selftests/bpf: Add test for bpf_get_link_xdp_id

Add xdp_info selftest that makes sure that bpf_get_link_xdp_id returns
valid prog_id for different input modes:

  * w/ and w/o flags when no program is attached;
  * w/ and w/o flags when one program is attached.
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/2a9a6d1ce33b91ccc1aa3de6dba2d309f2062811.1586236080.git.rdna@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/if_link.h>
+#include <test_progs.h>
+
+#define IFINDEX_LO 1
+
+void test_xdp_info(void)
+{
+	__u32 len = sizeof(struct bpf_prog_info), duration = 0, prog_id;
+	const char *file = "./xdp_dummy.o";
+	struct bpf_prog_info info = {};
+	struct bpf_object *obj;
+	int err, prog_fd;
+
+	/* Get prog_id for XDP_ATTACHED_NONE mode */
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &prog_id, 0);
+	if (CHECK(err, "get_xdp_none", "errno=%d\n", errno))
+		return;
+	if (CHECK(prog_id, "prog_id_none", "unexpected prog_id=%u\n", prog_id))
+		return;
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &prog_id, XDP_FLAGS_SKB_MODE);
+	if (CHECK(err, "get_xdp_none_skb", "errno=%d\n", errno))
+		return;
+	if (CHECK(prog_id, "prog_id_none_skb", "unexpected prog_id=%u\n",
+		  prog_id))
+		return;
+
+	/* Setup prog */
+
+	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj, &prog_fd);
+	if (CHECK_FAIL(err))
+		return;
+
+	err = bpf_obj_get_info_by_fd(prog_fd, &info, &len);
+	if (CHECK(err, "get_prog_info", "errno=%d\n", errno))
+		goto out_close;
+
+	err = bpf_set_link_xdp_fd(IFINDEX_LO, prog_fd, XDP_FLAGS_SKB_MODE);
+	if (CHECK(err, "set_xdp_skb", "errno=%d\n", errno))
+		goto out_close;
+
+	/* Get prog_id for single prog mode */
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &prog_id, 0);
+	if (CHECK(err, "get_xdp", "errno=%d\n", errno))
+		goto out;
+	if (CHECK(prog_id != info.id, "prog_id", "prog_id not available\n"))
+		goto out;
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &prog_id, XDP_FLAGS_SKB_MODE);
+	if (CHECK(err, "get_xdp_skb", "errno=%d\n", errno))
+		goto out;
+	if (CHECK(prog_id != info.id, "prog_id_skb", "prog_id not available\n"))
+		goto out;
+
+	err = bpf_get_link_xdp_id(IFINDEX_LO, &prog_id, XDP_FLAGS_DRV_MODE);
+	if (CHECK(err, "get_xdp_drv", "errno=%d\n", errno))
+		goto out;
+	if (CHECK(prog_id, "prog_id_drv", "unexpected prog_id=%u\n", prog_id))
+		goto out;
+
+out:
+	bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);
+out_close:
+	bpf_object__close(obj);
+}