bpftool: Add C++-specific open/load/etc skeleton wrappers

Add C++-specific static methods for code-generated BPF skeleton for each
skeleton operation: open, open_opts, open_and_load, load, attach,
detach, destroy, and elf_bytes. This is to facilitate easier C++
templating on top of pure C BPF skeleton.

In C, open/load/destroy/etc "methods" are of the form
<skeleton_name>__<method>() to avoid name collision with similar
"methods" of other skeletons withint the same application. This works
well, but is very inconvenient for C++ applications that would like to
write generic (templated) wrappers around BPF skeleton to fit in with
C++ code base and take advantage of destructors and other convenient C++
constructs.

This patch makes it easier to build such generic templated wrappers by
additionally defining C++ static methods for skeleton's struct with
fixed names. This allows to refer to, say, open method as `T::open()`
instead of having to somehow generate `T__open()` function call.

Next patch adds an example template to test_cpp selftest to demonstrate
how it's possible to have all the operations wrapped in a generic
Skeleton<my_skeleton> type without explicitly passing function references.

An example of generated declaration section without %1$s placeholders:

  #ifdef __cplusplus
      static struct test_attach_probe *open(const struct bpf_object_open_opts *opts = nullptr);
      static struct test_attach_probe *open_and_load();
      static int load(struct test_attach_probe *skel);
      static int attach(struct test_attach_probe *skel);
      static void detach(struct test_attach_probe *skel);
      static void destroy(struct test_attach_probe *skel);
      static const void *elf_bytes(size_t *sz);
  #endif /* __cplusplus */

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220212055733.539056-2-andrii@kernel.org

tools/bpf/bpftool/gen.c

@@ -831,6 +831,16 @@ static int do_skeleton(int argc, char **argv)
 
 	codegen("\
 		\n\
+									    \n\
+		#ifdef __cplusplus					    \n\
+			static struct %1$s *open(const struct bpf_object_open_opts *opts = nullptr);\n\
+			static struct %1$s *open_and_load();		    \n\
+			static int load(struct %1$s *skel);		    \n\
+			static int attach(struct %1$s *skel);		    \n\
+			static void detach(struct %1$s *skel);		    \n\
+			static void destroy(struct %1$s *skel);		    \n\
+			static const void *elf_bytes(size_t *sz);	    \n\
+		#endif /* __cplusplus */				    \n\
 		};							    \n\
 									    \n\
 		static void						    \n\
@@ -1025,9 +1035,19 @@ static int do_skeleton(int argc, char **argv)
 		\";							    \n\
 		}							    \n\
 									    \n\
-		#endif /* %s */						    \n\
+		#ifdef __cplusplus					    \n\
+		struct %1$s *%1$s::open(const struct bpf_object_open_opts *opts) { return %1$s__open_opts(opts); }\n\
+		struct %1$s *%1$s::open_and_load() { return %1$s__open_and_load(); }	\n\
+		int %1$s::load(struct %1$s *skel) { return %1$s__load(skel); }		\n\
+		int %1$s::attach(struct %1$s *skel) { return %1$s__attach(skel); }	\n\
+		void %1$s::detach(struct %1$s *skel) { %1$s__detach(skel); }		\n\
+		void %1$s::destroy(struct %1$s *skel) { %1$s__destroy(skel); }		\n\
+		const void *%1$s::elf_bytes(size_t *sz) { return %1$s__elf_bytes(sz); } \n\
+		#endif /* __cplusplus */				    \n\
+									    \n\
+		#endif /* %2$s */					    \n\
 		",
-		header_guard);
+		obj_name, header_guard);
 	err = 0;
 out:
 	bpf_object__close(obj);