selftests/bpf: Test creating dynptr from dynptr data and slice

The verifier currently does not allow creating dynptr from dynptr data
or slice. Add a selftest to test this explicitly.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260529014936.2811085-11-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: ameryhung

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

@@ -705,6 +705,48 @@ int dynptr_from_mem_invalid_api(void *ctx)
 	return 0;
 }
 
+/* Cannot create dynptr from dynptr data */
+SEC("?raw_tp")
+__failure __msg("Unsupported reg type mem for bpf_dynptr_from_mem data")
+int dynptr_from_dynptr_data(void *ctx)
+{
+	struct bpf_dynptr ptr, ptr2;
+	__u8 *data;
+
+	if (get_map_val_dynptr(&ptr))
+		return 0;
+
+	data = bpf_dynptr_data(&ptr, 0, sizeof(__u32));
+	if (!data)
+		return 0;
+
+	/* this should fail */
+	bpf_dynptr_from_mem(data, sizeof(__u32), 0, &ptr2);
+
+	return 0;
+}
+
+/* Cannot create dynptr from dynptr slice */
+SEC("?tc")
+__failure __msg("Unsupported reg type mem for bpf_dynptr_from_mem data")
+int dynptr_from_dynptr_slice(struct __sk_buff *skb)
+{
+	struct bpf_dynptr ptr, ptr2;
+	struct ethhdr *hdr;
+	char buffer[sizeof(*hdr)] = {};
+
+	bpf_dynptr_from_skb(skb, 0, &ptr);
+
+	hdr = bpf_dynptr_slice_rdwr(&ptr, 0, buffer, sizeof(buffer));
+	if (!hdr)
+		return SK_DROP;
+
+	/* this should fail */
+	bpf_dynptr_from_mem(hdr, sizeof(*hdr), 0, &ptr2);
+
+	return SK_PASS;
+}
+
 SEC("?tc")
 __failure __msg("cannot overwrite referenced dynptr") __log_level(2)
 int dynptr_pruning_overwrite(struct __sk_buff *ctx)