selftests/bpf: Update bpf_clone_redirect expected return code

Commit 151e887 ("veth: Fixing transmit return status for dropped
packets") started propagating proper NET_XMIT_DROP error to the caller
which means it's now possible to get positive error code when calling
bpf_clone_redirect() in this particular test. Update the test to reflect
that.

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230911194731.286342-2-sdf@google.com

Author: fomichev

tools/testing/selftests/bpf/prog_tests/empty_skb.c

@@ -24,6 +24,7 @@ void test_empty_skb(void)
 		int *ifindex;
 		int err;
 		int ret;
+		int lwt_egress_ret; /* expected retval at lwt/egress */
 		bool success_on_tc;
 	} tests[] = {
 		/* Empty packets are always rejected. */
@@ -57,6 +58,7 @@ void test_empty_skb(void)
 			.data_size_in = sizeof(eth_hlen),
 			.ifindex = &veth_ifindex,
 			.ret = -ERANGE,
+			.lwt_egress_ret = -ERANGE,
 			.success_on_tc = true,
 		},
 		{
@@ -70,6 +72,7 @@ void test_empty_skb(void)
 			.data_size_in = sizeof(eth_hlen),
 			.ifindex = &ipip_ifindex,
 			.ret = -ERANGE,
+			.lwt_egress_ret = -ERANGE,
 		},
 
 		/* ETH_HLEN+1-sized packet should be redirected. */
@@ -79,6 +82,7 @@ void test_empty_skb(void)
 			.data_in = eth_hlen_pp,
 			.data_size_in = sizeof(eth_hlen_pp),
 			.ifindex = &veth_ifindex,
+			.lwt_egress_ret = 1, /* veth_xmit NET_XMIT_DROP */
 		},
 		{
 			.msg = "ipip ETH_HLEN+1 packet ingress",
@@ -108,8 +112,12 @@ void test_empty_skb(void)
 
 	for (i = 0; i < ARRAY_SIZE(tests); i++) {
 		bpf_object__for_each_program(prog, bpf_obj->obj) {
-			char buf[128];
+			bool at_egress = strstr(bpf_program__name(prog), "egress") != NULL;
 			bool at_tc = !strncmp(bpf_program__section_name(prog), "tc", 2);
+			int expected_ret;
+			char buf[128];
+
+			expected_ret = at_egress && !at_tc ? tests[i].lwt_egress_ret : tests[i].ret;
 
 			tattr.data_in = tests[i].data_in;
 			tattr.data_size_in = tests[i].data_size_in;
@@ -128,7 +136,7 @@ void test_empty_skb(void)
 			if (at_tc && tests[i].success_on_tc)
 				ASSERT_GE(bpf_obj->bss->ret, 0, buf);
 			else
-				ASSERT_EQ(bpf_obj->bss->ret, tests[i].ret, buf);
+				ASSERT_EQ(bpf_obj->bss->ret, expected_ret, buf);
 		}
 	}