Re: [RFC PATCH 2/2] kobject: Fix -Wstringop-truncation warning

[Eric Biggers]

On Sat, Jun 23, 2018 at 11:07:53AM +0900, Stafford Horne wrote:
> When compiling with GCC 9.0.0 I am seeing the following warning:
> 
>     In function âfill_kobj_pathâ,
> 	inlined from âkobject_get_pathâ at lib/kobject.c:155:2:
>     lib/kobject.c:128:3: warning: âstrncpyâ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
>        strncpy(path + length, kobject_name(parent), cur);
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     lib/kobject.c: In function âkobject_get_pathâ:
>     lib/kobject.c:125:13: note: length computed here
>        int cur = strlen(kobject_name(parent));
> 		 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This is pointing out a bug that the strncpy limit is the source string not the
> destination buffer remaining length.  Fix it.
> 
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Stafford Horne <shorne@xxxxxxxxx>
> ---
>  lib/kobject.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/kobject.c b/lib/kobject.c
> index 18989b5b3b56..15338e5a96f2 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -125,7 +125,7 @@ static void fill_kobj_path(struct kobject *kobj, char *path, int length)
>  		int cur = strlen(kobject_name(parent));
>  		/* back up enough to print this name with '/' */
>  		length -= cur;
> -		strncpy(path + length, kobject_name(parent), cur);
> +		strncpy(path + length, kobject_name(parent), length);
>  		*(path + --length) = '/';
>  	}

It should be replaced with memcpy(), AFAICS; it wouldn't change the behavior but
it would quiet the gcc warning.  Your proposed "fix" is heavily broken: notice
that the code is building a string backwards (end to beginning).

- Eric