Re: [PATCH] xfs: check v5 superblock features early

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 28, 2026 at 08:26:28AM -0700, Darrick J. Wong wrote:
> On Tue, Jul 28, 2026 at 10:04:35AM +0200, Christoph Hellwig wrote:
> > When working on a new features that reuses the existing pad in the
> > superblock, I noticed that mounting such a file system on an old kernel
> > logs a rather confusing warning:
> > 
> >     XFS (vdc): Metadir superblock padding fields must be zero.
> > 
> > This is because we only validate the various feature fields in v5
> > superblocks after the common superblock validation helper is called.
> > 
> > Fix this by calling the feature validation first.  To make this more
> > obvious, rename xfs_validate_sb_read to xfs_validate_sb_features and only
> > call it for v5 file systems.
> 
> Shouldn't it be called xfs_validate_v5_sb_features then?
> 
> Oh.  There's already a xfs_sb_validate_v5_features function that checks
> that the v4 feature bits are set correctly for a v5 filesystem, and it
> would be confusing to have both.

Or just stick to the old name :)

> Could we move the code in
> xfs_validate_sb_read into xfs_sb_validate_v5_features instead?

But yes, that does look sensible as well.

> If you do that, then xfs_sb_good_version will validate the feature bit
> recognition, which it doesn't do now.  I'm not sure what weird side
> effects might result from that though.

None in the kernel, but userspace has a lot more callers that need a
careful look.