new helper: restore_altstack()

to be used by rt_sigreturn instances

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/signal.c

@@ -3103,6 +3103,13 @@ out:
 	return error;
 }
 
+int restore_altstack(const stack_t __user *uss)
+{
+	int err = do_sigaltstack(uss, NULL, current_user_stack_pointer());
+	/* squash all but EFAULT for now */
+	return err == -EFAULT ? err : 0;
+}
+
 #ifdef __ARCH_WANT_SYS_SIGPENDING
 
 /**