selftests/bpf: Add testcase for xdp attaching failure tracepoint

Add a test case for the tracepoint of xdp attaching failure by bpf
tracepoint when attach XDP to a device with invalid flags option.

The bpf tracepoint retrieves error message from the tracepoint, and
then put the error message to a perf buffer. The testing code receives
error message from perf buffer, and then ASSERT "Invalid XDP flags for
BPF link attachment".
Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
Link: https://lore.kernel.org/r/20230801142621.7925-3-hffilwlqm@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
+#include "test_xdp_attach_fail.skel.h"
 
 #define IFINDEX_LO 1
 #define XDP_FLAGS_REPLACE		(1U << 4)
@@ -85,10 +86,74 @@ static void test_xdp_attach(const char *file)
 	bpf_object__close(obj1);
 }
 
+#define ERRMSG_LEN 64
+
+struct xdp_errmsg {
+	char msg[ERRMSG_LEN];
+};
+
+static void on_xdp_errmsg(void *ctx, int cpu, void *data, __u32 size)
+{
+	struct xdp_errmsg *ctx_errmg = ctx, *tp_errmsg = data;
+
+	memcpy(&ctx_errmg->msg, &tp_errmsg->msg, ERRMSG_LEN);
+}
+
+static const char tgt_errmsg[] = "Invalid XDP flags for BPF link attachment";
+
+static void test_xdp_attach_fail(const char *file)
+{
+	struct test_xdp_attach_fail *skel = NULL;
+	struct xdp_errmsg errmsg = {};
+	struct perf_buffer *pb = NULL;
+	struct bpf_object *obj = NULL;
+	int err, fd_xdp;
+
+	LIBBPF_OPTS(bpf_link_create_opts, opts);
+
+	skel = test_xdp_attach_fail__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_xdp_attach_fail__open_and_load"))
+		goto out_close;
+
+	err = test_xdp_attach_fail__attach(skel);
+	if (!ASSERT_EQ(err, 0, "test_xdp_attach_fail__attach"))
+		goto out_close;
+
+	/* set up perf buffer */
+	pb = perf_buffer__new(bpf_map__fd(skel->maps.xdp_errmsg_pb), 1,
+			      on_xdp_errmsg, NULL, &errmsg, NULL);
+	if (!ASSERT_OK_PTR(pb, "perf_buffer__new"))
+		goto out_close;
+
+	err = bpf_prog_test_load(file, BPF_PROG_TYPE_XDP, &obj, &fd_xdp);
+	if (!ASSERT_EQ(err, 0, "bpf_prog_test_load"))
+		goto out_close;
+
+	opts.flags = 0xFF; // invalid flags to fail to attach XDP prog
+	err = bpf_link_create(fd_xdp, IFINDEX_LO, BPF_XDP, &opts);
+	if (!ASSERT_EQ(err, -EINVAL, "bpf_link_create"))
+		goto out_close;
+
+	/* read perf buffer */
+	err = perf_buffer__poll(pb, 100);
+	if (!ASSERT_GT(err, -1, "perf_buffer__poll"))
+		goto out_close;
+
+	ASSERT_STRNEQ((const char *) errmsg.msg, tgt_errmsg,
+		      42 /* strlen(tgt_errmsg) */, "check error message");
+
+out_close:
+	perf_buffer__free(pb);
+	bpf_object__close(obj);
+	test_xdp_attach_fail__destroy(skel);
+}
+
 void serial_test_xdp_attach(void)
 {
 	if (test__start_subtest("xdp_attach"))
 		test_xdp_attach("./test_xdp.bpf.o");
 	if (test__start_subtest("xdp_attach_dynptr"))
 		test_xdp_attach("./test_xdp_dynptr.bpf.o");
+	if (test__start_subtest("xdp_attach_failed"))
+		test_xdp_attach_fail("./xdp_dummy.bpf.o");
 }

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright Leon Hwang */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+#define ERRMSG_LEN 64
+
+struct xdp_errmsg {
+	char msg[ERRMSG_LEN];
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
+	__type(key, int);
+	__type(value, int);
+} xdp_errmsg_pb SEC(".maps");
+
+struct xdp_attach_error_ctx {
+	unsigned long unused;
+
+	/*
+	 * bpf does not support tracepoint __data_loc directly.
+	 *
+	 * Actually, this field is a 32 bit integer whose value encodes
+	 * information on where to find the actual data. The first 2 bytes is
+	 * the size of the data. The last 2 bytes is the offset from the start
+	 * of the tracepoint struct where the data begins.
+	 * -- https://github.com/iovisor/bpftrace/pull/1542
+	 */
+	__u32 msg; // __data_loc char[] msg;
+};
+
+/*
+ * Catch the error message at the tracepoint.
+ */
+
+SEC("tp/xdp/bpf_xdp_link_attach_failed")
+int tp__xdp__bpf_xdp_link_attach_failed(struct xdp_attach_error_ctx *ctx)
+{
+	char *msg = (void *)(__u64) ((void *) ctx + (__u16) ctx->msg);
+	struct xdp_errmsg errmsg = {};
+
+	bpf_probe_read_kernel_str(&errmsg.msg, ERRMSG_LEN, msg);
+	bpf_perf_event_output(ctx, &xdp_errmsg_pb, BPF_F_CURRENT_CPU, &errmsg,
+			      ERRMSG_LEN);
+	return 0;
+}
+
+/*
+ * Reuse the XDP program in xdp_dummy.c.
+ */
+
+char LICENSE[] SEC("license") = "GPL";