selftests/bpf: Add tests for attaching XDP programs

This adds tests for the various replacement operations using
IFLA_XDP_EXPECTED_FD.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158515700967.92963.15098921624731968356.stgit@toke.dk

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+#define IFINDEX_LO 1
+#define XDP_FLAGS_REPLACE		(1U << 4)
+
+void test_xdp_attach(void)
+{
+	struct bpf_object *obj1, *obj2, *obj3;
+	const char *file = "./test_xdp.o";
+	int err, fd1, fd2, fd3;
+	__u32 duration = 0;
+	DECLARE_LIBBPF_OPTS(bpf_xdp_set_link_opts, opts,
+			    .old_fd = -1);
+
+	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj1, &fd1);
+	if (CHECK_FAIL(err))
+		return;
+	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj2, &fd2);
+	if (CHECK_FAIL(err))
+		goto out_1;
+	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj3, &fd3);
+	if (CHECK_FAIL(err))
+		goto out_2;
+
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd1, XDP_FLAGS_REPLACE,
+				       &opts);
+	if (CHECK(err, "load_ok", "initial load failed"))
+		goto out_close;
+
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd2, XDP_FLAGS_REPLACE,
+				       &opts);
+	if (CHECK(!err, "load_fail", "load with expected id didn't fail"))
+		goto out;
+
+	opts.old_fd = fd1;
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd2, 0, &opts);
+	if (CHECK(err, "replace_ok", "replace valid old_fd failed"))
+		goto out;
+
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, fd3, 0, &opts);
+	if (CHECK(!err, "replace_fail", "replace invalid old_fd didn't fail"))
+		goto out;
+
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, -1, 0, &opts);
+	if (CHECK(!err, "remove_fail", "remove invalid old_fd didn't fail"))
+		goto out;
+
+	opts.old_fd = fd2;
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, -1, 0, &opts);
+	if (CHECK(err, "remove_ok", "remove valid old_fd failed"))
+		goto out;
+
+out:
+	bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);
+out_close:
+	bpf_object__close(obj3);
+out_2:
+	bpf_object__close(obj2);
+out_1:
+	bpf_object__close(obj1);
+}