On Wed, Feb 08, 2017 at 07:22:45AM -0800, James Bottomley wrote: > On Tue, 2017-02-07 at 17:54 -0800, Josh Triplett wrote: > > On Tue, Feb 07, 2017 at 11:49:33AM -0800, Christoph Hellwig wrote: > > > On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote: > > > > > Another option would be to require something like a project > > > > > as used > > > > > for project quotas as the root. This would also be conveniant > > > > > as it > > > > > could storge the used remapping tables. > > > > > > > > So this would be like the current project quota except set on a > > > > subtree? I could see it being done that way but I don't see what > > > > advantage it has over using flags in the subtree itself (the > > > > mapping is > > > > known based on the mount namespace, so there's really only a > > > > single bit > > > > of information to store). > > > > > > projects (which are the underling concept for project quotas) are > > > per-subtree in practice - the flag is set on an inode and then > > > all directories and files underneath inherit the project ID, > > > hardlinking outside a project is prohinited. > > > > I'm interested in having a VFS-level way to do more than just a > > shift; I'd like to be able to arbitrarily remap IDs between what's on > > disk and the system IDs. > > OK, so the shift is effectively an arbitrary remap because it allows > multiple ranges to be mapped (althought the userns currently imposes a > maximum number of five extents but that limit is a bit arbitrary just > to try to limit the amount of space the parametrisation takes). See > kernel/user_namespace.c:map_id_up/down() > > > If we're talking about developing a VFS-level solution for this, > > I'd like to avoid limiting it to just a shift. (A shift/range > > would definitely be the simplest solution for many common container > > cases, but not all.) > > I assume the above satisfies you on this point, but raises the > question: do you want an arbitrary shift not parametrised by a user > namespace? If so how many such shifts do you want ... giving some > details of the use case would be helpful. The limit of five extents means this may not work in the most general case, no. One use case: given an on-disk filesystem, its name-to-number mapping, and your host name-to-number mapping, mount the filesystem with all the UIDs bidirectionally mapped to those on your host system. Another use case: given an on-disk filesystem with potentially arbitrary UIDs (not necessarily in a clean contiguous block), and a pile of unprivileged UIDs, mount the filesystem such that every on-disk UID gets a unique unprivileged UID. (I have some additional use cases, but they would require the ability to extend the mapping on the fly without remounting.)
