selftests/bpf: Fix flaky selftest lwt_redirect/lwt_reroute

Recently, when running './test_progs -j', I occasionally hit the
following errors:

test_lwt_redirect:PASS:pthread_create 0 nsec
test_lwt_redirect_run:FAIL:netns_create unexpected error: 256 (errno 0)
#142/2 lwt_redirect/lwt_redirect_normal_nomac:FAIL
#142 lwt_redirect:FAIL
test_lwt_reroute:PASS:pthread_create 0 nsec
test_lwt_reroute_run:FAIL:netns_create unexpected error: 256 (errno 0)
test_lwt_reroute:PASS:pthread_join 0 nsec
#143/2 lwt_reroute/lwt_reroute_qdisc_dropped:FAIL
#143 lwt_reroute:FAIL

The netns_create() definition looks like below:

#define NETNS "ns_lwt"
static inline int netns_create(void)
{
return system("ip netns add " NETNS);
}

One possibility is that both lwt_redirect and lwt_reroute create
netns with the same name "ns_lwt" which may cause conflict. I tried
the following example:
$ sudo ip netns add abc
$ echo $?
0
$ sudo ip netns add abc
Cannot create namespace file "/var/run/netns/abc": File exists
$ echo $?
1
$

The return code for above netns_create() is 256. The internet search
suggests that the return value for 'ip netns add ns_lwt' is 1, which
matches the above 'sudo ip netns add abc' example.

This patch tried to use different netns names for two tests to avoid
'ip netns add <name>' failure.

I ran './test_progs -j' 10 times and all succeeded with
lwt_redirect/lwt_reroute tests.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240205052914.1742687-1-yonghong.song@linux.dev

selftests/bpf: Fix compiler warnings reported in -O2 mode

Fix a bunch of potentially unitialized variable usage warnings that are
reported by GCC in -O2 mode. Also silence overzealous stringop-truncation
class of warnings.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20231006175744.3136675-1-andrii@kernel.org

selftests/bpf: Add lwt_xmit tests for BPF_REDIRECT

There is no lwt_xmit test case for BPF_REDIRECT yet. Add test cases for
both normal and abnormal situations. For abnormal test cases, devices
are set down or have its carrier set down. Without proper fixes,
BPF_REDIRECT to either ingress or egress of such device would panic the
kernel.

Signed-off-by: Yan Zhai <yan@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/96bf435243641939d9c9da329fab29cb45f7df22.1692326837.git.yan@cloudflare.com