selftests/bpf: Add test to access integer type of variable array

Add prog test for accessing integer type of variable array in tracing
program.
In addition, hook load_balance function to access sd->span[0], only
to confirm whether the load is successful. Because there is no direct
way to trigger load_balance call.

Co-developed-by: Chengming Zhou <[e-mail]>
Signed-off-by: Chengming Zhou <[e-mail]>
Signed-off-by: Feng Zhou <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

···
 	long b;
 };
 
+struct bpf_testmod_struct_arg_3 {
+	int a;
+	int b[];
+};
+
 __diag_push();
 __diag_ignore_all("-Wmissing-prototypes",
 		  "Global functions as their definitions will be in bpf_testmod.ko BTF");
···
 noinline int
 bpf_testmod_test_struct_arg_5(void) {
 	bpf_testmod_test_struct_arg_result = 1;
+	return bpf_testmod_test_struct_arg_result;
+}
+
+noinline int
+bpf_testmod_test_struct_arg_6(struct bpf_testmod_struct_arg_3 *a) {
+	bpf_testmod_test_struct_arg_result = a->b[0];
 	return bpf_testmod_test_struct_arg_result;
 }
 
···
 	};
 	struct bpf_testmod_struct_arg_1 struct_arg1 = {10};
 	struct bpf_testmod_struct_arg_2 struct_arg2 = {2, 3};
+	struct bpf_testmod_struct_arg_3 *struct_arg3;
 	int i = 1;
 
 	while (bpf_testmod_return_ptr(i))
···
 	(void)bpf_testmod_test_struct_arg_3(1, 4, struct_arg2);
 	(void)bpf_testmod_test_struct_arg_4(struct_arg1, 1, 2, 3, struct_arg2);
 	(void)bpf_testmod_test_struct_arg_5();
+
+	struct_arg3 = kmalloc((sizeof(struct bpf_testmod_struct_arg_3) +
+				sizeof(int)), GFP_KERNEL);
+	if (struct_arg3 != NULL) {
+		struct_arg3->b[0] = 1;
+		(void)bpf_testmod_test_struct_arg_6(struct_arg3);
+		kfree(struct_arg3);
+	}
 
 	/* This is always true. Use the check to make sure the compiler
 	 * doesn't remove bpf_testmod_loop_test.

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Bytedance */
+
+#include <test_progs.h>
+#include "test_access_variable_array.skel.h"
+
+void test_access_variable_array(void)
+{
+	struct test_access_variable_array *skel;
+
+	skel = test_access_variable_array__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_access_variable_array__open_and_load"))
+		return;
+
+	test_access_variable_array__destroy(skel);
+}

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

···
 
 	ASSERT_EQ(skel->bss->t5_ret, 1, "t5 ret");
 
+	ASSERT_EQ(skel->bss->t6, 1, "t6 ret");
+
 	tracing_struct__detach(skel);
 destroy_skel:
 	tracing_struct__destroy(skel);

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Bytedance */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+unsigned long span = 0;
+
+SEC("fentry/load_balance")
+int BPF_PROG(fentry_fentry, int this_cpu, struct rq *this_rq,
+		struct sched_domain *sd)
+{
+	span = sd->span[0];
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";

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

···
 	long b;
 };
 
+struct bpf_testmod_struct_arg_3 {
+	int a;
+	int b[];
+};
+
 long t1_a_a, t1_a_b, t1_b, t1_c, t1_ret, t1_nregs;
 __u64 t1_reg0, t1_reg1, t1_reg2, t1_reg3;
 long t2_a, t2_b_a, t2_b_b, t2_c, t2_ret;
 long t3_a, t3_b, t3_c_a, t3_c_b, t3_ret;
 long t4_a_a, t4_b, t4_c, t4_d, t4_e_a, t4_e_b, t4_ret;
 long t5_ret;
+int t6;
 
 SEC("fentry/bpf_testmod_test_struct_arg_1")
 int BPF_PROG2(test_struct_arg_1, struct bpf_testmod_struct_arg_2, a, int, b, int, c)
···
 int BPF_PROG2(test_struct_arg_10, int, ret)
 {
 	t5_ret = ret;
+	return 0;
+}
+
+SEC("fentry/bpf_testmod_test_struct_arg_6")
+int BPF_PROG2(test_struct_arg_11, struct bpf_testmod_struct_arg_3 *, a)
+{
+	t6 = a->b[0];
 	return 0;
 }