Re: [PATCH] xfs: Verify DA node btree hash order

[Carlos Maiolino]

On Thu, May 01, 2025 at 07:12:49AM -0700, Darrick J. Wong wrote:
> On Wed, Apr 30, 2025 at 11:23:57AM +0200, Carlos Maiolino wrote:
> > On Sat, Apr 12, 2025 at 08:03:57PM +0000, Charalampos Mitrodimas wrote:
> > > The xfs_da3_node_verify() function checks the integrity of directory
> > > and attribute B-tree node blocks. However, it was missing a check to
> > > ensure that the hash values of the btree entries within the node are
> > > strictly increasing, as required by the B-tree structure.
> > >
> > > Add a loop to iterate through the btree entries and verify that each
> > > entry's hash value is greater than the previous one. If an
> > > out-of-order hash value is detected, return failure to indicate
> > > corruption.
> > >
> > > This addresses the "XXX: hash order check?" comment and improves
> > > corruption detection for DA node blocks.
> > >
> > > Signed-off-by: Charalampos Mitrodimas <charmitro@xxxxxxxxxx>
> > > ---
> > >  fs/xfs/libxfs/xfs_da_btree.c | 11 ++++++++++-
> > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_da_btree.c b/fs/xfs/libxfs/xfs_da_btree.c
> > > index 17d9e6154f1978ce5a5cb82176eea4d6b9cd768d..6c748911e54619c3ceae9b81f55cf61da6735f01 100644
> > > --- a/fs/xfs/libxfs/xfs_da_btree.c
> > > +++ b/fs/xfs/libxfs/xfs_da_btree.c
> > > @@ -247,7 +247,16 @@ xfs_da3_node_verify(
> > >  	    ichdr.count > mp->m_attr_geo->node_ents)
> > >  		return __this_address;
> > >
> > > -	/* XXX: hash order check? */
> > > +	/* Check hash order */
> > > +	uint32_t prev_hash = be32_to_cpu(ichdr.btree[0].hashval);
> > > +
> > > +	for (int i = 1; i < ichdr.count; i++) {
> > > +		uint32_t curr_hash = be32_to_cpu(ichdr.btree[i].hashval);
> > > +
> > > +		if (curr_hash <= prev_hash)
> > > +			return __this_address;
> > > +		prev_hash = curr_hash;
> > > +	}
> >
> > Hmmm. Do you have any numbers related to the performance impact of this patch?
> >
> > IIRC for very populated directories we can end up having many entries here. It's
> > not uncommon to have filesystems with millions of entries in a single directory.
> > Now we'll be looping over all those entries here during verification, which could
> > scale to many interactions on this loop.
> > I'm not sure if I'm right here, but this seems to add a big performance penalty
> > for directory writes, so I'm curious about the performance implications of this
> > patch.
> 
> It's only a single dabtree block, which will likely be warm in cache
> due to the crc32c validation.

I assumed this could be called to verify a leaf block, a look at the code
seems that's not the case, so, this is perhaps harmless related to performance.

> 
> But if memory serves, one can create a large enough dir (or xattr)
> structure such that a dabtree node gets written out with a bunch of
> entries with the same hashval.  That was the subject of the correction
> made in commit b7b81f336ac02f ("xfs_repair: fix incorrect dabtree
> hashval comparison") so I've been wondering if this passes the xfs/599
> test?  Or am I just being dumb?
> 
> --D
> 
> > >
> > >  	return NULL;
> > >  }
> > >
> > > ---
> > > base-commit: ecd5d67ad602c2c12e8709762717112ef0958767
> > > change-id: 20250412-xfs-hash-check-be7397881a2c
> > >
> > > Best regards,
> > > --
> > > Charalampos Mitrodimas <charmitro@xxxxxxxxxx>
> > >
> >