Add a test to make sure implicit struct_ops association does not
break backward compatibility nor return incorrect struct_ops.
struct_ops programs should still be allowed to be reused in
different struct_ops map. The associated struct_ops map set implicitly
however will be poisoned. Trying to read it through the helper
bpf_prog_get_assoc_struct_ops() should result in a NULL pointer.

While recursion of test_1() cannot happen due to the associated
struct_ops being ambiguois, explicitly check for it to prevent stack
overflow if the test regresses.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
 .../bpf/prog_tests/test_struct_ops_assoc.c    | 38 ++++++++++
 .../bpf/progs/struct_ops_assoc_reuse.c        | 75 +++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_assoc_reuse.c

diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_assoc.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_assoc.c
index 29e8b58a14fa..f69306cb8974 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_assoc.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_assoc.c
@@ -2,6 +2,7 @@
 
 #include <test_progs.h>
 #include "struct_ops_assoc.skel.h"
+#include "struct_ops_assoc_reuse.skel.h"
 
 static void test_st_ops_assoc(void)
 {
@@ -65,8 +66,45 @@ static void test_st_ops_assoc(void)
 	struct_ops_assoc__destroy(skel);
 }
 
+static void test_st_ops_assoc_reuse(void)
+{
+	struct struct_ops_assoc_reuse *skel = NULL;
+	int err;
+
+	skel = struct_ops_assoc_reuse__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "struct_ops_assoc_reuse__open"))
+		goto out;
+
+	err = bpf_program__assoc_struct_ops(skel->progs.syscall_prog_a,
+					    skel->maps.st_ops_map_a, NULL);
+	ASSERT_OK(err, "bpf_program__assoc_struct_ops");
+
+	err = bpf_program__assoc_struct_ops(skel->progs.syscall_prog_b,
+					    skel->maps.st_ops_map_b, NULL);
+	ASSERT_OK(err, "bpf_program__assoc_struct_ops");
+
+	err = struct_ops_assoc_reuse__attach(skel);
+	if (!ASSERT_OK(err, "struct_ops_assoc__attach"))
+		goto out;
+
+	/* run syscall_prog that calls .test_1 and checks return */
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.syscall_prog_a), NULL);
+	ASSERT_OK(err, "bpf_prog_test_run_opts");
+
+	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.syscall_prog_b), NULL);
+	ASSERT_OK(err, "bpf_prog_test_run_opts");
+
+	ASSERT_EQ(skel->bss->test_err_a, 0, "skel->bss->test_err_a");
+	ASSERT_EQ(skel->bss->test_err_b, 0, "skel->bss->test_err_b");
+
+out:
+	struct_ops_assoc_reuse__destroy(skel);
+}
+
 void test_struct_ops_assoc(void)
 {
 	if (test__start_subtest("st_ops_assoc"))
 		test_st_ops_assoc();
+	if (test__start_subtest("st_ops_assoc_reuse"))
+		test_st_ops_assoc_reuse();
 }
diff --git a/tools/testing/selftests/bpf/progs/struct_ops_assoc_reuse.c b/tools/testing/selftests/bpf/progs/struct_ops_assoc_reuse.c
new file mode 100644
index 000000000000..caaa45bdccc2
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/struct_ops_assoc_reuse.c
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+#include "../test_kmods/bpf_testmod.h"
+#include "../test_kmods/bpf_testmod_kfunc.h"
+
+char _license[] SEC("license") = "GPL";
+
+#define MAP_A_MAGIC 1234
+int test_err_a;
+int recur;
+
+/*
+ * test_1_a is reused. The kfunc should not be able to get the associated
+ * struct_ops and call test_1 recursively as it is ambiguous.
+ */
+SEC("struct_ops")
+int BPF_PROG(test_1_a, struct st_ops_args *args)
+{
+	int ret;
+
+	if (!recur) {
+		recur++;
+		ret = bpf_kfunc_multi_st_ops_test_1_prog_arg(args, NULL);
+		if (ret != -1)
+			test_err_a++;
+		recur--;
+	}
+
+	return MAP_A_MAGIC;
+}
+
+/* Programs associated with st_ops_map_a */
+
+SEC("syscall")
+int syscall_prog_a(void *ctx)
+{
+	struct st_ops_args args = {};
+	int ret;
+
+	ret = bpf_kfunc_multi_st_ops_test_1_prog_arg(&args, NULL);
+	if (ret != MAP_A_MAGIC)
+		test_err_a++;
+
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_multi_st_ops st_ops_map_a = {
+	.test_1 = (void *)test_1_a,
+};
+
+/* Programs associated with st_ops_map_b */
+
+int test_err_b;
+
+SEC("syscall")
+int syscall_prog_b(void *ctx)
+{
+	struct st_ops_args args = {};
+	int ret;
+
+	ret = bpf_kfunc_multi_st_ops_test_1_prog_arg(&args, NULL);
+	if (ret != MAP_A_MAGIC)
+		test_err_b++;
+
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_multi_st_ops st_ops_map_b = {
+	.test_1 = (void *)test_1_a,
+};
-- 
2.47.3