selftests/bpf: Add BPF XDP link selftests

Add selftest validating all the attachment logic around BPF XDP link. Test
also link updates and get_obj_info() APIs.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722064603.3350758-9-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <uapi/linux/if_link.h>
+#include <test_progs.h>
+#include "test_xdp_link.skel.h"
+
+#define IFINDEX_LO 1
+
+void test_xdp_link(void)
+{
+	__u32 duration = 0, id1, id2, id0 = 0, prog_fd1, prog_fd2, err;
+	DECLARE_LIBBPF_OPTS(bpf_xdp_set_link_opts, opts, .old_fd = -1);
+	struct test_xdp_link *skel1 = NULL, *skel2 = NULL;
+	struct bpf_link_info link_info;
+	struct bpf_prog_info prog_info;
+	struct bpf_link *link;
+	__u32 link_info_len = sizeof(link_info);
+	__u32 prog_info_len = sizeof(prog_info);
+
+	skel1 = test_xdp_link__open_and_load();
+	if (CHECK(!skel1, "skel_load", "skeleton open and load failed\n"))
+		goto cleanup;
+	prog_fd1 = bpf_program__fd(skel1->progs.xdp_handler);
+
+	skel2 = test_xdp_link__open_and_load();
+	if (CHECK(!skel2, "skel_load", "skeleton open and load failed\n"))
+		goto cleanup;
+	prog_fd2 = bpf_program__fd(skel2->progs.xdp_handler);
+
+	memset(&prog_info, 0, sizeof(prog_info));
+	err = bpf_obj_get_info_by_fd(prog_fd1, &prog_info, &prog_info_len);
+	if (CHECK(err, "fd_info1", "failed %d\n", -errno))
+		goto cleanup;
+	id1 = prog_info.id;
+
+	memset(&prog_info, 0, sizeof(prog_info));
+	err = bpf_obj_get_info_by_fd(prog_fd2, &prog_info, &prog_info_len);
+	if (CHECK(err, "fd_info2", "failed %d\n", -errno))
+		goto cleanup;
+	id2 = prog_info.id;
+
+	/* set initial prog attachment */
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, prog_fd1, XDP_FLAGS_REPLACE, &opts);
+	if (CHECK(err, "fd_attach", "initial prog attach failed: %d\n", err))
+		goto cleanup;
+
+	/* validate prog ID */
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	CHECK(err || id0 != id1, "id1_check",
+	      "loaded prog id %u != id1 %u, err %d", id0, id1, err);
+
+	/* BPF link is not allowed to replace prog attachment */
+	link = bpf_program__attach_xdp(skel1->progs.xdp_handler, IFINDEX_LO);
+	if (CHECK(!IS_ERR(link), "link_attach_fail", "unexpected success\n")) {
+		bpf_link__destroy(link);
+		/* best-effort detach prog */
+		opts.old_fd = prog_fd1;
+		bpf_set_link_xdp_fd_opts(IFINDEX_LO, -1, XDP_FLAGS_REPLACE, &opts);
+		goto cleanup;
+	}
+
+	/* detach BPF program */
+	opts.old_fd = prog_fd1;
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, -1, XDP_FLAGS_REPLACE, &opts);
+	if (CHECK(err, "prog_detach", "failed %d\n", err))
+		goto cleanup;
+
+	/* now BPF link should attach successfully */
+	link = bpf_program__attach_xdp(skel1->progs.xdp_handler, IFINDEX_LO);
+	if (CHECK(IS_ERR(link), "link_attach", "failed: %ld\n", PTR_ERR(link)))
+		goto cleanup;
+	skel1->links.xdp_handler = link;
+
+	/* validate prog ID */
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	if (CHECK(err || id0 != id1, "id1_check",
+		  "loaded prog id %u != id1 %u, err %d", id0, id1, err))
+		goto cleanup;
+
+	/* BPF prog attach is not allowed to replace BPF link */
+	opts.old_fd = prog_fd1;
+	err = bpf_set_link_xdp_fd_opts(IFINDEX_LO, prog_fd2, XDP_FLAGS_REPLACE, &opts);
+	if (CHECK(!err, "prog_attach_fail", "unexpected success\n"))
+		goto cleanup;
+
+	/* Can't force-update when BPF link is active */
+	err = bpf_set_link_xdp_fd(IFINDEX_LO, prog_fd2, 0);
+	if (CHECK(!err, "prog_update_fail", "unexpected success\n"))
+		goto cleanup;
+
+	/* Can't force-detach when BPF link is active */
+	err = bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);
+	if (CHECK(!err, "prog_detach_fail", "unexpected success\n"))
+		goto cleanup;
+
+	/* BPF link is not allowed to replace another BPF link */
+	link = bpf_program__attach_xdp(skel2->progs.xdp_handler, IFINDEX_LO);
+	if (CHECK(!IS_ERR(link), "link_attach_fail", "unexpected success\n")) {
+		bpf_link__destroy(link);
+		goto cleanup;
+	}
+
+	bpf_link__destroy(skel1->links.xdp_handler);
+	skel1->links.xdp_handler = NULL;
+
+	/* new link attach should succeed */
+	link = bpf_program__attach_xdp(skel2->progs.xdp_handler, IFINDEX_LO);
+	if (CHECK(IS_ERR(link), "link_attach", "failed: %ld\n", PTR_ERR(link)))
+		goto cleanup;
+	skel2->links.xdp_handler = link;
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &id0, 0);
+	if (CHECK(err || id0 != id2, "id2_check",
+		  "loaded prog id %u != id2 %u, err %d", id0, id1, err))
+		goto cleanup;
+
+	/* updating program under active BPF link works as expected */
+	err = bpf_link__update_program(link, skel1->progs.xdp_handler);
+	if (CHECK(err, "link_upd", "failed: %d\n", err))
+		goto cleanup;
+
+	memset(&link_info, 0, sizeof(link_info));
+	err = bpf_obj_get_info_by_fd(bpf_link__fd(link), &link_info, &link_info_len);
+	if (CHECK(err, "link_info", "failed: %d\n", err))
+		goto cleanup;
+
+	CHECK(link_info.type != BPF_LINK_TYPE_XDP, "link_type",
+	      "got %u != exp %u\n", link_info.type, BPF_LINK_TYPE_XDP);
+	CHECK(link_info.prog_id != id1, "link_prog_id",
+	      "got %u != exp %u\n", link_info.prog_id, id1);
+	CHECK(link_info.xdp.ifindex != IFINDEX_LO, "link_ifindex",
+	      "got %u != exp %u\n", link_info.xdp.ifindex, IFINDEX_LO);
+
+cleanup:
+	test_xdp_link__destroy(skel1);
+	test_xdp_link__destroy(skel2);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char LICENSE[] SEC("license") = "GPL";
+
+SEC("xdp/handler")
+int xdp_handler(struct xdp_md *xdp)
+{
+	return 0;
+}