bpftool: Make skeleton C code compilable with C++ compiler

When auto-generated BPF skeleton C code is included from C++ application, it
triggers compilation error due to void * being implicitly casted to whatever
target pointer type. This is supported by C, but not C++. To solve this
problem, add explicit casts, where necessary.

To ensure issues like this are captured going forward, add skeleton usage in
test_cpp test.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191226210253.3132060-1-andriin@fb.com

tools/bpf/bpftool/gen.c

@@ -397,7 +397,7 @@ static int do_skeleton(int argc, char **argv)
 		{							    \n\
 			struct %1$s *obj;				    \n\
 									    \n\
-			obj = calloc(1, sizeof(*obj));			    \n\
+			obj = (typeof(obj))calloc(1, sizeof(*obj));	    \n\
 			if (!obj)					    \n\
 				return NULL;				    \n\
 			if (%1$s__create_skeleton(obj))			    \n\
@@ -461,7 +461,7 @@ static int do_skeleton(int argc, char **argv)
 		{							    \n\
 			struct bpf_object_skeleton *s;			    \n\
 									    \n\
-			s = calloc(1, sizeof(*s));			    \n\
+			s = (typeof(s))calloc(1, sizeof(*s));		    \n\
 			if (!s)						    \n\
 				return -1;				    \n\
 			obj->skeleton = s;				    \n\
@@ -479,7 +479,7 @@ static int do_skeleton(int argc, char **argv)
 				/* maps */				    \n\
 				s->map_cnt = %zu;			    \n\
 				s->map_skel_sz = sizeof(*s->maps);	    \n\
-				s->maps = calloc(s->map_cnt, s->map_skel_sz);\n\
+				s->maps = (typeof(s->maps))calloc(s->map_cnt, s->map_skel_sz);\n\
 				if (!s->maps)				    \n\
 					goto err;			    \n\
 			",
@@ -515,7 +515,7 @@ static int do_skeleton(int argc, char **argv)
 				/* programs */				    \n\
 				s->prog_cnt = %zu;			    \n\
 				s->prog_skel_sz = sizeof(*s->progs);	    \n\
-				s->progs = calloc(s->prog_cnt, s->prog_skel_sz);\n\
+				s->progs = (typeof(s->progs))calloc(s->prog_cnt, s->prog_skel_sz);\n\
 				if (!s->progs)				    \n\
 					goto err;			    \n\
 			",
@@ -538,7 +538,7 @@ static int do_skeleton(int argc, char **argv)
 		\n\
 									    \n\
 			s->data_sz = %d;				    \n\
-			s->data = \"\\					    \n\
+			s->data = (void *)\"\\				    \n\
 		",
 		file_sz);
 

tools/testing/selftests/bpf/Makefile

@@ -371,7 +371,7 @@ $(OUTPUT)/test_verifier: test_verifier.c verifier/tests.h $(BPFOBJ) | $(OUTPUT)
 	$(CC) $(CFLAGS) $(filter %.a %.o %.c,$^) $(LDLIBS) -o $@
 
 # Make sure we are able to include and link libbpf against c++.
-$(OUTPUT)/test_cpp: test_cpp.cpp $(BPFOBJ)
+$(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
 	$(call msg,        CXX,,$@)
 	$(CXX) $(CFLAGS) $^ $(LDLIBS) -o $@
 

tools/testing/selftests/bpf/test_cpp.cpp

 /* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+#include <iostream>
 #include "libbpf.h"
 #include "bpf.h"
 #include "btf.h"
+#include "test_core_extern.skel.h"
 
 /* do nothing, just make sure we can link successfully */
 
 int main(int argc, char *argv[])
 {
+	struct test_core_extern *skel;
+
 	/* libbpf.h */
 	libbpf_set_print(NULL);
 
@@ -16,5 +20,11 @@ int main(int argc, char *argv[])
 	/* btf.h */
 	btf__new(NULL, 0);
 
+	/* BPF skeleton */
+	skel = test_core_extern__open_and_load();
+	test_core_extern__destroy(skel);
+
+	std::cout << "DONE!" << std::endl;
+
 	return 0;
 }