On Thu, Oct 29, 2020 at 05:05:02PM +0100, Lennart Poettering wrote: > On Do, 29.10.20 10:47, Eric W. Biederman (ebiederm@xxxxxxxxxxxx) wrote: > > > Is that the use case you are looking at removing the need for > > systemd-homed to avoid chowning after lugging encrypted home directories > > from one system to another? Why would it be desirable to avoid the > > chown? > > Yes, I am very interested in seeing Christian's work succeed, for the > usecase in systemd-homed. In systemd-homed each user gets their own > private file system, and these fs shall be owned by the user's local > UID, regardless in which system it is used. The UID should be an > artifact of the local, individual system in this model, and thus > the UID on of the same user/home on system A might be picked as 1010 > and on another as 1543, and on a third as 1323, and it shouldn't > matter. This way, home directories become migratable without having to > universially sync UID assignments: it doesn't matter anymore what the > local UID is. > > Right now we do a recursive chown() at login time to ensure the home > dir is properly owned. This has two disadvantages: > > 1. It's slow. In particular on large home dirs, it takes a while to go > through the whole user's homedir tree and chown/adjust ACLs for > everything. > > 2. Because it is so slow we take a shortcut right now: if the > top-level home dir inode itself is owned by the correct user, we > skip the recursive chowning. This means in the typical case where a > user uses the same system most of the time, and thus the UID is > stable we can avoid the slowness. But this comes at a drawback: if > the user for some reason ends up with files in their homedir owned > by an unrelated user, then we'll never notice or readjust. > > > If the goal is to solve fragmented administration of uid assignment I > > suggest that it might be better to solve the administration problem so > > that all of the uids of interest get assigned the same way on all of the > > systems of interest. > > Well, the goal is to make things simple and be able to use the home > dir everywhere without any prior preparation, without central UID > assignment authority. > > The goal is to have a scheme that requires no administration, by > making the UID management problem go away. Hence, if you suggest > solving this by having a central administrative authority: this is > exactly what the model wants to get away from. > > Or to say this differently: just because I personally use three > different computers, I certainly don't want to set up LDAP or sync > UIDs manually. > > Lennart > > -- > Lennart Poettering, Berlin Can you help me understand systemd-homed a little bit? In the man page it says: systemd-homed is a system service that may be used to create, remove, change or inspect home areas (directories and network mounts and real or loopback block devices with a filesystem, optionally encrypted). It seems that the "underlay?" (If you'll call it that, maybe there is a better term) can either be a standalone block device (this sounds close to systemd machined?), a btrfs subvolume (which receives its own superblock (IIRC?, I might be wrong. It's been a while since I've used btrfs), or just be a directory that's mapped? What decides whether it's just a directory and bind-mounted (or a similar vfsmount), or an actual superblock? How is the mapping of "real UIDs" to "namespace UIDs" works when it's just a bind mount? From the perspective of multiple user namespaces, are all "underlying" UIDs mapped through, or if I try to look at another user's home directory will they not show up? Is there a reason you can't / don't / wont use overlayfs instead of bind mounts?
