selftests/bpf: freplace tests for tracking of changes_packet_data

Try different combinations of global functions replacement:
- replace function that changes packet data with one that doesn't;
- replace function that changes packet data with one that does;
- replace function that doesn't change packet data with one that does;
- replace function that doesn't change packet data with one that doesn't;

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20241210041100.1898468-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "bpf/libbpf.h"
+#include "changes_pkt_data_freplace.skel.h"
+#include "changes_pkt_data.skel.h"
+#include <test_progs.h>
+
+static void print_verifier_log(const char *log)
+{
+	if (env.verbosity >= VERBOSE_VERY)
+		fprintf(stdout, "VERIFIER LOG:\n=============\n%s=============\n", log);
+}
+
+static void test_aux(const char *main_prog_name, const char *freplace_prog_name, bool expect_load)
+{
+	struct changes_pkt_data_freplace *freplace = NULL;
+	struct bpf_program *freplace_prog = NULL;
+	LIBBPF_OPTS(bpf_object_open_opts, opts);
+	struct changes_pkt_data *main = NULL;
+	char log[16*1024];
+	int err;
+
+	opts.kernel_log_buf = log;
+	opts.kernel_log_size = sizeof(log);
+	if (env.verbosity >= VERBOSE_SUPER)
+		opts.kernel_log_level = 1 | 2 | 4;
+	main = changes_pkt_data__open_opts(&opts);
+	if (!ASSERT_OK_PTR(main, "changes_pkt_data__open"))
+		goto out;
+	err = changes_pkt_data__load(main);
+	print_verifier_log(log);
+	if (!ASSERT_OK(err, "changes_pkt_data__load"))
+		goto out;
+	freplace = changes_pkt_data_freplace__open_opts(&opts);
+	if (!ASSERT_OK_PTR(freplace, "changes_pkt_data_freplace__open"))
+		goto out;
+	freplace_prog = bpf_object__find_program_by_name(freplace->obj, freplace_prog_name);
+	if (!ASSERT_OK_PTR(freplace_prog, "freplace_prog"))
+		goto out;
+	bpf_program__set_autoload(freplace_prog, true);
+	bpf_program__set_autoattach(freplace_prog, true);
+	bpf_program__set_attach_target(freplace_prog,
+				       bpf_program__fd(main->progs.dummy),
+				       main_prog_name);
+	err = changes_pkt_data_freplace__load(freplace);
+	print_verifier_log(log);
+	if (expect_load) {
+		ASSERT_OK(err, "changes_pkt_data_freplace__load");
+	} else {
+		ASSERT_ERR(err, "changes_pkt_data_freplace__load");
+		ASSERT_HAS_SUBSTR(log, "Extension program changes packet data", "error log");
+	}
+
+out:
+	changes_pkt_data_freplace__destroy(freplace);
+	changes_pkt_data__destroy(main);
+}
+
+/* There are two global subprograms in both changes_pkt_data.skel.h:
+ * - one changes packet data;
+ * - another does not.
+ * It is ok to freplace subprograms that change packet data with those
+ * that either do or do not. It is only ok to freplace subprograms
+ * that do not change packet data with those that do not as well.
+ * The below tests check outcomes for each combination of such freplace.
+ */
+void test_changes_pkt_data_freplace(void)
+{
+	if (test__start_subtest("changes_with_changes"))
+		test_aux("changes_pkt_data", "changes_pkt_data", true);
+	if (test__start_subtest("changes_with_doesnt_change"))
+		test_aux("changes_pkt_data", "does_not_change_pkt_data", true);
+	if (test__start_subtest("doesnt_change_with_changes"))
+		test_aux("does_not_change_pkt_data", "changes_pkt_data", false);
+	if (test__start_subtest("doesnt_change_with_doesnt_change"))
+		test_aux("does_not_change_pkt_data", "does_not_change_pkt_data", true);
+}

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

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+__noinline
+long changes_pkt_data(struct __sk_buff *sk, __u32 len)
+{
+	return bpf_skb_pull_data(sk, len);
+}
+
+__noinline __weak
+long does_not_change_pkt_data(struct __sk_buff *sk, __u32 len)
+{
+	return 0;
+}
+
+SEC("tc")
+int dummy(struct __sk_buff *sk)
+{
+	changes_pkt_data(sk, 0);
+	does_not_change_pkt_data(sk, 0);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";

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

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("?freplace")
+long changes_pkt_data(struct __sk_buff *sk, __u32 len)
+{
+	return bpf_skb_pull_data(sk, len);
+}
+
+SEC("?freplace")
+long does_not_change_pkt_data(struct __sk_buff *sk, __u32 len)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";