Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes: > On Mon, Mar 12, 2018 at 03:23:44PM -0500, Eric W. Biederman wrote: > >> Of the two code paths you are concert about: >> >> For path path_connected looking at s_root is a heuristic to avoid >> calling is_subdir every time we need to do that check. If the heuristic >> fails we still have is_subdir which should remain accurate. If >> is_subdir fails the path is genuinely not connected at that moment >> and failing is the correct thing to do. > > Umm... That might be not good enough - the logics is "everything's > reachable from ->s_root anyway, so we might as well not bother checking". > For NFS it's simply not true. If I am parsing the code correctly path_connected is broken for nfsv2 and nfsv3 when NFS_MOUNT_UNSHARED is not set. nfsv4 appears to make a kernel mount of the real root of the filesystem properly setting s_root and then finds the child it is mounting. > We can mount server:/foo/bar/baz on /tmp/a, then server:/foo on /tmp/b > and we'll have ->s_root pointing to a subtree of what's reachable at > /tmp/b. Play with renames under /tmp/b and you just might end up with > a problem. And mount on /tmp/a will be (mistakenly) considered to > be safe, since it satisfies the heuristics in path_connected(). Agreed. Which means that if you mount server:/foo/bar/baz first and then mount server:/foo with an appropriate rename you might be able to see all of server:/foo or possibly server:/ Hmm.. Given that nfs_kill_super calls generic_shutdown_super and generic_shutdown_super calls shrink_dcache_for_umount I would argue that nfsv2 and nfsv3 are buggy in the same case, as shrink_dcache_for_umount is called on something that is not the root of the filesystem's dentry tree. Eric
