2022-04-26 12:59:04

by Jiabing Wan

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

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 <[email protected]>
---
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

2022-04-26 13:22:47

by Matthew Wilcox

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

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.

2022-04-27 10:53:16

by Matthew Wilcox

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 <[email protected]> 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?"

2022-04-27 11:02:50

by Andrew Morton

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

On Tue, 26 Apr 2022 13:08:08 +0100 Matthew Wilcox <[email protected]> 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?