selftests/bpf: Accept mem from dynptr in helper funcs

This ensures that buffers retrieved from dynptr_data are allowed to be
passed in to helpers that take mem, like bpf_strncmp

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20230506013134.2492210-6-drosen@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -27,6 +27,7 @@ static struct {
 	{"test_dynptr_is_rdonly", SETUP_SKB_PROG},
 	{"test_dynptr_clone", SETUP_SKB_PROG},
 	{"test_dynptr_skb_no_buff", SETUP_SKB_PROG},
+	{"test_dynptr_skb_strcmp", SETUP_SKB_PROG},
 };
 
 static void verify_success(const char *prog_name, enum test_setup_type setup_type)

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

@@ -522,3 +522,24 @@ int test_dynptr_skb_no_buff(struct __sk_buff *skb)
 
 	return !!data;
 }
+
+SEC("?cgroup_skb/egress")
+int test_dynptr_skb_strcmp(struct __sk_buff *skb)
+{
+	struct bpf_dynptr ptr;
+	char *data;
+
+	if (bpf_dynptr_from_skb(skb, 0, &ptr)) {
+		err = 1;
+		return 1;
+	}
+
+	/* This may return NULL. SKB may require a buffer */
+	data = bpf_dynptr_slice(&ptr, 0, NULL, 10);
+	if (data) {
+		bpf_strncmp(data, 10, "foo");
+		return 1;
+	}
+
+	return 1;
+}