linux-kernel - Re: [PATCH] bcachefs: Fix NULL ptr dereference in btree_node_iter_and_journal

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Message-ID: <6uve5q7c6o4bvuifa2klcwpussxpy5cvcjf2pqdezfuvneqjaz@kdj67a2ehnrf>
Date: Fri, 25 Oct 2024 20:16:24 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Alan Huang <mmpgouride@...il.com>
Cc: Piotr Zalewski <pZ010001011111@...ton.me>, 
	linux-bcachefs@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, skhan@...uxfoundation.org, 
	syzbot+005ef9aa519f30d97657@...kaller.appspotmail.com
Subject: Re: [PATCH] bcachefs: Fix NULL ptr dereference in
 btree_node_iter_and_journal_peek

On Fri, Oct 25, 2024 at 08:11:50PM -0400, Kent Overstreet wrote:
> On Wed, Oct 23, 2024 at 03:33:22PM +0800, Alan Huang wrote:
> > On Oct 23, 2024, at 15:21, Piotr Zalewski <pZ010001011111@...ton.me> wrote:
> > > 
> > > Add NULL check for key returned from bch2_btree_and_journal_iter_peek in
> > > btree_node_iter_and_journal_peek to avoid NULL ptr dereference in
> > > bch2_bkey_buf_reassemble.
> > 
> > It would be helpful if the commit message explained why k.k is null in this case
> 
> This code is only for iterating over interior btree nodes - k.k is only
> null when we have a bad btree topology (gaps).
> 
> Piotr, could you add a comment to that effect?

Actually, not just that - when this happens we should flag the
filesystem as having topology repairs, and possibly start topology
repair.

Calling bch2_topology_error() will do that.

We definitely want to log an error message, too; it should reference the
btree node we're iterating over and explain that it's missing child
nodes.