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    Subject[PATCH 3.16 008/305] xfs: disallow rw remount on fs with unknown ro-compat features
    3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Sandeen <sandeen@redhat.com>

    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>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/xfs/xfs_super.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    --- a/fs/xfs/xfs_super.c
    +++ b/fs/xfs/xfs_super.c
    @@ -1250,6 +1250,16 @@ xfs_fs_remount(

    /* ro -> rw */
    if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(*flags & MS_RDONLY)) {
    + 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;

    /*
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      Last update: 2016-09-17 09:56    [W:4.037 / U:0.024 seconds]
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