Re: [PATCH bpf-next v4 6/7] libbpf: Add a support for getting xdp prog id on ifindex

[John Fastabend <john.fastabend@gmail.com>]

On 1/28/19 11:16 AM, Maciej Fijalkowski wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> 
> Since we have a dedicated netlink attributes for xdp setup on a
> particular interface, it is now possible to retrieve the program id that
> is currently attached to the interface. The use case is targeted for
> sample xdp programs, which will store the program id just after loading
> bpf program onto iface. On shutdown, the sample will make sure that it
> can unload the program by querying again the iface and verifying that
> both program id's matches.
> 
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> ---

small nit.

> +
> +int bpf_get_link_xdp_id(int ifindex, __u32 *prog_id, __u32 flags)
> +{
> +	struct xdp_id_md xdp_id = {};
> +	int sock, ret;
> +	__u32 nl_pid;
> +	__u32 mask;
> +
> +	if (flags & ~XDP_FLAGS_MASK)
> +		return -EINVAL;
> +
> +	/* Check whether the single {HW,DRV,SKB} mode is set */
> +	flags &= XDP_FLAGS_MODES;
> +	mask = flags - 1;
> +	if (flags && flags & mask)
> +		return -EINVAL;
> +
> +	sock = libbpf_netlink_open(&nl_pid);
> +	if (sock < 0)
> +		return sock;
> +
> +	xdp_id.ifindex = ifindex;
> +	xdp_id.flags = flags;
> +
> +	ret = libbpf_nl_get_link(sock, nl_pid, get_xdp_id, &xdp_id);
> +	*prog_id = xdp_id.id;

just a nit but should we really set prog_id from user if there is
an error. Probably friendlier not to set caller data in error
case.

> +
> +	close(sock);
> +	return ret;
> +}
> +
>  int libbpf_nl_get_link(int sock, unsigned int nl_pid,
>  		       libbpf_dump_nlmsg_t dump_link_nlmsg, void *cookie)
>  {
>