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SubjectRe: [RFC PATCH 2/2] kobject: Fix -Wstringop-truncation warning
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 ‘fill_kobj_path’,
> inlined from ‘kobject_get_path’ at lib/kobject.c:155:2:
> lib/kobject.c:128:3: warning: ‘strncpy’ 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 ‘kobject_get_path’:
> 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@linuxfoundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Stafford Horne <shorne@gmail.com>
> ---
> 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

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