selftests/bpf: Add testcase for updating attached freplace prog to prog_array map

Add a selftest to confirm the issue, which gets -EINVAL when update
attached freplace prog to prog_array map, has been fixed.

cd tools/testing/selftests/bpf; ./test_progs -t tailcalls
328/25  tailcalls/tailcall_freplace:OK
328     tailcalls:OK
Summary: 1/25 PASSED, 0 SKIPPED, 0 FAILED
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20240728114612.48486-3-leon.hwang@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -5,7 +5,8 @@
 #include "tailcall_poke.skel.h"
 #include "tailcall_bpf2bpf_hierarchy2.skel.h"
 #include "tailcall_bpf2bpf_hierarchy3.skel.h"
-
+#include "tailcall_freplace.skel.h"
+#include "tc_bpf2bpf.skel.h"
 
 /* test_tailcall_1 checks basic functionality by patching multiple locations
  * in a single program for a single tail call slot with nop->jmp, jmp->nop
@@ -1495,6 +1496,66 @@ static void test_tailcall_bpf2bpf_hierarchy_3(void)
 	RUN_TESTS(tailcall_bpf2bpf_hierarchy3);
 }
 
+/* test_tailcall_freplace checks that the attached freplace prog is OK to
+ * update the prog_array map.
+ */
+static void test_tailcall_freplace(void)
+{
+	struct tailcall_freplace *freplace_skel = NULL;
+	struct bpf_link *freplace_link = NULL;
+	struct bpf_program *freplace_prog;
+	struct tc_bpf2bpf *tc_skel = NULL;
+	int prog_fd, map_fd;
+	char buff[128] = {};
+	int err, key;
+
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		    .data_in = buff,
+		    .data_size_in = sizeof(buff),
+		    .repeat = 1,
+	);
+
+	freplace_skel = tailcall_freplace__open();
+	if (!ASSERT_OK_PTR(freplace_skel, "tailcall_freplace__open"))
+		return;
+
+	tc_skel = tc_bpf2bpf__open_and_load();
+	if (!ASSERT_OK_PTR(tc_skel, "tc_bpf2bpf__open_and_load"))
+		goto out;
+
+	prog_fd = bpf_program__fd(tc_skel->progs.entry_tc);
+	freplace_prog = freplace_skel->progs.entry_freplace;
+	err = bpf_program__set_attach_target(freplace_prog, prog_fd, "subprog");
+	if (!ASSERT_OK(err, "set_attach_target"))
+		goto out;
+
+	err = tailcall_freplace__load(freplace_skel);
+	if (!ASSERT_OK(err, "tailcall_freplace__load"))
+		goto out;
+
+	freplace_link = bpf_program__attach_freplace(freplace_prog, prog_fd,
+						     "subprog");
+	if (!ASSERT_OK_PTR(freplace_link, "attach_freplace"))
+		goto out;
+
+	map_fd = bpf_map__fd(freplace_skel->maps.jmp_table);
+	prog_fd = bpf_program__fd(freplace_prog);
+	key = 0;
+	err = bpf_map_update_elem(map_fd, &key, &prog_fd, BPF_ANY);
+	if (!ASSERT_OK(err, "update jmp_table"))
+		goto out;
+
+	prog_fd = bpf_program__fd(tc_skel->progs.entry_tc);
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+	ASSERT_OK(err, "test_run");
+	ASSERT_EQ(topts.retval, 34, "test_run retval");
+
+out:
+	bpf_link__destroy(freplace_link);
+	tc_bpf2bpf__destroy(tc_skel);
+	tailcall_freplace__destroy(freplace_skel);
+}
+
 void test_tailcalls(void)
 {
 	if (test__start_subtest("tailcall_1"))
@@ -1543,4 +1604,6 @@ void test_tailcalls(void)
 		test_tailcall_bpf2bpf_hierarchy_fentry_entry();
 	test_tailcall_bpf2bpf_hierarchy_2();
 	test_tailcall_bpf2bpf_hierarchy_3();
+	if (test__start_subtest("tailcall_freplace"))
+		test_tailcall_freplace();
 }

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

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__uint(value_size, sizeof(__u32));
+} jmp_table SEC(".maps");
+
+int count = 0;
+
+SEC("freplace")
+int entry_freplace(struct __sk_buff *skb)
+{
+	count++;
+	bpf_tail_call_static(skb, &jmp_table, 0);
+	return count;
+}
+
+char __license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/tc_bpf2bpf.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+__noinline
+int subprog(struct __sk_buff *skb)
+{
+	int ret = 1;
+
+	__sink(ret);
+	return ret;
+}
+
+SEC("tc")
+int entry_tc(struct __sk_buff *skb)
+{
+	return subprog(skb);
+}
+
+char __license[] SEC("license") = "GPL";