of the original implementation that I didn't touch with
these updates. Imo the mark mount is kind of kludgy, and I'd like to see
it done a different way.

A couple of alternatives have been suggested. One was to use xattrs for
marking, or I did a PoC with an older version of the new mount API
patches where an fsfd was passed to the less privileged context that it
could attach to its mount tree:

https://lkml.kernel.org/r/20180717133847.GB15620@ubuntu-xps13

Either of these can accomplish the same things as the mark mount with
better control over who can create an id-shifted mount of the subtree.

However if the mark mount is kept then no-op inode operations seems
reasonable to me.

Thanks,
Seth