Re: [PATCH v2 1/7] mm/gup: fix wrongly calculated returned value in fault_in_safe_writeable()

[David Hildenbrand]

On 31.03.25 10:13, Baoquan He wrote:
> Not like fault_in_readable() or fault_in_writeable(), in
> fault_in_safe_writeable() local variable 'start' is increased page
> by page to loop till the whole address range is handled. However,
> it mistakenly calcalates the size of handled range with 'uaddr - start'.
>
> Fix it here.

Fixes: ? Do we know of user-visible effects?

> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
> v1->v2:
> - Fix a patch log typo caused by copy-and-paste error. Thanks
>    to Yanjun.
>
>   mm/gup.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 855ab860f88b..73777b1de679 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2207,8 +2207,8 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
>   	} while (start != end);
>   	mmap_read_unlock(mm);
>   
> -	if (size > (unsigned long)uaddr - start)
> -		return size - ((unsigned long)uaddr - start);
> +	if (size > start - (unsigned long)uaddr)
> +		return size - (start - (unsigned long)uaddr);
>   	return 0;
>   }
>   EXPORT_SYMBOL(fault_in_safe_writeable);

Can we instead just use the uaddr and start variables like in 
fault_in_readable?

That is, turn "start" into a const and adjust uaddr instead.

(maybe we should also handle the types of these variables similar to as 
in fault_in_readable)

--
Cheers,

David / dhildenb