Re: [PATCH v3] xfs: deprecate the V4 format

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

On Mon, Sep 14, 2020 at 08:29:09AM +0100, Christoph Hellwig wrote:
> On Fri, Sep 11, 2020 at 09:43:11AM -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>
> > ---
> > v3: be a little more helpful about old xfsprogs and warn more loudly
> > about deprecation
> > v2: define what is a V4 filesystem, update the administrator guide
> 
> Whie this patch itself looks good, I think the ifdef as is is rather
> silly as it just prevents mounting v4 file systems without reaping any
> benefits from that.
> 
> So at very least we should add a little helper like this:
> 
> static inline bool xfs_sb_is_v4(truct xfs_sb *sbp)
> {
> 	if (IS_ENABLED(CONFIG_XFS_SUPPORT_V4))
> 		return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4;
> 	return false;
> }
> 
> and use it in all the feature test macros to let the compile eliminate
> all the dead code.

Oh, wait, you meant as a means for future patches to make various bits
of code disappear, not just as a weird one-off thing for this particular
patch?

I mean... maybe we should just stuff that into the hascrc predicate,
like Eric sort of implied on irc.  Hmm, I'll look into that.

--D