linux-kernel - Re: [syzbot] [udf?] BUG: unable to handle kernel NULL pointer dereference in _

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Date:   Mon, 23 Jan 2023 08:36:09 +0100
From:   Christoph Hellwig <hch@....de>
To:     syzbot <syzbot+c27475eb921c46bbdc62@...kaller.appspotmail.com>
Cc:     akpm@...ux-foundation.org, hch@....de, jack@...e.com, jack@...e.cz,
        linkinjeon@...il.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com, willy@...radead.org
Subject: Re: [syzbot] [udf?] BUG: unable to handle kernel NULL pointer
 dereference in __writepage

I looked into this and got really confused.  We should never end
up in generic_writepages if ->writepages is set, which this patch
obviously does.

Then I took a closer look at udf, and it seems to switch a_aops around
at run time, and it seems like we're hitting just that case, and the
patch just seems to narrow down that window.

I suspect the right fix is to remove this runtime switching of aops,
and just do conditionals inside the methods.