On Wed, Jan 25, 2023 at 9:45 PM Giuseppe Scrivano <gscrivan@xxxxxxxxxx> wrote: > > Amir Goldstein <amir73il@xxxxxxxxx> writes: > > >> >> I previously mentioned my wish of using it from a user namespace, the > >> >> goal seems more challenging with EROFS or any other block devices. I > >> >> don't know about the difficulty of getting overlay metacopy working in a > >> >> user namespace, even though it would be helpful for other use cases as > >> >> well. > >> >> > >> > > >> > There is no restriction of metacopy in user namespace. > >> > overlayfs needs to be mounted with -o userxattr and the overlay > >> > xattrs needs to use user.overlay. prefix. > >> > >> if I specify both userxattr and metacopy=on then the mount ends up in > >> the following check: > >> > >> if (config->userxattr) { > >> [...] > >> if (config->metacopy && metacopy_opt) { > >> pr_err("conflicting options: userxattr,metacopy=on\n"); > >> return -EINVAL; > >> } > >> } > >> > > > > Right, my bad. > > > >> to me it looks like it was done on purpose to prevent metacopy from a > >> user namespace, but I don't know the reason for sure. > >> > > > > With hand crafted metacopy, an unpriv user can chmod > > any files to anything by layering another file with different > > mode on top of it.... > > I might be missing something obvious about metacopy, so please correct > me if I am wrong, but I don't see how it is any different than just > copying the file and chowning it. Of course, as long as overlay uses > the same security model so that a file that wasn't originally possible > to access must be still blocked, even if referenced through metacopy. > You're right. The reason for mutual exclusion maybe related to the comment in ovl_check_metacopy_xattr() about EACCES. Need to check with Vivek or Miklos. But get this - you do not need metacopy=on to follow lower inode. It should work without metacopy=on. metacopy=on only instructs overlayfs whether to copy up data or only metadata when changing metadata of lower object, so it is not relevant for readonly mount. Thanks, Amir.