Re: [PATCH] xfs: deprecate the V4 format

[Dave Chinner <david@fromorbit.com>]

On Thu, Sep 10, 2020 at 11:27:06AM -0700, Darrick J. Wong wrote:
> 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

Fine by me.

You forgot to add the V4 format to the deprecation schedule in
Documentation/filesystems/.....

<nggggh>

(where TF has this bit of XFS documentation been moved to???)

in Documentation/admin-guide/xfs.rst.

-Dave.

[ Why did that file get moved rather than just linked from it's
original spot in all the -filesystem- documentation?  Can we move it
back, please, like all the other <filesystem>.rst files in
Documentation/filesystems/ ? ]


-- 
Dave Chinner
david@fromorbit.com