Re: [PATCH] xfs: deprecate the V4 format

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

On Fri, Sep 11, 2020 at 07:39:51AM +1000, Dave Chinner wrote:
> 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.

Aha, I knew I was missing something!  I looked for that this morning but
couldn't find the file.

> -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/ ? ]

I think it got moved to the administration guide since mount options and
feature discussion is an ... administrative concern?

--D

> 
> -- 
> Dave Chinner
> david@fromorbit.com