[PATCH] bpftool: output map/prog indices on `gen skeleton`

[Marcelo Juchem <juchem@gmail.com>]

The skeleton generated by `bpftool` makes it easy to attach and load bpf
objects as a whole. Some BPF programs are not directly portable across kernel
versions, though, and require some cherry-picking on which programs to
load/attach. The skeleton makes this cherry-picking possible, but not entirely
friendly in some cases.

For example, an useful feature is `attach_with_fallback` so that one
program can be attempted, and fallback programs tried subsequently until
one works (think `tcp_recvmsg` interface changing on kernel 5.19).

Being able to represent a set of probes programatically in a way that is both
descriptive, compile-time validated, runtime efficient and custom library
friendly is quite desirable for application developers. A very simple way to
represent a set of probes is with an array of indices.

This patch creates a couple of enums under the `__cplusplus` section to
represent the program and map indices inside the skeleton object, that can be
used to refer to the proper program/map object.

This is the code generated for the `__cplusplus` section of `profiler.skel.h`:
```
  enum map_idxs: size_t {
    events = 0,
    fentry_readings = 1,
    accum_readings = 2,
    counts = 3,
    rodata = 4
  };
  enum prog_idxs: size_t {
    fentry_XXX = 0,
    fexit_XXX = 1
  };
  static inline struct profiler_bpf *open(const struct bpf_object_open_opts *opts = nullptr);
  static inline struct profiler_bpf *open_and_load();
  static inline int load(struct profiler_bpf *skel);
  static inline int attach(struct profiler_bpf *skel);
  static inline void detach(struct profiler_bpf *skel);
  static inline void destroy(struct profiler_bpf *skel);
  static inline const void *elf_bytes(size_t *sz);
```
---
 src/gen.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/gen.c b/src/gen.c
index 7070dcf..7e28dc7 100644
--- a/src/gen.c
+++ b/src/gen.c
@@ -1086,6 +1086,38 @@ static int do_skeleton(int argc, char **argv)
 		\n\
 									    \n\
 		#ifdef __cplusplus					    \n\
+		"
+	);
+
+	{
+		size_t i = 0;
+		printf("\tenum map_index: size_t {");
+		bpf_object__for_each_map(map, obj) {
+			if (!get_map_ident(map, ident, sizeof(ident)))
+				continue;
+			if (i) {
+				printf(",");
+			}
+			printf("\n\t\t%s = %lu", ident, i);
+			++i;
+		}
+		printf("\n\t};\n");
+	}
+	{
+		size_t i = 0;
+		printf("\tenum prog_index: size_t {");
+		bpf_object__for_each_program(prog, obj) {
+			if (i) {
+				printf(",");
+			}
+			printf("\n\t\t%s = %lu", bpf_program__name(prog), i);
+			++i;
+		}
+		printf("\n\t};\n");
+	}
+
+	codegen("\
+		\n\
 			static inline struct %1$s *open(const struct bpf_object_open_opts *opts = nullptr);\n\
 			static inline struct %1$s *open_and_load();	    \n\
 			static inline int load(struct %1$s *skel);	    \n\
-- 
2.37.2