selftests/bpf: add cgroup skb direct packet access test

This verifies that programs of BPF_PROG_TYPE_CGROUP_SKB can access
skb->data_end with direct packet access when being run with
BPF_PROG_TEST_RUN.

Signed-off-by: Mahe Tardy <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "cgroup_skb_direct_packet_access.skel.h"
+
+void test_cgroup_skb_prog_run_direct_packet_access(void)
+{
+	int err;
+	struct cgroup_skb_direct_packet_access *skel;
+	char test_skb[64] = {};
+
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		.data_in = test_skb,
+		.data_size_in = sizeof(test_skb),
+	);
+
+	skel = cgroup_skb_direct_packet_access__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "cgroup_skb_direct_packet_access__open_and_load"))
+		return;
+
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.direct_packet_access), &topts);
+	ASSERT_OK(err, "bpf_prog_test_run_opts err");
+	ASSERT_EQ(topts.retval, 1, "retval");
+
+	ASSERT_NEQ(skel->bss->data_end, 0, "data_end");
+
+	cgroup_skb_direct_packet_access__destroy(skel);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+__u32 data_end;
+
+SEC("cgroup_skb/ingress")
+int direct_packet_access(struct __sk_buff *skb)
+{
+	data_end = skb->data_end;
+	return 1;
+}
+
+char _license[] SEC("license") = "GPL";