libbpf: add stub for offset-related skeleton padding

Add a stub function for reporting in which offset within a mapping
libbpf places the map's data. This will be used in a subsequent
patch to support offsetting arena variables within the mapped region.

Adjust skeleton generation to account for the new arena memory layout
by adding padding corresponding to the offset into the arena map. Add
a libbbpf API function to get the data offset within the map's mapping
during skeleton generation.

Signed-off-by: Emil Tsalapatis <[email protected]>

Author: etsal

tools/bpf/bpftool/gen.c

@@ -148,7 +148,8 @@ static int codegen_datasec_def(struct bpf_object *obj,
 			       struct btf *btf,
 			       struct btf_dump *d,
 			       const struct btf_type *sec,
-			       const char *obj_name)
+			       const char *obj_name,
+			       int var_off)
 {
 	const char *sec_name = btf__name_by_offset(btf, sec->name_off);
 	const struct btf_var_secinfo *sec_var = btf_var_secinfos(sec);
@@ -163,6 +164,17 @@ static int codegen_datasec_def(struct bpf_object *obj,
 		strip_mods = true;
 
 	printf("	struct %s__%s {\n", obj_name, sec_ident);
+
+	/*
+	 * Arena variables may be placed in an offset within the section.
+	 * Represent this in the skeleton using a padding struct.
+	 */
+	if (var_off > 0) {
+		printf("\t\tchar __pad%d[%d];\n",
+			pad_cnt, var_off);
+		pad_cnt++;
+	}
+
 	for (i = 0; i < vlen; i++, sec_var++) {
 		const struct btf_type *var = btf__type_by_id(btf, sec_var->type);
 		const char *var_name = btf__name_by_offset(btf, var->name_off);
@@ -279,6 +291,7 @@ static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
 	struct bpf_map *map;
 	const struct btf_type *sec;
 	char map_ident[256];
+	int var_off;
 	int err = 0;
 
 	d = btf_dump__new(btf, codegen_btf_dump_printf, NULL, NULL);
@@ -303,7 +316,13 @@ static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
 			printf("	struct %s__%s {\n", obj_name, map_ident);
 			printf("	} *%s;\n", map_ident);
 		} else {
-			err = codegen_datasec_def(obj, btf, d, sec, obj_name);
+			var_off = bpf_map__data_offset(map);
+			if (var_off < 0)  {
+				p_err("bpf_map__data_offset called on unmapped map\n");
+				err = var_off;
+				goto out;
+			}
+			err = codegen_datasec_def(obj, btf, d, sec, obj_name, var_off);
 			if (err)
 				goto out;
 		}

tools/lib/bpf/libbpf.c

@@ -10552,6 +10552,16 @@ const char *bpf_map__name(const struct bpf_map *map)
 	return map->name;
 }
 
+int bpf_map__data_offset(const struct bpf_map *map)
+{
+	if (!map->mmaped)
+		return -EINVAL;
+
+	/* No offsetting for now. */
+	return 0;
+}
+
+
 enum bpf_map_type bpf_map__type(const struct bpf_map *map)
 {
 	return map->def.type;

tools/lib/bpf/libbpf.h

@@ -1314,6 +1314,15 @@ LIBBPF_API int bpf_map__set_exclusive_program(struct bpf_map *map, struct bpf_pr
  */
 LIBBPF_API struct bpf_program *bpf_map__exclusive_program(struct bpf_map *map);
 
+/*
+ * @brief **bpf_map__data_offset** returns the offset of the map's data
+ * within the address mapping.
+ * @param BPF map whose variable offset we are looking into.
+ * @return the offset >= 0 of the map's contents within its mapping; negative
+ * error code, otherwise.
+ */
+LIBBPF_API int bpf_map__data_offset(const struct bpf_map *map);
+
 struct bpf_xdp_set_link_opts {
 	size_t sz;
 	int old_fd;

tools/lib/bpf/libbpf.map

@@ -451,4 +451,5 @@ LIBBPF_1.7.0 {
 	global:
 		bpf_map__set_exclusive_program;
 		bpf_map__exclusive_program;
+		bpf_map__data_offset;
 } LIBBPF_1.6.0;