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SubjectRe: [PATCH linux-next] mm/folio-compat: fix potential NULL pointer access
On Mon, Sep 13, 2021 at 08:11:13AM +0000, cgel.zte@gmail.com wrote:
> From: xu xin <xu.xin16@zte.com.cn>
>
> The pointer 'folio' might be NULL, but the structure it points to is accessed.
> Accordingly, we add a check of NULL pointer by 'if (!folio)'.
> Secondly, there is no need to check if folio is pointer or value with 'xa_is_value(folio)'
> because folio is alwayse pointer.
>
> Reported-by: Zeal Robot <zealci@zte.com.cn>

Your robot is garbage.

> 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))
> return &folio->page;

&folio->page *does not access folio*. It does pointer arithmetic on
folio. Specifically, it adds zero to folio, because page is the first
element of folio.

Secondly, __filemap_get_folio() absolutely can return an xa_is_value()
result if FGP_ENTRY entry is set. It's right there in the first few
lines:

folio = mapping_get_entry(mapping, index);
if (xa_is_value(folio)) {
if (fgp_flags & FGP_ENTRY)
return folio;

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