selftests/bpf: Add tailcall epilogue test

This patch adds a gen_epilogue test to test a main prog
using a bpf_tail_call.

A non test_loader test is used. The tailcall target program,
"test_epilogue_subprog", needs to be used in a struct_ops map
before it can be loaded. Another struct_ops map is also needed
to host the actual "test_epilogue_tailcall" struct_ops program
that does the bpf_tail_call. The earlier test_loader patch
will attach all struct_ops maps but the bpf_testmod.c does
not support >1 attached struct_ops.

The earlier patch used the test_loader which has already covered
checking for the patched pro/epilogue instructions. This is done
by the __xlated tag.

This patch goes for the regular skel load and syscall test to do
the tailcall test that can also allow to directly pass the
the "struct st_ops_args *args" as ctx_in to the
SEC("syscall") program.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240829210833.388152-8-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -3,8 +3,54 @@
 
 #include <test_progs.h>
 #include "pro_epilogue.skel.h"
+#include "epilogue_tailcall.skel.h"
+
+struct st_ops_args {
+	__u64 a;
+};
+
+static void test_tailcall(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	struct epilogue_tailcall *skel;
+	struct st_ops_args args;
+	int err, prog_fd;
+
+	skel = epilogue_tailcall__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "epilogue_tailcall__open_and_load"))
+		return;
+
+	topts.ctx_in = &args;
+	topts.ctx_size_in = sizeof(args);
+
+	skel->links.epilogue_tailcall =
+		bpf_map__attach_struct_ops(skel->maps.epilogue_tailcall);
+	if (!ASSERT_OK_PTR(skel->links.epilogue_tailcall, "attach_struct_ops"))
+		goto done;
+
+	/* Both test_epilogue_tailcall and test_epilogue_subprog are
+	 * patched with epilogue. When syscall_epilogue_tailcall()
+	 * is run, test_epilogue_tailcall() is triggered.
+	 * It executes a tail call and control is transferred to
+	 * test_epilogue_subprog(). Only test_epilogue_subprog()
+	 * does args->a += 1, thus final args.a value of 10001
+	 * guarantees that only the epilogue of the
+	 * test_epilogue_subprog is executed.
+	 */
+	memset(&args, 0, sizeof(args));
+	prog_fd = bpf_program__fd(skel->progs.syscall_epilogue_tailcall);
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+	ASSERT_OK(err, "bpf_prog_test_run_opts");
+	ASSERT_EQ(args.a, 10001, "args.a");
+	ASSERT_EQ(topts.retval, 10001 * 2, "topts.retval");
+
+done:
+	epilogue_tailcall__destroy(skel);
+}
 
 void test_pro_epilogue(void)
 {
 	RUN_TESTS(pro_epilogue);
+	if (test__start_subtest("tailcall"))
+		test_tailcall();
 }

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

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+#include "../bpf_testmod/bpf_testmod.h"
+#include "../bpf_testmod/bpf_testmod_kfunc.h"
+
+char _license[] SEC("license") = "GPL";
+
+static __noinline __used int subprog(struct st_ops_args *args)
+{
+	args->a += 1;
+	return args->a;
+}
+
+SEC("struct_ops/test_epilogue_subprog")
+int BPF_PROG(test_epilogue_subprog, struct st_ops_args *args)
+{
+	subprog(args);
+	return args->a;
+}
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__uint(value_size, sizeof(__u32));
+	__array(values, void (void));
+} epilogue_map SEC(".maps") = {
+	.values = {
+		[0] = (void *)&test_epilogue_subprog,
+	}
+};
+
+SEC("struct_ops/test_epilogue_tailcall")
+int test_epilogue_tailcall(unsigned long long *ctx)
+{
+	bpf_tail_call(ctx, &epilogue_map, 0);
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_st_ops epilogue_tailcall = {
+	.test_epilogue = (void *)test_epilogue_tailcall,
+};
+
+SEC(".struct_ops.link")
+struct bpf_testmod_st_ops epilogue_subprog = {
+	.test_epilogue = (void *)test_epilogue_subprog,
+};
+
+SEC("syscall")
+int syscall_epilogue_tailcall(struct st_ops_args *args)
+{
+	return bpf_kfunc_st_ops_test_epilogue(args);
+}