bpftool: Cast pointers for shadow types explicitly.

According to a report, skeletons fail to assign shadow pointers when being
compiled with C++ programs. Unlike C doing implicit casting for void
pointers, C++ requires an explicit casting.

To support C++, we do explicit casting for each shadow pointer.

Also add struct_ops_module.skel.h to test_cpp to validate C++
compilation as part of BPF selftests.
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20240312013726.1780720-1-thinker.li@gmail.com

tools/bpf/bpftool/gen.c

@@ -1131,7 +1131,8 @@ static void gen_st_ops_shadow_init(struct btf *btf, struct bpf_object *obj)
 			continue;
 		codegen("\
 			\n\
-				obj->struct_ops.%1$s = bpf_map__initial_value(obj->maps.%1$s, NULL);\n\
+				obj->struct_ops.%1$s = (typeof(obj->struct_ops.%1$s))\n\
+					bpf_map__initial_value(obj->maps.%1$s, NULL);\n\
 			\n\
 			", ident);
 	}

tools/testing/selftests/bpf/test_cpp.cpp

@@ -7,6 +7,7 @@
 #include <bpf/bpf.h>
 #include <bpf/btf.h>
 #include "test_core_extern.skel.h"
+#include "struct_ops_module.skel.h"
 
 template <typename T>
 class Skeleton {
@@ -98,6 +99,7 @@ int main(int argc, char *argv[])
 {
 	struct btf_dump_opts opts = { };
 	struct test_core_extern *skel;
+	struct struct_ops_module *skel2;
 	struct btf *btf;
 	int fd;
 
@@ -118,6 +120,9 @@ int main(int argc, char *argv[])
 	skel = test_core_extern__open_and_load();
 	test_core_extern__destroy(skel);
 
+	skel2 = struct_ops_module__open_and_load();
+	struct_ops_module__destroy(skel2);
+
 	fd = bpf_enable_stats(BPF_STATS_RUN_TIME);
 	if (fd < 0)
 		std::cout << "FAILED to enable stats: " << fd << std::endl;