Re: [PATCH v10 06/12] fs, arm64: untag user pointers in copy_mount_options

[Dave Hansen]

On 2/22/19 4:53 AM, Andrey Konovalov wrote:
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2730,7 +2730,7 @@ void *copy_mount_options(const void __user * data)
>  	 * the remainder of the page.
>  	 */
>  	/* copy_from_user cannot cross TASK_SIZE ! */
> -	size = TASK_SIZE - (unsigned long)data;
> +	size = TASK_SIZE - (unsigned long)untagged_addr(data);
>  	if (size > PAGE_SIZE)
>  		size = PAGE_SIZE;

I would have thought that copy_from_user() *is* entirely capable of
detecting and returning an error in the case that its arguments cross
TASK_SIZE.  It will fail and return an error, but that's what it's
supposed to do.

I'd question why this code needs to be doing its own checking in the
first place.  Is there something subtle going on?