xfs: deprecate the V4 format

The V4 filesystem format contains known weaknesses in the on-disk format
that make metadata verification diffiult.  In addition, the format does
not support dates past 2038 and will not be upgraded to do so.  We
should start the process of retiring the old format to close off attack
surfaces and to encourage users to migrate onto V5.

Therefore, make XFS V4 support a configurable option.  For the first
period it will be default Y in case some distributors want to withdraw
support early; for the second period it will be default N so that anyone
who wishes to continue support can do so; and after that, support will
be removed from the kernel.  Dates for these events have been added to
the upstream kernel.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>

fs/xfs/xfs_super.c

@@ -1450,6 +1450,19 @@ xfs_fc_fill_super(
 	if (error)
 		goto out_free_sb;
 
+	/* V4 support is undergoing deprecation. */
+	if (!xfs_sb_version_hascrc(&mp->m_sb)) {
+#ifdef CONFIG_XFS_SUPPORT_V4
+		xfs_warn_once(mp,
+	"Deprecated V4 format (crc=0) will not be supported after September 2030.");
+#else
+		xfs_warn(mp,
+	"Deprecated V4 format (crc=0) not supported by kernel.");
+		error = -EINVAL;
+		goto out_free_sb;
+#endif
+	}
+
 	/*
 	 * XFS block mappings use 54 bits to store the logical block offset.
 	 * This should suffice to handle the maximum file size that the VFS