Re: [PATCH v4 1/4] mm/gup: fix wrongly calculated returned value in fault_in_safe_writeable()

[Oscar Salvador <osalvador@suse.de>]

On Thu, Apr 10, 2025 at 11:57:14AM +0800, 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.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Fixes: fe673d3f5bf1 ("mm: gup: make fault_in_safe_writeable() use fixup_user_fault()")

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> ---
>  mm/gup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 92351e2fa876..84461d384ae2 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);
> -- 
> 2.41.0
> 

-- 
Oscar Salvador
SUSE Labs