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

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

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 <maciej.fijalkowski@intel.com>
> ---
>  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.

> +	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. 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.

> +}
> +