bpftool: Bpf skeletons assert type sizes

When emitting type declarations in skeletons, bpftool will now also emit
static assertions on the size of the data/bss/rodata/etc fields. This
ensures that in situations where userspace and kernel types have the same
name but differ in size we do not silently produce incorrect results but
instead break the build.

This was reported in [1] and as expected the repro in [2] fails to build
on the new size assert after this change.

  [1]: Closes: libbpf/libbpf#433
  [2]: https://github.com/fuweid/iovisor-bcc-pr-3777

Signed-off-by: Delyan Kratunov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Tested-by: Hengqi Chen <[email protected]>
Acked-by: Hengqi Chen <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Delyan Kratunov

tools/bpf/bpftool/gen.c

@@ -209,15 +209,36 @@ static int codegen_datasec_def(struct bpf_object *obj,
 	return 0;
 }
 
+static const struct btf_type *find_type_for_map(struct btf *btf, const char *map_ident)
+{
+	int n = btf__type_cnt(btf), i;
+	char sec_ident[256];
+
+	for (i = 1; i < n; i++) {
+		const struct btf_type *t = btf__type_by_id(btf, i);
+		const char *name;
+
+		if (!btf_is_datasec(t))
+			continue;
+
+		name = btf__str_by_offset(btf, t->name_off);
+		if (!get_datasec_ident(name, sec_ident, sizeof(sec_ident)))
+			continue;
+
+		if (strcmp(sec_ident, map_ident) == 0)
+			return t;
+	}
+	return NULL;
+}
+
 static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
 {
 	struct btf *btf = bpf_object__btf(obj);
-	int n = btf__type_cnt(btf);
 	struct btf_dump *d;
 	struct bpf_map *map;
 	const struct btf_type *sec;
-	char sec_ident[256], map_ident[256];
-	int i, err = 0;
+	char map_ident[256];
+	int err = 0;
 
 	d = btf_dump__new(btf, codegen_btf_dump_printf, NULL, NULL);
 	err = libbpf_get_error(d);
@@ -234,23 +255,7 @@ static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
 		if (!get_map_ident(map, map_ident, sizeof(map_ident)))
 			continue;
 
-		sec = NULL;
-		for (i = 1; i < n; i++) {
-			const struct btf_type *t = btf__type_by_id(btf, i);
-			const char *name;
-
-			if (!btf_is_datasec(t))
-				continue;
-
-			name = btf__str_by_offset(btf, t->name_off);
-			if (!get_datasec_ident(name, sec_ident, sizeof(sec_ident)))
-				continue;
-
-			if (strcmp(sec_ident, map_ident) == 0) {
-				sec = t;
-				break;
-			}
-		}
+		sec = find_type_for_map(btf, map_ident);
 
 		/* In some cases (e.g., sections like .rodata.cst16 containing
 		 * compiler allocated string constants only) there will be
@@ -363,6 +368,73 @@ static size_t bpf_map_mmap_sz(const struct bpf_map *map)
 	return map_sz;
 }
 
+/* Emit type size asserts for all top-level fields in memory-mapped internal maps. */
+static void codegen_asserts(struct bpf_object *obj, const char *obj_name)
+{
+	struct btf *btf = bpf_object__btf(obj);
+	struct bpf_map *map;
+	struct btf_var_secinfo *sec_var;
+	int i, vlen;
+	const struct btf_type *sec;
+	char map_ident[256], var_ident[256];
+
+	codegen("\
+		\n\
+		__attribute__((unused)) static void			    \n\
+		%1$s__assert(struct %1$s *s)				    \n\
+		{							    \n\
+		#ifdef __cplusplus					    \n\
+		#define _Static_assert static_assert			    \n\
+		#endif							    \n\
+		", obj_name);
+
+	bpf_object__for_each_map(map, obj) {
+		if (!bpf_map__is_internal(map))
+			continue;
+		if (!(bpf_map__map_flags(map) & BPF_F_MMAPABLE))
+			continue;
+		if (!get_map_ident(map, map_ident, sizeof(map_ident)))
+			continue;
+
+		sec = find_type_for_map(btf, map_ident);
+		if (!sec) {
+			/* best effort, couldn't find the type for this map */
+			continue;
+		}
+
+		sec_var = btf_var_secinfos(sec);
+		vlen =  btf_vlen(sec);
+
+		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);
+			long var_size;
+
+			/* static variables are not exposed through BPF skeleton */
+			if (btf_var(var)->linkage == BTF_VAR_STATIC)
+				continue;
+
+			var_size = btf__resolve_size(btf, var->type);
+			if (var_size < 0)
+				continue;
+
+			var_ident[0] = '\0';
+			strncat(var_ident, var_name, sizeof(var_ident) - 1);
+			sanitize_identifier(var_ident);
+
+			printf("\t_Static_assert(sizeof(s->%s->%s) == %ld, \"unexpected size of '%s'\");\n",
+			       map_ident, var_ident, var_size, var_ident);
+		}
+	}
+	codegen("\
+		\n\
+		#ifdef __cplusplus					    \n\
+		#undef _Static_assert					    \n\
+		#endif							    \n\
+		}							    \n\
+		");
+}
+
 static void codegen_attach_detach(struct bpf_object *obj, const char *obj_name)
 {
 	struct bpf_program *prog;
@@ -639,8 +711,11 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 			}						    \n\
 			return skel;					    \n\
 		}							    \n\
+									    \n\
 		", obj_name);
 
+	codegen_asserts(obj, obj_name);
+
 	codegen("\
 		\n\
 									    \n\
@@ -1046,9 +1121,17 @@ static int do_skeleton(int argc, char **argv)
 		const void *%1$s::elf_bytes(size_t *sz) { return %1$s__elf_bytes(sz); } \n\
 		#endif /* __cplusplus */				    \n\
 									    \n\
-		#endif /* %2$s */					    \n\
 		",
-		obj_name, header_guard);
+		obj_name);
+
+	codegen_asserts(obj, obj_name);
+
+	codegen("\
+		\n\
+									    \n\
+		#endif /* %1$s */					    \n\
+		",
+		header_guard);
 	err = 0;
 out:
 	bpf_object__close(obj);