Re: [PATCH bpf-next] selftests/bpf: Wait for links to come up in lwt_ip_encap

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Sep 16, 2026 at 11:24 PM UTC, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> lwt_ip_encap creates three fresh netns with veth pairs across them and
> then immediately pings over IPv6. The peer ends of the first veth pair
> end up with the same ifindex in their respective netns, so linkwatch
> does not treat the carrier-on as urgent and may deliver NETDEV_CHANGE up
> to one second later. Until that happens addrconf considers the link not
> ready: no ff00::/8 multicast route and no link-local address are
> installed, and the neighbour solicitation for the next hop is dropped
> as a no-route packet. The NS retransmit a second later succeeds, but
> ping -W1 has already given up:
>
>   check_ping_ok:FAIL:ip netns exec ns-lwt-ip-encap-1-... ping -6 -c 1 -W1 -I veth1 fb04::1 > /dev/null unexpected error: 256 (errno 2)
>   lwt_ip_encap:FAIL:ping OK unexpected error: -1 (errno 2)
>   #226/1   lwt_ip_encap_ipv4/egress:FAIL
>
> This reproduces reliably when the test is run standalone on an idle
> system where setup completes well within a second. The original shell
> script had a "sleep 1 # reduce flakiness" after setup which was lost in
> the conversion to test_progs.
>
> Instead of sleeping, poll SIOCGIFFLAGS for IFF_RUNNING on all veths at
> the end of setup_network() so traffic is only sent once every link is
> operationally up.
>
> Fixes: f5e288943e2c ("selftests/bpf: Move test_lwt_ip_encap to test_progs")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  .../selftests/bpf/prog_tests/lwt_ip_encap.c   | 53 +++++++++++++++++++
>  1 file changed, 53 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c b/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
> index 39e8a3b8b6af..5c5560d45c5b 100644
> --- a/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
> +++ b/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
> @@ -1,5 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0-only
> +#include <net/if.h>
>  #include <netinet/in.h>
> +#include <sys/ioctl.h>
>  
>  #include "network_helpers.h"
>  #include "test_progs.h"
> @@ -141,6 +143,54 @@ static int set_bottom_addr(const char *ns1, const char *ns2, const char *ns3)
>  	return 1;
>  }
>  
> +/*
> + * A veth whose peer sits in another netns with the same ifindex gets its
> + * carrier-on handled as a non-urgent linkwatch event, i.e. up to 1s late.
> + * Until then IPv6 considers the link not ready (no ff00::/8 route, no
> + * link-local address) and silently drops neighbour solicitations, so wait
> + * for all links to be operationally up (IFF_RUNNING) before sending traffic.
> + */
> +static int wait_for_oper_up(const char *ns, const char *dev)
> +{
> +	struct nstoken *nstoken;
> +	struct ifreq ifr = {};
> +	int i, fd, ret = -1;
> +
> +	nstoken = open_netns(ns);
> +	if (!ASSERT_OK_PTR(nstoken, "open ns"))
> +		return -1;
> +
> +	fd = socket(AF_INET, SOCK_DGRAM, 0);
> +	if (!ASSERT_OK_FD(fd, "socket"))
> +		goto out;
> +
> +	strncpy(ifr.ifr_name, dev, IFNAMSIZ - 1);
> +	for (i = 0; i < 50; i++) {
> +		if (!ASSERT_OK(ioctl(fd, SIOCGIFFLAGS, &ifr), "SIOCGIFFLAGS"))
> +			break;
> +		if (ifr.ifr_flags & IFF_RUNNING) {
> +			ret = 0;
> +			break;
> +		}
> +		usleep(100000);
> +	}
> +	close(fd);
> +out:
> +	ASSERT_OK(ret, dev);
> +	close_netns(nstoken);
> +	return ret;
> +}
> +
> +static int wait_for_links(const char *ns1, const char *ns2, const char *ns3)
> +{
> +	if (wait_for_oper_up(ns1, "veth1") || wait_for_oper_up(ns1, "veth5") ||
> +	    wait_for_oper_up(ns2, "veth2") || wait_for_oper_up(ns2, "veth3") ||
> +	    wait_for_oper_up(ns2, "veth6") || wait_for_oper_up(ns2, "veth7") ||
> +	    wait_for_oper_up(ns3, "veth4") || wait_for_oper_up(ns3, "veth8"))
> +		return -1;
> +	return 0;
> +}
> +
>  static int configure_vrf(const char *ns1, const char *ns2)
>  {
>  	if (!ns1 || !ns2)
> @@ -304,6 +354,9 @@ static int setup_network(char *ns1, char *ns2, char *ns3, const char *vrf)
>  	if (!ASSERT_OK(configure_ns3(ns3), "configure ns3 routes"))
>  		goto fail;
>  
> +	if (!ASSERT_OK(wait_for_links(ns1, ns2, ns3), "wait for links"))
> +		goto fail;
> +
>  	/* Link bottom route to the GRE tunnels */
>  	SYS(fail, "ip -n %s route add %s/32 dev veth5 via %s %s",
>  	    ns1, IP4_ADDR_GRE, IP4_ADDR_6, vrf);