bpf, selftests: Test probe_* helpers from SCHED_CLS

Lets test using probe* in SCHED_CLS network programs as well just
to be sure these keep working. Its cheap to add the extra test
and provides a second context to test outside of sk_msg after
we generalized probe* helpers to all networking types.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159033911685.12355.15951980509828906214.stgit@john-Precision-5820-TowerSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <network_helpers.h>
+
+void test_skb_helpers(void)
+{
+	struct __sk_buff skb = {
+		.wire_len = 100,
+		.gso_segs = 8,
+		.gso_size = 10,
+	};
+	struct bpf_prog_test_run_attr tattr = {
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.ctx_in = &skb,
+		.ctx_size_in = sizeof(skb),
+		.ctx_out = &skb,
+		.ctx_size_out = sizeof(skb),
+	};
+	struct bpf_object *obj;
+	int err;
+
+	err = bpf_prog_load("./test_skb_helpers.o", BPF_PROG_TYPE_SCHED_CLS, &obj,
+			    &tattr.prog_fd);
+	if (CHECK_ATTR(err, "load", "err %d errno %d\n", err, errno))
+		return;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err, "len", "err %d errno %d\n", err, errno);
+	bpf_object__close(obj);
+}

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

+// SPDX-License-Identifier: GPL-2.0-only
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+#define TEST_COMM_LEN 16
+
+struct {
+	__uint(type, BPF_MAP_TYPE_CGROUP_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, u32);
+	__type(value, u32);
+} cgroup_map SEC(".maps");
+
+char _license[] SEC("license") = "GPL";
+
+SEC("classifier/test_skb_helpers")
+int test_skb_helpers(struct __sk_buff *skb)
+{
+	struct task_struct *task;
+	char comm[TEST_COMM_LEN];
+	__u32 tpid;
+
+	task = (struct task_struct *)bpf_get_current_task();
+	bpf_probe_read_kernel(&tpid , sizeof(tpid), &task->tgid);
+	bpf_probe_read_kernel_str(&comm, sizeof(comm), &task->comm);
+	return 0;
+}