selftests/bpf: Test allowing NULL buffer in dynptr slice

bpf_dynptr_slice(_rw) no longer requires a buffer for verification. If the
buffer is needed, but not present, the function will return NULL.
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20230506013134.2492210-3-drosen@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -26,6 +26,7 @@ static struct {
 	{"test_dynptr_is_null", SETUP_SYSCALL_SLEEP},
 	{"test_dynptr_is_rdonly", SETUP_SKB_PROG},
 	{"test_dynptr_clone", SETUP_SKB_PROG},
+	{"test_dynptr_skb_no_buff", 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

@@ -505,3 +505,20 @@ int test_dynptr_clone(struct __sk_buff *skb)
 
 	return 0;
 }
+
+SEC("?cgroup_skb/egress")
+int test_dynptr_skb_no_buff(struct __sk_buff *skb)
+{
+	struct bpf_dynptr ptr;
+	__u64 *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, 1);
+
+	return !!data;
+}