Merge branch 'Make BPF skeleton easier to use from C++ code'

Andrii Nakryiko says:

====================

Add minimal C++-specific additions to BPF skeleton codegen to facilitate
easier use of C skeletons in C++ applications. These additions don't add any
extra ongoing maintenance and allows C++ users to fit pure C skeleton better
into their C++ code base. All that without the need to design, implement and
support a separate C++ BPF skeleton implementation.

v1->v2:
  - use default argument values in T::open() (Alexei).
====================
Signed-off-by: Alexei Starovoitov <ast@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);

tools/testing/selftests/bpf/test_cpp.cpp

@@ -5,18 +5,100 @@
 #include <bpf/btf.h>
 #include "test_core_extern.skel.h"
 
-/* do nothing, just make sure we can link successfully */
+template <typename T>
+class Skeleton {
+private:
+	T *skel;
+public:
+	Skeleton(): skel(nullptr) { }
+
+	~Skeleton() { if (skel) T::destroy(skel); }
+
+	int open(const struct bpf_object_open_opts *opts = nullptr)
+	{
+		int err;
+
+		if (skel)
+			return -EBUSY;
+
+		skel = T::open(opts);
+		err = libbpf_get_error(skel);
+		if (err) {
+			skel = nullptr;
+			return err;
+		}
+
+		return 0;
+	}
+
+	int load() { return T::load(skel); }
+
+	int attach() { return T::attach(skel); }
+
+	void detach() { return T::detach(skel); }
+
+	const T* operator->() const { return skel; }
+
+	T* operator->() { return skel; }
+
+	const T *get() const { return skel; }
+};
 
 static void dump_printf(void *ctx, const char *fmt, va_list args)
 {
 }
 
+static void try_skeleton_template()
+{
+	Skeleton<test_core_extern> skel;
+	std::string prog_name;
+	int err;
+	LIBBPF_OPTS(bpf_object_open_opts, opts);
+
+	err = skel.open(&opts);
+	if (err) {
+		fprintf(stderr, "Skeleton open failed: %d\n", err);
+		return;
+	}
+
+	skel->data->kern_ver = 123;
+	skel->data->int_val = skel->data->ushort_val;
+
+	err = skel.load();
+	if (err) {
+		fprintf(stderr, "Skeleton load failed: %d\n", err);
+		return;
+	}
+
+	if (!skel->kconfig->CONFIG_BPF_SYSCALL)
+		fprintf(stderr, "Seems like CONFIG_BPF_SYSCALL isn't set?!\n");
+
+	err = skel.attach();
+	if (err) {
+		fprintf(stderr, "Skeleton attach failed: %d\n", err);
+		return;
+	}
+
+	prog_name = bpf_program__name(skel->progs.handle_sys_enter);
+	if (prog_name != "handle_sys_enter")
+		fprintf(stderr, "Unexpected program name: %s\n", prog_name.c_str());
+
+	bpf_link__destroy(skel->links.handle_sys_enter);
+	skel->links.handle_sys_enter = bpf_program__attach(skel->progs.handle_sys_enter);
+
+	skel.detach();
+
+	/* destructor will destory underlying skeleton */
+}
+
 int main(int argc, char *argv[])
 {
 	struct btf_dump_opts opts = { };
 	struct test_core_extern *skel;
 	struct btf *btf;
 
+	try_skeleton_template();
+
 	/* libbpf.h */
 	libbpf_set_print(NULL);
 
@@ -25,7 +107,8 @@ int main(int argc, char *argv[])
 
 	/* btf.h */
 	btf = btf__new(NULL, 0);
-	btf_dump__new(btf, dump_printf, nullptr, &opts);
+	if (!libbpf_get_error(btf))
+		btf_dump__new(btf, dump_printf, nullptr, &opts);
 
 	/* BPF skeleton */
 	skel = test_core_extern__open_and_load();