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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Apr 22, 2025 at 6:15 AM 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.
>
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---
>  mm/maccess.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

Reviewed-by: Andrii Nakryiko <andrii@kernel.org>

> diff --git a/mm/maccess.c b/mm/maccess.c
> index 8f0906180a94..831b4dd7296c 100644
> --- 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++;
>         }
>
> --
> 2.49.0
>