utils: improve lxc_switch_uid_gid()

src/lxc/utils.c

@@ -1353,19 +1353,27 @@ int lxc_preserve_ns(const int pid, const char *ns)
 
 int lxc_switch_uid_gid(uid_t uid, gid_t gid)
 {
-	if (setgid(gid) < 0) {
-		SYSERROR("Failed to switch to gid %d.", gid);
-		return -errno;
+	int ret = 0;
+
+	if (gid != LXC_INVALID_GID) {
+		ret = setgid(gid);
+		if (ret < 0) {
+			SYSERROR("Failed to switch to gid %d", gid);
+			return -1;
+		}
+		NOTICE("Switched to gid %d", gid);
 	}
-	NOTICE("Switched to gid %d.", gid);
 
-	if (setuid(uid) < 0) {
-		SYSERROR("Failed to switch to uid %d.", uid);
-		return -errno;
+	if (uid != LXC_INVALID_UID) {
+		ret = setuid(uid);
+		if (ret < 0) {
+			SYSERROR("Failed to switch to uid %d", uid);
+			return -1;
+		}
+		NOTICE("Switched to uid %d", uid);
 	}
-	NOTICE("Switched to uid %d.", uid);
 
-	return 0;
+	return ret;
 }
 
 /* Simple covenience function which enables uniform logging. */

src/lxc/utils.h

@@ -358,7 +358,9 @@ extern int lxc_preserve_ns(const int pid, const char *ns);
 /* Check whether a signal is blocked by a process. */
 extern bool task_blocks_signal(pid_t pid, int signal);
 
-/* Switch to a new uid and gid. */
+/* Switch to a new uid and gid.
+ * If LXC_INVALID_{G,U}ID is passed then the set{g,u}id() will not be called.
+ */
 extern int lxc_switch_uid_gid(uid_t uid, gid_t gid);
 extern int lxc_setgroups(int size, gid_t list[]);