xfs: disallow rw remount on fs with unknown ro-compat features

commit d0a58e833931234c44e515b5b8bede32bd4e6eed upstream.

Today, a kernel which refuses to mount a filesystem read-write
due to unknown ro-compat features can still transition to read-write
via the remount path.  The old kernel is most likely none the wiser,
because it's unaware of the new feature, and isn't using it.  However,
writing to the filesystem may well corrupt metadata related to that
new feature, and moving to a newer kernel which understand the feature
will have problems.

Right now the only ro-compat feature we have is the free inode btree,
which showed up in v3.16.  It would be good to push this back to
all the active stable kernels, I think, so that if anyone is using
newer mkfs (which enables the finobt feature) with older kernel
releases, they'll be protected.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 36bd8825bfb03cd2faafbecd202730869f8321d3..ef64a1e1a66a3c85b0c49cb5d74a14aaaff94c78 100644 (file)
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1233,6 +1233,16 @@ xfs_fs_remount(
                        return -EINVAL;
                }
 
+               if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5 &&
+                   xfs_sb_has_ro_compat_feature(sbp,
+                                       XFS_SB_FEAT_RO_COMPAT_UNKNOWN)) {
+                       xfs_warn(mp,
+"ro->rw transition prohibited on unknown (0x%x) ro-compat filesystem",
+                               (sbp->sb_features_ro_compat &
+                                       XFS_SB_FEAT_RO_COMPAT_UNKNOWN));
+                       return -EINVAL;
+               }
+
                mp->m_flags &= ~XFS_MOUNT_RDONLY;
 
                /*