[PATCH bpf-next] selftests/bpf: Fix implementation-defined behavior in sk_lookup test

[Jakub Sitnicki <jakub@cloudflare.com>]

Shifting 16-bit type by 16 bits is implementation-defined for BPF programs.
Don't rely on it in case it is causing the test failures we are seeing on
s390x z15 target.

Fixes: 2ed0dc5937d3 ("selftests/bpf: Cover 4-byte load from remote_port in bpf_sk_lookup")
Reported-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---

I don't have a dev env for s390x/z15 set up yet, so can't definitely confirm the fix.
That said, it seems worth fixing either way.

 tools/testing/selftests/bpf/progs/test_sk_lookup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/test_sk_lookup.c b/tools/testing/selftests/bpf/progs/test_sk_lookup.c
index bf5b7caefdd0..7d47276a8964 100644
--- a/tools/testing/selftests/bpf/progs/test_sk_lookup.c
+++ b/tools/testing/selftests/bpf/progs/test_sk_lookup.c
@@ -65,6 +65,7 @@ static const __u32 KEY_SERVER_A = SERVER_A;
 static const __u32 KEY_SERVER_B = SERVER_B;
 
 static const __u16 SRC_PORT = bpf_htons(8008);
+static const __u32 SRC_PORT_U32 = bpf_htonl(8008U << 16);
 static const __u32 SRC_IP4 = IP4(127, 0, 0, 2);
 static const __u32 SRC_IP6[] = IP6(0xfd000000, 0x0, 0x0, 0x00000002);
 
@@ -421,7 +422,7 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 
 	/* Load from remote_port field with zero padding (backward compatibility) */
 	val_u32 = *(__u32 *)&ctx->remote_port;
-	if (val_u32 != bpf_htonl(bpf_ntohs(SRC_PORT) << 16))
+	if (val_u32 != SRC_PORT_U32)
 		return SK_DROP;
 
 	/* Narrow loads from local_port field. Expect DST_PORT. */
-- 
2.35.1