selftests/bpf: Add a test for attaching a bpf fentry/fexit trace to an XDP program

Add a test that will attach a FENTRY and FEXIT program to the XDP test
program. It will also verify data from the XDP context on FENTRY and
verifies the return code on exit.
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157909410480.47481.11202505690938004673.stgit@xdp-tutorial

tools/testing/selftests/bpf/prog_tests/xdp_bpf2bpf.c

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <net/if.h>
+#include "test_xdp.skel.h"
+#include "test_xdp_bpf2bpf.skel.h"
+
+void test_xdp_bpf2bpf(void)
+{
+	__u32 duration = 0, retval, size;
+	char buf[128];
+	int err, pkt_fd, map_fd;
+	struct iphdr *iph = (void *)buf + sizeof(struct ethhdr);
+	struct iptnl_info value4 = {.family = AF_INET};
+	struct test_xdp *pkt_skel = NULL;
+	struct test_xdp_bpf2bpf *ftrace_skel = NULL;
+	struct vip key4 = {.protocol = 6, .family = AF_INET};
+	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+
+	/* Load XDP program to introspect */
+	pkt_skel = test_xdp__open_and_load();
+	if (CHECK(!pkt_skel, "pkt_skel_load", "test_xdp skeleton failed\n"))
+		return;
+
+	pkt_fd = bpf_program__fd(pkt_skel->progs._xdp_tx_iptunnel);
+
+	map_fd = bpf_map__fd(pkt_skel->maps.vip2tnl);
+	bpf_map_update_elem(map_fd, &key4, &value4, 0);
+
+	/* Load trace program */
+	opts.attach_prog_fd = pkt_fd,
+	ftrace_skel = test_xdp_bpf2bpf__open_opts(&opts);
+	if (CHECK(!ftrace_skel, "__open", "ftrace skeleton failed\n"))
+		goto out;
+
+	err = test_xdp_bpf2bpf__load(ftrace_skel);
+	if (CHECK(err, "__load", "ftrace skeleton failed\n"))
+		goto out;
+
+	err = test_xdp_bpf2bpf__attach(ftrace_skel);
+	if (CHECK(err, "ftrace_attach", "ftrace attach failed: %d\n", err))
+		goto out;
+
+	/* Run test program */
+	err = bpf_prog_test_run(pkt_fd, 1, &pkt_v4, sizeof(pkt_v4),
+				buf, &size, &retval, &duration);
+
+	if (CHECK(err || retval != XDP_TX || size != 74 ||
+		  iph->protocol != IPPROTO_IPIP, "ipv4",
+		  "err %d errno %d retval %d size %d\n",
+		  err, errno, retval, size))
+		goto out;
+
+	/* Verify test results */
+	if (CHECK(ftrace_skel->bss->test_result_fentry != if_nametoindex("lo"),
+		  "result", "fentry failed err %llu\n",
+		  ftrace_skel->bss->test_result_fentry))
+		goto out;
+
+	CHECK(ftrace_skel->bss->test_result_fexit != XDP_TX, "result",
+	      "fexit failed err %llu\n", ftrace_skel->bss->test_result_fexit);
+
+out:
+	test_xdp__destroy(pkt_skel);
+	test_xdp_bpf2bpf__destroy(ftrace_skel);
+}

tools/testing/selftests/bpf/progs/test_xdp_bpf2bpf.c

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include "bpf_helpers.h"
+#include "bpf_trace_helpers.h"
+
+struct net_device {
+	/* Structure does not need to contain all entries,
+	 * as "preserve_access_index" will use BTF to fix this...
+	 */
+	int ifindex;
+} __attribute__((preserve_access_index));
+
+struct xdp_rxq_info {
+	/* Structure does not need to contain all entries,
+	 * as "preserve_access_index" will use BTF to fix this...
+	 */
+	struct net_device *dev;
+	__u32 queue_index;
+} __attribute__((preserve_access_index));
+
+struct xdp_buff {
+	void *data;
+	void *data_end;
+	void *data_meta;
+	void *data_hard_start;
+	unsigned long handle;
+	struct xdp_rxq_info *rxq;
+} __attribute__((preserve_access_index));
+
+__u64 test_result_fentry = 0;
+SEC("fentry/_xdp_tx_iptunnel")
+int BPF_PROG(trace_on_entry, struct xdp_buff *xdp)
+{
+	test_result_fentry = xdp->rxq->dev->ifindex;
+	return 0;
+}
+
+__u64 test_result_fexit = 0;
+SEC("fexit/_xdp_tx_iptunnel")
+int BPF_PROG(trace_on_exit, struct xdp_buff *xdp, int ret)
+{
+	test_result_fexit = ret;
+	return 0;
+}