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

Darrick J. Wong
2019-12-12T22:46:26Z

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,
> >
>