On Fri, 2020-01-17 at 13:19 -0800, Tycho Andersen wrote: > On Fri, Jan 17, 2020 at 08:25:42AM -0800, James Bottomley wrote: > > On Fri, 2020-01-17 at 09:44 -0600, Serge E. Hallyn wrote: > > > On Thu, Jan 16, 2020 at 08:29:33AM -0800, James Bottomley wrote: > > > I guess I figured we would have privileged task in the owning > > > namespace (presumably init_user_ns) mark a bind mount as > > > shiftable > > > > Yes, that's what I've got today in the prototype. It mirrors the > > original shiftfs mechanism. However, I have also heard people say > > they want a permanent mark, like an xattr for this. > > Please, no. mount() failures are already hard to reason about, I > would rather not add another temporary (or worse, permanent) non- > obvious failure mode. I'm not particularly bothered either way ... although using xattrs always seems to end up biting us for nesting, so I wasn't wildly enthusiastic about it. > What if we make shifted bind mounts always readonly? That will force > people to use an overlay (or something else) on top, but they > probably want to do that anyway so they can avoid tainting the > original container image with writes. That really causes problems for the mutable (non-docker) container use case which is pretty much the way I always use containers. Who wants to bother with overlayfs when their image is expected to mutate: it's just a huge hassle. > > > Oh - I consider the detail of whether we pass a userid or userns > > > nsfd as more of an implementation detail which we can hash out > > > after the more general shift-mount api is decided upon. Anyway, > > > passing nsfds just has a cool factor :) > > > > Well, yes, won't aruge on the cool factor-ness. > > It's not just the cool factor: if you're doing this, it's presumably > because you want to use it with a container in a user namespace. > Specifying the same parameters twice leaves room for error, causing > CVEs and more work. It depends. For the offset, we agreed there's no extant user_ns, so you have to create one specifically. That leads to a more error prone setup with no actual checking benefit. For the shift_ns, it depends whether you want one mount point per tenant, in which case the tenant user_ns might be a useful check, or one mount point with an ACL in which case you just backshift along the binding tenant user_ns. James
