linux-kernel - Re: [PATCH] mm/folio-compact: fix potential NULL pointer in pagecache_get

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Date:   Wed, 27 Jul 2022 11:40:38 +0000
From:   William Kucharski <william.kucharski@...cle.com>
To:     Yongqiang Liu <liuyongqiang13@...wei.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zhangxiaoxu5@...wei.com" <zhangxiaoxu5@...wei.com>,
        "yanaijie@...wei.com" <yanaijie@...wei.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "wangkefeng.wang@...wei.com" <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] mm/folio-compact: fix potential NULL pointer in
 pagecache_get_page

This isn't a NULL pointer dereference; returning &(0->page) is completely legal
as was discussed regarding this exact code back in April:

https://lore.kernel.org/lkml/YmfgqKcMmstgfz+0@casper.infradead.org/

> On Jul 27, 2022, at 12:46 AM, Yongqiang Liu <liuyongqiang13@...wei.com> wrote:
> 
> When __filemap_get_folio() failed and returned NULL, we would
> get a NULL pointer dereference in pagecache_get_page.
> 
> Fixes: 3f0c6a07fee6 ("mm/filemap: Add filemap_get_folio")
> Signed-off-by: Yongqiang Liu <liuyongqiang13@...wei.com>
> Cc: <stable@...r.kernel.org> # 5.16
> ---
> 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 20bc15b57d93..7b21393480e0 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);
> }
> -- 
> 2.25.1
> 
>