selftests/bpf: Cover 4-byte load from remote_port in bpf_sk_lookup

Extend the context access tests for sk_lookup prog to cover the surprising
case of a 4-byte load from the remote_port field, where the expected value
is actually shifted by 16 bits.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220209184333.654927-3-jakub@cloudflare.com

tools/include/uapi/linux/bpf.h

@@ -6453,7 +6453,8 @@ struct bpf_sk_lookup {
 	__u32 protocol;		/* IP protocol (IPPROTO_TCP, IPPROTO_UDP) */
 	__u32 remote_ip4;	/* Network byte order */
 	__u32 remote_ip6[4];	/* Network byte order */
-	__u32 remote_port;	/* Network byte order */
+	__be16 remote_port;	/* Network byte order */
+	__u16 :16;		/* Zero padding */
 	__u32 local_ip4;	/* Network byte order */
 	__u32 local_ip6[4];	/* Network byte order */
 	__u32 local_port;	/* Host byte order */

tools/testing/selftests/bpf/progs/test_sk_lookup.c

@@ -392,6 +392,7 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 {
 	struct bpf_sock *sk;
 	int err, family;
+	__u32 val_u32;
 	bool v4;
 
 	v4 = (ctx->family == AF_INET);
@@ -418,6 +419,11 @@ int ctx_narrow_access(struct bpf_sk_lookup *ctx)
 	if (LSW(ctx->remote_port, 0) != SRC_PORT)
 		return SK_DROP;
 
+	/* 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))
+		return SK_DROP;
+
 	/* Narrow loads from local_port field. Expect DST_PORT. */
 	if (LSB(ctx->local_port, 0) != ((DST_PORT >> 0) & 0xff) ||
 	    LSB(ctx->local_port, 1) != ((DST_PORT >> 8) & 0xff) ||