linux-kernel - Re: [PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get

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Message-ID: <YmhhcQuXO7ZKjolH@casper.infradead.org>
Date:   Tue, 26 Apr 2022 22:17:37 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wan Jiabing <wanjiabing@...o.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Howells <dhowells@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Julia Lawall <Julia.Lawall@...ia.fr>
Subject: Re: [PATCH] mm/filemap: Fix NULL pointer dereference in
 pagecache_get_page

On Tue, Apr 26, 2022 at 02:06:41PM -0700, Andrew Morton wrote:
> On Tue, 26 Apr 2022 13:08:08 +0100 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Tue, Apr 26, 2022 at 04:25:48PM +0800, Wan Jiabing wrote:
> > > Fix following coccicheck error:
> > > mm/folio-compat.c:128:17-21: ERROR: folio is NULL but dereferenced.
> > >  	folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
> > > -	if ((fgp_flags & FGP_HEAD) || !folio || xa_is_value(folio))
> > > +	if (!folio)
> > > +		return NULL;
> > > +	if ((fgp_flags & FGP_HEAD) || xa_is_value(folio))
> > >  		return &folio->page;
> > 
> > That doesn't dereference the folio.  Coccicheck is wrong.
> 
> Doing
> 
> 		return &(0->page);
> 
> is a rather obscure way of doing `return NULL;'.
> 
> I agree the patch doesn't fix anything, but it results in saner-looking code?

I suppose that's in the eye of the beholder?  The original code makes
more sense to me.  Besides, it's in the folio-compat file; nobody
should be looking at that except to figure out "What function should I
be transitioning to?"