netdev - Re: [PATCH bpf-next] libbpf: fix GCC8 warning for strncpy

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Date:   Mon, 1 Jul 2019 23:09:45 -0700
From:   Y Song <ys114321@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     andrii.nakryiko@...il.com, bpf <bpf@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Magnus Karlsson <magnus.karlsson@...el.com>
Subject: Re: [PATCH bpf-next] libbpf: fix GCC8 warning for strncpy

On Mon, Jul 1, 2019 at 10:47 PM Andrii Nakryiko <andriin@...com> wrote:
>
> GCC8 started emitting warning about using strncpy with number of bytes
> exactly equal destination size, which is generally unsafe, as can lead
> to non-zero terminated string being copied. Use IFNAMSIZ - 1 as number
> of bytes to ensure name is always zero-terminated.
>
> Cc: Magnus Karlsson <magnus.karlsson@...el.com>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
>  tools/lib/bpf/xsk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index bf15a80a37c2..9588e7f87d0b 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -327,7 +327,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
>
>         channels.cmd = ETHTOOL_GCHANNELS;
>         ifr.ifr_data = (void *)&channels;
> -       strncpy(ifr.ifr_name, xsk->ifname, IFNAMSIZ);
> +       strncpy(ifr.ifr_name, xsk->ifname, IFNAMSIZ - 1);

To accommodate the xsk->ifname string length FNAMSIZ - 1, we need to have
    ifr.ifr_name[FNAMSIZ - 1] = '\0';
right?

>         err = ioctl(fd, SIOCETHTOOL, &ifr);
>         if (err && errno != EOPNOTSUPP) {
>                 ret = -errno;
> --
> 2.17.1
>