Re: [PATCH] mm: Avoid calling folio_page() with an out-of-bounds index

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/25/26 10:26, Li Zhe wrote:
> In folio_zero_user(), the page pointer is calculated via folio_page()
> before checking if the number of pages to be cleared is greater than zero.
> Furthermore, folio_page() does not verify that the page number lies
> within folio.
> 
> When 'addr_hint' is near the end of a large folio, the range 'r[0]'
> represents an empty interval. In this scenario, 'nr_pages' will be
> calculated as 0 and 'r[0].start' can be an index that is out-of-bounds
> for folio_page(). The code unconditionally calls folio_page() on a wrong
> index, even though the subsequent clearing logic is correctly skipped.
> 
> While this does not cause a functional bug today, calculating a page
> pointer for an out-of-bounds index is logically unsound and fragile. It
> could pose a risk for future refactoring or trigger warnings from static
> analysis tools.
> 
> To fix this, move the call to folio_page() inside the 'if (nr_pages > 0)'
> block. This ensures that the page pointer is only calculated when it is
> actually needed for a valid, non-empty range of pages, thus making the code
> more robust and logically correct.
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> ---
>  mm/memory.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 07778814b4a8..6f8c55d604b5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -7343,12 +7343,14 @@ void folio_zero_user(struct folio *folio, unsigned long addr_hint)
>  	r[0] = DEFINE_RANGE(r[2].end + 1, pg.end);
>  
>  	for (i = 0; i < ARRAY_SIZE(r); i++) {
> -		const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
>  		const long nr_pages = (long)range_len(&r[i]);
> -		struct page *page = folio_page(folio, r[i].start);
>  
> -		if (nr_pages > 0)
> +		if (nr_pages > 0) {
> +			const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
> +			struct page *page = folio_page(folio, r[i].start);
> +
>  			clear_contig_highpages(page, addr, nr_pages);
> +		}
>  	}
>  }
>  

It's all just arithmetic operations. Just like Willy says, I'd assume
that the compiler can just move them around as it pleases. No dependencies.

So I don't see any real benefit with this patch.

-- 
Cheers,

David