[PATCH] xfs: deprecate the V4 format

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

The V4 filesystem format contains known weaknesses in the on-disk format
that make metadata verification diffiult.  In addition, the format will
does not support dates past 2038 and will not be upgraded to do so.
Therefore, 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.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/xfs/Kconfig     |   18 ++++++++++++++++++
 fs/xfs/xfs_mount.c |   11 +++++++++++
 2 files changed, 29 insertions(+)

diff --git a/fs/xfs/Kconfig b/fs/xfs/Kconfig
index e685299eb3d2..db54ca9914c7 100644
--- a/fs/xfs/Kconfig
+++ b/fs/xfs/Kconfig
@@ -22,6 +22,24 @@ config XFS_FS
 	  system of your root partition is compiled as a module, you'll need
 	  to use an initial ramdisk (initrd) to boot.
 
+config XFS_SUPPORT_V4
+	bool "Support deprecated V4 format"
+	default y
+	help
+	  The V4 filesystem format lacks certain features that are supported
+	  by the V5 format, such as metadata checksumming, strengthened
+	  metadata verification, and the ability to store timestamps past the
+	  year 2038.  Because of this, the V4 format is deprecated.  All users
+	  should upgrade by backing up their files, reformatting, and restoring
+	  from the backup.
+
+	  This option will become default N in September 2025.  Support for the
+	  V4 format will be removed entirely in September 2030.  Distributors
+	  can say N here to withdraw support earlier.
+
+	  To continue supporting the old V4 format, say Y.
+	  To close off an attack surface, say N.
+
 config XFS_QUOTA
 	bool "XFS Quota support"
 	depends on XFS_FS
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index ed69c4bfda71..48c0175b9457 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -315,6 +315,17 @@ xfs_readsb(
 		goto release_buf;
 	}
 
+#ifndef CONFIG_XFS_SUPPORT_V4
+	/* V4 support is undergoing deprecation. */
+	if (!xfs_sb_version_hascrc(sbp)) {
+		if (loud)
+			xfs_warn(mp,
+	"Deprecated V4 format not supported by kernel.");
+		error = -EINVAL;
+		goto release_buf;
+	}
+#endif
+
 	/*
 	 * We must be able to do sector-sized and sector-aligned IO.
 	 */