conf: rework lxc_map_ids()

src/lxc/conf.c

@@ -3473,16 +3473,35 @@ cleanup:
 	return fret;
 }
 
+int lxc_map_ids_exec_wrapper(void *args)
+{
+	execl("/bin/sh", "sh", "-c", (char *)args, (char *)NULL);
+	return -1;
+}
+
 int lxc_map_ids(struct lxc_list *idmap, pid_t pid)
 {
 	struct id_map *map;
 	struct lxc_list *iterator;
 	enum idtype type;
+	char u_or_g;
 	char *pos;
-	int euid;
-	int ret = 0, use_shadow = 0;
-	int uidmap = 0, gidmap = 0;
-	char *buf = NULL;
+	int euid, fill, left;
+	char cmd_output[MAXPATHLEN];
+	/* strlen("new@idmap") = 9
+	 * +
+	 * strlen(" ") = 1
+	 * +
+	 * LXC_NUMSTRLEN64
+	 * +
+	 * strlen(" ") = 1
+	 *
+	 * We add some additional space to make sure that we really have
+	 * LXC_IDMAPLEN bytes available for our the {g,u]id mapping.
+	 */
+	char mapbuf[9 + 1 + LXC_NUMSTRLEN64 + 1 + LXC_IDMAPLEN] = {0};
+	int ret = 0, uidmap = 0, gidmap = 0;
+	bool use_shadow = false, had_entry = false;
 
 	euid = geteuid();
 
@@ -3507,17 +3526,12 @@ int lxc_map_ids(struct lxc_list *idmap, pid_t pid)
 		return -1;
 	}
 
-	for (type = ID_TYPE_UID; type <= ID_TYPE_GID; type++) {
-		int left, fill;
-		bool had_entry = false;
-		if (!buf) {
-			buf = pos = malloc(LXC_IDMAPLEN);
-			if (!buf)
-				return -ENOMEM;
-		}
-		pos = buf;
+	for (type = ID_TYPE_UID, u_or_g = 'u'; type <= ID_TYPE_GID;
+	     type++, u_or_g = 'g') {
+		pos = mapbuf;
+
 		if (use_shadow)
-			pos += sprintf(buf, "new%cidmap %d", type == ID_TYPE_UID ? 'u' : 'g', pid);
+			pos += sprintf(mapbuf, "new%cidmap %d", u_or_g, pid);
 
 		lxc_list_for_each(iterator, idmap) {
 			/* The kernel only takes <= 4k for writes to
@@ -3529,7 +3543,7 @@ int lxc_map_ids(struct lxc_list *idmap, pid_t pid)
 
 			had_entry = true;
 
-			left = LXC_IDMAPLEN - (pos - buf);
+			left = LXC_IDMAPLEN - (pos - mapbuf);
 			fill = snprintf(pos, left, "%s%lu %lu %lu%s",
 					use_shadow ? " " : "", map->nsid,
 					map->hostid, map->range,
@@ -3542,22 +3556,28 @@ int lxc_map_ids(struct lxc_list *idmap, pid_t pid)
 		if (!had_entry)
 			continue;
 
-		if (!use_shadow) {
-			ret = write_id_mapping(type, pid, buf, pos - buf);
+		/* Try to catch the ouput of new{g,u}idmap to make debugging
+		 * easier.
+		 */
+		if (use_shadow) {
+			ret = run_command(cmd_output, sizeof(cmd_output),
+					  lxc_map_ids_exec_wrapper,
+					  (void *)mapbuf);
+			if (ret < 0) {
+				ERROR("new%cidmap failed to write mapping: %s",
+				      u_or_g, cmd_output);
+				return -1;
+			}
 		} else {
-			left = LXC_IDMAPLEN - (pos - buf);
-			fill = snprintf(pos, left, "\n");
-			if (fill <= 0 || fill >= left)
-				SYSERROR("Too many {g,u}id mappings defined.");
-			pos += fill;
-			ret = system(buf);
+			ret = write_id_mapping(type, pid, mapbuf, pos - mapbuf);
+			if (ret < 0)
+				return -1;
 		}
-		if (ret)
-			break;
+
+		memset(mapbuf, 0, sizeof(mapbuf));
 	}
 
-	free(buf);
-	return ret;
+	return 0;
 }
 
 /*