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

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 6f2e20be0c62fb6e821bbaa995367452cedbf31c..022f30490567d6ef5899c06df4b291061aedbb3c 100644 (file)
--- a/tools/bpf/bpftool/gen.c
+++ b/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);