Subject: Re: [PATCH bpf-next 1/3] libbpf: xsk: use bpf_link

From: Maciej Fijalkowski <[email-masked]>
Date: Tue, 16 Feb 2021 03:38:33 +0100

On Mon, Feb 15, 2021 at 12:49:27PM -0800, John Fastabend wrote:
> Maciej Fijalkowski wrote:
> > Currently, if there are multiple xdpsock instances running on a single
> > interface and in case one of the instances is terminated, the rest of
> > them are left in an inoperable state due to the fact of unloaded XDP
> > prog from interface.
> > 
> > To address that, step away from setting bpf prog in favour of bpf_link.
> > This means that refcounting of BPF resources will be done automatically
> > by bpf_link itself.
> > 
> > When setting up BPF resources during xsk socket creation, check whether
> > bpf_link for a given ifindex already exists via set of calls to
> > bpf_link_get_next_id -> bpf_link_get_fd_by_id -> bpf_obj_get_info_by_fd
> > and comparing the ifindexes from bpf_link and xsk socket.
> > 
> > If there's no bpf_link yet, create one for a given XDP prog and unload
> > explicitly existing prog if XDP_FLAGS_UPDATE_IF_NOEXIST is not set.
> > 
> > If bpf_link is already at a given ifindex and underlying program is not
> > AF-XDP one, bail out or update the bpf_link's prog given the presence of
> > XDP_FLAGS_UPDATE_IF_NOEXIST.
> > 
> > Signed-off-by: Maciej Fijalkowski <[email-masked]>
> > ---
> >  tools/lib/bpf/xsk.c | 143 +++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 122 insertions(+), 21 deletions(-)
> 
> [...]
> 
> > +static int xsk_create_bpf_link(struct xsk_socket *xsk)
> > +{
> > +	/* bpf_link only accepts XDP_FLAGS_MODES, but xsk->config.xdp_flags
> > +	 * might have set XDP_FLAGS_UPDATE_IF_NOEXIST
> > +	 */
> > +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
> > +			    .flags = (xsk->config.xdp_flags & XDP_FLAGS_MODES));
> > +	struct xsk_ctx *ctx = xsk->ctx;
> > +	__u32 prog_id;
> > +	int link_fd;
> > +	int err;
> > +
> > +	/* for !XDP_FLAGS_UPDATE_IF_NOEXIST, unload the program first, if any,
> > +	 * so that bpf_link can be attached
> > +	 */
> > +	if (!(xsk->config.xdp_flags & XDP_FLAGS_UPDATE_IF_NOEXIST)) {
> > +		err = bpf_get_link_xdp_id(ctx->ifindex, &prog_id, xsk->config.xdp_flags);
> > +		if (err) {
> > +			pr_warn("getting XDP prog id failed\n");
> > +			return err;
> > +		}
> > +		if (prog_id) {
> > +			err = bpf_set_link_xdp_fd(ctx->ifindex, -1, 0);
> > +			if (err < 0) {
> > +				pr_warn("detaching XDP prog failed\n");
> > +				return err;
> > +			}
> > +		}
> >  	}
> >  
> > -	ctx->prog_fd = prog_fd;
> > +	link_fd = bpf_link_create(ctx->prog_fd, xsk->ctx->ifindex, BPF_XDP, &opts);
> > +	if (link_fd < 0) {
> > +		pr_warn("bpf_link_create failed: %s\n", strerror(errno));
> > +		return link_fd;
> > +	}
> > +
> 
> This can leave the system in a bad state where it unloaded the XDP program
> above, but then failed to create the link. So we should somehow fix that
> if possible or at minimum put a note somewhere so users can't claim they
> shouldn't know this.
> 
> Also related, its not good for real systems to let XDP program go missing
> for some period of time. I didn't check but we should make
> XDP_FLAGS_UPDATE_IF_NOEXIST the default if its not already.

Old way of attaching prog is mutual exclusive with bpf_link, right?
What I'm saying is in order to use one of the two, you need to wipe out
the current one in favour of the second that you would like to load.

> 
> > +	ctx->link_fd = link_fd;
> >  	return 0;
> >  }
> >  
> 
> [...]
> 
> > +static int xsk_link_lookup(struct xsk_ctx *ctx, __u32 *prog_id)
> > +{
> > +	__u32 link_len = sizeof(struct bpf_link_info);
> > +	struct bpf_link_info link_info;
> > +	__u32 id = 0;
> > +	int err;
> > +	int fd;
> > +
> > +	while (true) {
> > +		err = bpf_link_get_next_id(id, &id);
> > +		if (err) {
> > +			if (errno == ENOENT)
> > +				break;
> > +			pr_warn("can't get next link: %s\n", strerror(errno));
> > +			break;
> > +		}
> > +
> > +		fd = bpf_link_get_fd_by_id(id);
> > +		if (fd < 0) {
> > +			if (errno == ENOENT)
> > +				continue;
> > +			pr_warn("can't get link by id (%u): %s\n", id, strerror(errno));
> > +			break;
> > +		}
> > +
> > +		memset(&link_info, 0, link_len);
> > +		err = bpf_obj_get_info_by_fd(fd, &link_info, &link_len);
> > +		if (err) {
> > +			pr_warn("can't get link info: %s\n", strerror(errno));
> > +			close(fd);
> > +			break;
> > +		}
> > +		if (link_info.xdp.ifindex == ctx->ifindex) {
> > +			ctx->link_fd = fd;
> > +			*prog_id = link_info.prog_id;
> > +			break;
> > +		}
> > +		close(fd);
> > +	}
> > +
> > +	return errno == ENOENT ? 0 : err;
> 
> But, err wont be set in fd < 0 case? I guess we don't want to return 0 if
> bpf_link_get_fd_by_id fails.

Good catch!

> Although I really don't like the construct
> here that much. I think just `return err` and ensuring err is set correctly
> would be more clear. At least the fd error case needs to be handled
> though.

FWIW, this was inspired by tools/bpf/bpftool/link.c:do_show()

> > +}
> > +