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: <a504f547-dc3c-418b-acb0-111b372d88ce@redhat.com>
Date: Tue, 1 Apr 2025 16:37:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Baoquan He <bhe@...hat.com>
Cc: akpm@...ux-foundation.org, 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 01.04.25 16:34, Baoquan He wrote:
> On 04/01/25 at 10:02am, David Hildenbrand wrote:
>> 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.
> 
> Right.
> 
>>
>> 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.
> 
> Sounds great to me, will put below change into this patch of v3 as suggested.
> Thanks.
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 9e4ed09c578b..d551da9549b1 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1427,10 +1427,10 @@ static long __get_user_pages(struct mm_struct *mm,
>   
>   	start = untagged_addr_remote(mm, start);
>   
> -	VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));
> +	VM_WARN_ON_ONCE(!!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)) ==
> +	if (VM_WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
>   			 (FOLL_PIN | FOLL_GET)))

That won't work (or shouldn't work :) ). Just drop the handling, it's a 
sanity check we don't expect to ever trigger, just like the old 
VM_BUG_ON above.

-- 
Cheers,

David / dhildenb