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

[Maciej Fijalkowski <maciejromanfijalkowski@gmail.com>]

On Mon, 28 Jan 2019 13:07:49 -0800
John Fastabend <john.fastabend@gmail.com> wrote:

> 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.
>
Agree, besides checking the return value from libbpf_nl_get_link() we should
also check that we have assigned a value to xdp_id.id so that would mean we
have gone through whole get_xdp_id(). I'll post a v5 within several hours with
following check:

if (!ret && xdp_id.id)
	*prog_id = xdp_id;

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