Re: [PATCH v2] maccess: fix strncpy_from_user_nofault empty string handling

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 22 Apr 2025 14:14:49 +0100 Mykyta Yatsenko <mykyta.yatsenko5@gmail.com> wrote:

> From: Mykyta Yatsenko <yatsenko@meta.com>
> 
> strncpy_from_user_nofault should return the length of the copied string
> including the trailing NUL, but if the argument unsafe_addr points to
> an empty string ({'\0'}), the return value is 0.
> 
> This happens as strncpy_from_user copies terminal symbol into dst
> and returns 0 (as expected), but strncpy_from_user_nofault does not
> modify ret as it is not equal to count and not greater than 0, so 0 is
> returned, which contradicts the contract.
> 
> ...
>

Thanks.

Does this fix any known runtime issue?  If so, please fully describe this?

> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
>  	if (ret >= count) {
>  		ret = count;
>  		dst[ret - 1] = '\0';
> -	} else if (ret > 0) {
> +	} else if (ret >= 0) {
>  		ret++;
>  	}
>