Re: [PATCH 6/6] xfs_repair: check plausibility of root dir pointer before trashing it\

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

On Thu, Dec 05, 2019 at 09:38:58AM -0500, Brian Foster wrote:
> On Wed, Dec 04, 2019 at 09:05:02AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > If sb_rootino doesn't point to where we think mkfs should have allocated
> > the root directory, check to see if the alleged root directory actually
> > looks like a root directory.  If so, we'll let it live because someone
> > could have changed sunit since formatting time, and that changes the
> > root directory inode estimate.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  repair/xfs_repair.c |   45 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 45 insertions(+)
> > 
> > 
> > diff --git a/repair/xfs_repair.c b/repair/xfs_repair.c
> > index abd568c9..b0407f4b 100644
> > --- a/repair/xfs_repair.c
> > +++ b/repair/xfs_repair.c
> > @@ -426,6 +426,37 @@ _("would reset superblock %s inode pointer to %"PRIu64"\n"),
> >  	*ino = expected_ino;
> >  }
> >  
> > +/* Does the root directory inode look like a plausible root directory? */
> > +static bool
> > +has_plausible_rootdir(
> > +	struct xfs_mount	*mp)
> > +{
> > +	struct xfs_inode	*ip;
> > +	xfs_ino_t		ino;
> > +	int			error;
> > +	bool			ret = false;
> > +
> > +	error = -libxfs_iget(mp, NULL, mp->m_sb.sb_rootino, 0, &ip,
> > +			&xfs_default_ifork_ops);
> > +	if (error)
> > +		goto out;
> > +	if (!S_ISDIR(VFS_I(ip)->i_mode))
> > +		goto out_rele;
> > +
> > +	error = -libxfs_dir_lookup(NULL, ip, &xfs_name_dotdot, &ino, NULL);
> > +	if (error)
> > +		goto out_rele;
> > +
> > +	/* The root directory '..' entry points to the directory. */
> > +	if (ino == mp->m_sb.sb_rootino)
> > +		ret = true;
> > +
> > +out_rele:
> > +	libxfs_irele(ip);
> > +out:
> > +	return ret;
> > +}
> > +
> >  /*
> >   * Make sure that the first 3 inodes in the filesystem are the root directory,
> >   * the realtime bitmap, and the realtime summary, in that order.
> > @@ -436,6 +467,20 @@ calc_mkfs(
> >  {
> >  	xfs_ino_t		rootino = libxfs_ialloc_calc_rootino(mp, -1);
> >  
> > +	/*
> > +	 * If the root inode isn't where we think it is, check its plausibility
> > +	 * as a root directory.  It's possible that somebody changed sunit
> > +	 * since the filesystem was created, which can change the value of the
> > +	 * above computation.  Don't blow up the root directory if this is the
> > +	 * case.
> > +	 */
> > +	if (mp->m_sb.sb_rootino != rootino && has_plausible_rootdir(mp)) {
> > +		do_warn(
> > +_("sb root inode value %" PRIu64 " inconsistent with alignment (expected %"PRIu64")\n"),
> > +			mp->m_sb.sb_rootino, rootino);
> > +		rootino = mp->m_sb.sb_rootino;
> > +	}
> > +
> 
> A slightly unfortunate side effect of this is that there's seemingly no
> straightforward way for a user to "clear" this state/warning. We've
> solved the major problem by allowing repair to handle this condition,
> but AFAICT this warning will persist unless the stripe unit is changed
> back to its original value.

Heh, I apparently never replied to this. :(

> IOW, what if this problem exists simply because a user made a mistake
> and wants to undo it? It's probably easy enough for us to say "use
> whatever you did at mkfs time," but what if that's unknown or was set
> automatically? I feel like that is the type of thing that in practice
> could result in unnecessary bugs or error reports unless the tool can
> make a better suggestion to the end user. For example, could we check
> the geometry on secondary supers (if they exist) against the current
> rootino and use that as a secondary form of verification and/or suggest
> the user reset to that geometry (if desired)?

That sounds reasonable.

> OTOH, I guess we'd have to consider what happens if the filesystem was
> grown in that scenario too..  :/

I think it would be fine, so long as we're careful with the if-then
chain.  Specifically:

a. If we dislike the rootino that we compute with the ondisk sunit value,
and...

b. The thing sb_rootino points to actually does look like the root
directory, and...

c. One of the secondary supers has an sunit value that gives us a
rootino calculation that matches the sb_rootino that we just checked
out...

...then we'll propose correcting the primary sb_unit to the value we
found in (c).

> 
> (Actually on a quick test, it looks like growfs updates every super,
> even preexisting..).

I'll throw that onto the V3 series.

--D

> 
> Brian
> 
> >  	ensure_fixed_ino(&mp->m_sb.sb_rootino, rootino,
> >  			_("root"));
> >  	ensure_fixed_ino(&mp->m_sb.sb_rbmino, rootino + 1,
> > 
>