[PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get_page

[PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get_page

[Wan Jiabing]

Fix following coccicheck error:
mm/folio-compat.c:128:17-21: ERROR: folio is NULL but dereferenced.

'folio' would be NULL when getting a reference to a folio failed.
Add a NULL check before dereferring 'folio'.

Fixes: 3f0c6a07fee6 ("mm/filemap: Add filemap_get_folio")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
---
 mm/folio-compat.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/folio-compat.c b/mm/folio-compat.c
index 46fa179e32fb..54b17c1246c4 100644
--- a/mm/folio-compat.c
+++ b/mm/folio-compat.c
@@ -124,7 +124,9 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 	struct folio *folio;
 
 	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;
 	return folio_file_page(folio, index);
 }
-- 
2.35.3

Re: [PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get_page

[Matthew Wilcox]

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.

Re: [PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get_page

[Andrew Morton]

On Tue, 26 Apr 2022 13:08:08 +0100 Matthew Wilcox <willy@infradead.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?

Re: [PATCH] mm/filemap: Fix NULL pointer dereference in pagecache_get_page

[Matthew Wilcox]

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@infradead.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?"