On Thu, Jan 14, 2021 at 09:12:41AM -0800, Darrick J. Wong wrote: > On Tue, Jan 12, 2021 at 11:00:42PM +0100, Christian Brauner wrote: > > Hey everyone, > > > > The only major change is the inclusion of hch's patch to port XFS to > > support idmapped mounts. Thanks to Christoph for doing that work. > > Yay :) > > > (For a full list of major changes between versions see the end of this > > cover letter. > > Please also note the large xfstests testsuite in patch 42 that has been > > kept as part of this series. It verifies correct vfs behavior with and > > without idmapped mounts including covering newer vfs features such as > > io_uring. > > I currently still plan to target the v5.12 merge window.) > > > > With this patchset we make it possible to attach idmappings to mounts, > > i.e. simply put different bind mounts can expose the same file or > > directory with different ownership. > > Shifting of ownership on a per-mount basis handles a wide range of > > long standing use-cases. Here are just a few: > > - Shifting of a subset of ownership-less filesystems (vfat) for use by > > multiple users, effectively allowing for DAC on such devices > > (systemd, Android, ...) > > - Allow remapping uid/gid on external filesystems or paths (USB sticks, > > network filesystem, ...) to match the local system's user and groups. > > (David Howells intends to port AFS as a first candidate.) > > - Shifting of a container rootfs or base image without having to mangle > > every file (runc, Docker, containerd, k8s, LXD, systemd ...) > > - Sharing of data between host or privileged containers with > > unprivileged containers (runC, Docker, containerd, k8s, LXD, ...) > > - Data sharing between multiple user namespaces with incompatible maps > > (LXD, k8s, ...) > > That sounds neat. AFAICT, the VFS passes the filesystem a mount userns > structure, which is then carried down the call stack to whatever > functions actually care about mapping kernel [ug]ids to their ondisk > versions? Yes. This requires not too many changes to the actual filesystems as you can see from the xfs conversion that Christoph has done. > > Does quota still work after this patchset is applied? There isn't any > mention of that in the cover letter and I don't see a code patch, so > does that mean everything just works? I'm particularly curious about The most interesting quota codepaths I audited are dquot_transfer that transfers quota from one inode to another one during setattr. That happens via a struct iattr which will already contain correctly translated ia_uid and ia_gid values according to the mount the caller is coming from. I'll take another close look at that now and add tests for that if I can find some in xfstests. > whether there can exist processes with CAP_SYS_ADMIN and an idmapped > mount? Syscalls like bulkstat and quotactl present file [ug]ids to Yes, that should be possible. > programs, but afaict there won't be any translating going on? quotactl operates on the superblock. So the caller would need a mapping in the user namespace of the superblock. That doesn't need to change. But we could in the future extend this to be on a per-mount basis if this was a desired use-case. I don't think it needs to happen right now though. > > (To be fair, bulkstat is an xfs-only thing, but quota control isn't.) I'm certain we'll find more things to cover after the first version has landed. :) We for sure won't cover it all in the first iteration. > > I'll start skimming the patchset... Thanks! Christian
