Re: [PATCH] xfs: don't crash when read verifier reports errors with null b_ops

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

On Wed, Jan 16, 2019 at 07:23:36AM +1100, Dave Chinner wrote:
> On Tue, Jan 15, 2019 at 09:08:20AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > In xrep_findroot_block, we work out the btree type and correctness of a
> > given block by calling different btree verifiers on root block
> > candidates.  However, we leave the NULL b_ops while ->verify_read
> > validates the block, which means that if the verifier calls
> > xfs_buf_verifier_error it'll crash on the null b_ops.  Fix it to avoid
> > this crash.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> > FWIW I also thought about stuffing in a fake b_ops while xrep_findroot
> > is calling a candidate read verifier, but thought that might just
> > confuse things more?
> > ---
> >  fs/xfs/xfs_error.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_error.c b/fs/xfs/xfs_error.c
> > index 9866f542e77b..2a02be280769 100644
> > --- a/fs/xfs/xfs_error.c
> > +++ b/fs/xfs/xfs_error.c
> > @@ -359,7 +359,7 @@ xfs_buf_verifier_error(
> >  
> >  	xfs_alert(mp, "Metadata %s detected at %pS, %s block 0x%llx %s",
> >  		  bp->b_error == -EFSBADCRC ? "CRC error" : "corruption",
> > -		  fa, bp->b_ops->name, bp->b_bn, name);
> > +		  fa, bp->b_ops ? bp->b_ops->name : "unknown", bp->b_bn, name);
> 
> Looks fine, but can you add a comment to xfs_buf_verifier_error()
> that says that some callers might be running the verifiers
> themselves rather than through the b_ops structure and so it may be
> null...

Will do.

--D

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