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

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.

Cc: <stable@vger.kernel.org> # 3.10.x-
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>

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index d760934109b5d628891ea3e91fa1a9e3f68ec36b..ca058a153c15d35fb9568b41ea71dbead56d3149 100644 (file)
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -1276,6 +1276,16 @@ 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;
 
                /*