selftests/bpf: add CO-RE relocs ptr-as-array tests

Add test validating correct relocation handling for cases where pointer
to something is used as an array. E.g.:

int *ptr = ...;
int x = ptr[42];

Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	.output_len = sizeof(struct core_reloc_mods_output),		\
 }
 
+#define PTR_AS_ARR_CASE(name) {						\
+	.case_name = #name,						\
+	.bpf_obj_file = "test_core_reloc_ptr_as_arr.o",			\
+	.btf_src_file = "btf__core_reloc_" #name ".o",			\
+	.input = (const char *)&(struct core_reloc_##name []){		\
+		{ .a = 1 },						\
+		{ .a = 2 },						\
+		{ .a = 3 },						\
+	},								\
+	.input_len = 3 * sizeof(struct core_reloc_##name),		\
+	.output = STRUCT_TO_CHAR_PTR(core_reloc_ptr_as_arr) {		\
+		.a = 3,							\
+	},								\
+	.output_len = sizeof(struct core_reloc_ptr_as_arr),		\
+}
+
 struct core_reloc_test_case {
 	const char *case_name;
 	const char *bpf_obj_file;
···
 	MODS_CASE(mods),
 	MODS_CASE(mods___mod_swap),
 	MODS_CASE(mods___typedefs),
+
+	/* handling "ptr is an array" semantics */
+	PTR_AS_ARR_CASE(ptr_as_arr),
+	PTR_AS_ARR_CASE(ptr_as_arr___diff_sz),
 };
 
 struct data {

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

···
+#include "core_reloc_types.h"
+
+void f(struct core_reloc_ptr_as_arr x) {}

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

···
+#include "core_reloc_types.h"
+
+void f(struct core_reloc_ptr_as_arr___diff_sz x) {}

tools/testing/selftests/bpf/progs/core_reloc_types.h

···
 	int3_t b;
 	int3_t a;
 };
+
+/*
+ * PTR_AS_ARR
+ */
+struct core_reloc_ptr_as_arr {
+	int a;
+};
+
+struct core_reloc_ptr_as_arr___diff_sz {
+	int :32; /* padding */
+	char __some_more_padding;
+	int a;
+};

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

···
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2019 Facebook
+
+#include <linux/bpf.h>
+#include <stdint.h>
+#include "bpf_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+static volatile struct data {
+	char in[256];
+	char out[256];
+} data;
+
+struct core_reloc_ptr_as_arr {
+	int a;
+};
+
+SEC("raw_tracepoint/sys_enter")
+int test_core_ptr_as_arr(void *ctx)
+{
+	struct core_reloc_ptr_as_arr *in = (void *)&data.in;
+	struct core_reloc_ptr_as_arr *out = (void *)&data.out;
+
+	if (BPF_CORE_READ(&out->a, &in[2].a))
+		return 1;
+
+	return 0;
+}
+