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

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

On Mon, Jul 1, 2019 at 11:10 PM Y Song <ys114321@gmail.com> wrote:
>
> On Mon, Jul 1, 2019 at 10:47 PM Andrii Nakryiko <andriin@fb.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@intel.com>
> > Signed-off-by: Andrii Nakryiko <andriin@fb.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?

Yes. I somehow misread description of strncpy and assumed it does that
automatically (which would make sense), but it actually doesn't. Only
strlcpy does. v2 with fix is coming.

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