selftests: bpf: Check bpf_msg_push_data return value

bpf_msg_push_data may return a non-zero value to indicate an error. The
return value should be checked to prevent undetected errors.

To indicate an error, the BPF programs now perform a different action
than their intended one to make the userspace test program notice the
error, i.e., the programs supposed to pass/redirect drop, the program
supposed to drop passes.

Fixes: 84fbfe02 ("bpf: test_sockmap add options to use msg_push_data")
Signed-off-by: Felix Maurer <fmaurer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/89f767bb44005d6b4dd1f42038c438f76b3ebfad.1644601294.git.fmaurer@redhat.com

tools/testing/selftests/bpf/progs/test_sockmap_kern.h

@@ -235,7 +235,7 @@ SEC("sk_msg1")
 int bpf_prog4(struct sk_msg_md *msg)
 {
 	int *bytes, zero = 0, one = 1, two = 2, three = 3, four = 4, five = 5;
-	int *start, *end, *start_push, *end_push, *start_pop, *pop;
+	int *start, *end, *start_push, *end_push, *start_pop, *pop, err = 0;
 
 	bytes = bpf_map_lookup_elem(&sock_apply_bytes, &zero);
 	if (bytes)
@@ -249,8 +249,11 @@ int bpf_prog4(struct sk_msg_md *msg)
 		bpf_msg_pull_data(msg, *start, *end, 0);
 	start_push = bpf_map_lookup_elem(&sock_bytes, &two);
 	end_push = bpf_map_lookup_elem(&sock_bytes, &three);
-	if (start_push && end_push)
-		bpf_msg_push_data(msg, *start_push, *end_push, 0);
+	if (start_push && end_push) {
+		err = bpf_msg_push_data(msg, *start_push, *end_push, 0);
+		if (err)
+			return SK_DROP;
+	}
 	start_pop = bpf_map_lookup_elem(&sock_bytes, &four);
 	pop = bpf_map_lookup_elem(&sock_bytes, &five);
 	if (start_pop && pop)
@@ -263,6 +266,7 @@ int bpf_prog6(struct sk_msg_md *msg)
 {
 	int zero = 0, one = 1, two = 2, three = 3, four = 4, five = 5, key = 0;
 	int *bytes, *start, *end, *start_push, *end_push, *start_pop, *pop, *f;
+	int err = 0;
 	__u64 flags = 0;
 
 	bytes = bpf_map_lookup_elem(&sock_apply_bytes, &zero);
@@ -279,8 +283,11 @@ int bpf_prog6(struct sk_msg_md *msg)
 
 	start_push = bpf_map_lookup_elem(&sock_bytes, &two);
 	end_push = bpf_map_lookup_elem(&sock_bytes, &three);
-	if (start_push && end_push)
-		bpf_msg_push_data(msg, *start_push, *end_push, 0);
+	if (start_push && end_push) {
+		err = bpf_msg_push_data(msg, *start_push, *end_push, 0);
+		if (err)
+			return SK_DROP;
+	}
 
 	start_pop = bpf_map_lookup_elem(&sock_bytes, &four);
 	pop = bpf_map_lookup_elem(&sock_bytes, &five);
@@ -338,7 +345,7 @@ SEC("sk_msg5")
 int bpf_prog10(struct sk_msg_md *msg)
 {
 	int *bytes, *start, *end, *start_push, *end_push, *start_pop, *pop;
-	int zero = 0, one = 1, two = 2, three = 3, four = 4, five = 5;
+	int zero = 0, one = 1, two = 2, three = 3, four = 4, five = 5, err = 0;
 
 	bytes = bpf_map_lookup_elem(&sock_apply_bytes, &zero);
 	if (bytes)
@@ -352,8 +359,11 @@ int bpf_prog10(struct sk_msg_md *msg)
 		bpf_msg_pull_data(msg, *start, *end, 0);
 	start_push = bpf_map_lookup_elem(&sock_bytes, &two);
 	end_push = bpf_map_lookup_elem(&sock_bytes, &three);
-	if (start_push && end_push)
-		bpf_msg_push_data(msg, *start_push, *end_push, 0);
+	if (start_push && end_push) {
+		err = bpf_msg_push_data(msg, *start_push, *end_push, 0);
+		if (err)
+			return SK_PASS;
+	}
 	start_pop = bpf_map_lookup_elem(&sock_bytes, &four);
 	pop = bpf_map_lookup_elem(&sock_bytes, &five);
 	if (start_pop && pop)