linux-kernel - Re: [PATCH v2 2/7] mm/gup: check if both GUP_GET and GUP_PIN are set in __get_user

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Message-ID: <e6b8481f-0bf6-4acf-9ebc-9b4f28c0be08@redhat.com>
Date: Tue, 1 Apr 2025 10:02:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Baoquan He <bhe@...hat.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, yanjun.zhu@...ux.dev
Subject: Re: [PATCH v2 2/7] mm/gup: check if both GUP_GET and GUP_PIN are set
 in __get_user_pages() earlier

On 31.03.25 10:13, Baoquan He wrote:
> In __get_user_pages(), it will traverse page table and take a reference
> to the page the given user address corresponds to if GUP_GET or GUP_PIN
> is et. However, it's not supported both GUP_GET and GUP_PIN are set.
> This check should be done earlier, but not doing it till entering into
> follow_page_pte() and failed.
> 
> Here move the checking to the beginning of __get_user_pages().
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
> v1->v2:
> - Fix code bug caused by copy-and-paste error, this is reported by
>    lkp test robot.
> 
>   mm/gup.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 73777b1de679..f9bce14ed3cd 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -847,11 +847,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>   	pte_t *ptep, pte;
>   	int ret;
>   
> -	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> -	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> -			 (FOLL_PIN | FOLL_GET)))
> -		return ERR_PTR(-EINVAL);
> -
>   	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
>   	if (!ptep)
>   		return no_page_table(vma, flags, address);
> @@ -1434,6 +1429,11 @@ static long __get_user_pages(struct mm_struct *mm,
>   
>   	VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
>   
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
> +			 (FOLL_PIN | FOLL_GET)))
> +		return -EINVAL;
> +

We already have that check in is_valid_gup_args(), that catches all 
external users that could possibly mess this up.

So we can just convert that into a VM_WARN_ON_ONCE(), and while doing 
that, we should convert the VM_BUG_ON() above to a VM_WARN_ON_ONCE() as 
well.

-- 
Cheers,

David / dhildenb