• Eric W. Biederman's avatar
    fs: Teach path_connected to handle nfs filesystems with multiple roots. · 95dd7758
    Eric W. Biederman authored
    On nfsv2 and nfsv3 the nfs server can export subsets of the same
    filesystem and report the same filesystem identifier, so that the nfs
    client can know they are the same filesystem.  The subsets can be from
    disjoint directory trees.  The nfsv2 and nfsv3 filesystems provides no
    way to find the common root of all directory trees exported form the
    server with the same filesystem identifier.
    
    The practical result is that in struct super s_root for nfs s_root is
    not necessarily the root of the filesystem.  The nfs mount code sets
    s_root to the root of the first subset of the nfs filesystem that the
    kernel mounts.
    
    This effects the dcache invalidation code in generic_shutdown_super
    currently called shrunk_dcache_for_umount and that code for years
    has gone through an additional list of dentries that might be dentry
    trees that need to be freed to accomodate nfs.
    
    When I wrote path_connected I did not realize nfs was so special, and
    it's hueristic for avoiding calling is_subdir can fail.
    
    The practical case where this fails is when there is a move of a
    directory from the subtree exposed by one nfs mount to the subtree
    exposed by another nfs mount.  This move can happen either locally or
    remotely.  With the remote case requiring that the move directory be cached
    before the move and that after the move someone walks the path
    to where the move directory now exists and in so doing causes the
    already cached directory to be moved in the dcache through the magic
    of d_splice_alias.
    
    If someone whose working directory is in the move directory or a
    subdirectory and now starts calling .. from the initial mount of nfs
    (where s_root == mnt_root), then path_connected as a heuristic will
    not bother with the is_subdir check.  As s_root really is not the root
    of the nfs filesystem this heuristic is wrong, and the path may
    actually not be connected and path_connected can fail.
    
    The is_subdir function might be cheap enough that we can call it
    unconditionally.  Verifying that will take some benchmarking and
    the result may not be the same on all kernels this fix needs
    to be backported to.  So I am avoiding that for now.
    
    Filesystems with snapshots such as nilfs and btrfs do something
    similar.  But as the directory tree of the snapshots are disjoint
    from one another and from the main directory tree rename won't move
    things between them and this problem will not occur.
    
    Cc: stable@vger.kernel.org
    Reported-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
    Fixes: 397d425d ("vfs: Test for and handle paths that are unreachable from their mnt_root")
    Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    95dd7758
fs.h 109 KB