bpftool: Provide a helper method for accessing skeleton's embedded ELF data

This adds a skeleton method X__elf_bytes() which returns the binary data of
the compiled and embedded BPF object file. It additionally sets the size of
the return data to the provided size_t pointer argument.

The assignment to s->data is cast to void * to ensure no warning is issued if
compiled with a previous version of libbpf where the bpf_object_skeleton field
is void * instead of const void *
Signed-off-by: Matt Smith <alastorze@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210901194439.3853238-3-alastorze@fb.com

tools/bpf/bpftool/gen.c

@@ -238,8 +238,8 @@ static void codegen(const char *template, ...)
 		} else if (c == '\n') {
 			break;
 		} else {
-			p_err("unrecognized character at pos %td in template '%s'",
-			      src - template - 1, template);
+			p_err("unrecognized character at pos %td in template '%s': '%c'",
+			      src - template - 1, template, c);
 			free(s);
 			exit(-1);
 		}
@@ -406,7 +406,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 	}
 
 	bpf_object__for_each_map(map, obj) {
-		const char * ident;
+		const char *ident;
 
 		ident = get_map_ident(map);
 		if (!ident)
@@ -862,6 +862,8 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 									    \n\
+		static inline const void *%1$s__elf_bytes(size_t *sz);	    \n\
+									    \n\
 		static inline int					    \n\
 		%1$s__create_skeleton(struct %1$s *obj)			    \n\
 		{							    \n\
@@ -943,10 +945,20 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 									    \n\
-			s->data_sz = %d;				    \n\
-			s->data = (void *)\"\\				    \n\
-		",
-		file_sz);
+			s->data = (void *)%2$s__elf_bytes(&s->data_sz);	    \n\
+									    \n\
+			return 0;					    \n\
+		err:							    \n\
+			bpf_object__destroy_skeleton(s);		    \n\
+			return -ENOMEM;					    \n\
+		}							    \n\
+									    \n\
+		static inline const void *%2$s__elf_bytes(size_t *sz)	    \n\
+		{							    \n\
+			*sz = %1$d;					    \n\
+			return (const void *)\"\\			    \n\
+		"
+		, file_sz, obj_name);
 
 	/* embed contents of BPF object file */
 	print_hex(obj_data, file_sz);
@@ -954,11 +966,6 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 		\";							    \n\
-									    \n\
-			return 0;					    \n\
-		err:							    \n\
-			bpf_object__destroy_skeleton(s);		    \n\
-			return -ENOMEM;					    \n\
 		}							    \n\
 									    \n\
 		#endif /* %s */						    \n\