From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 8734 invoked by alias); 19 Feb 2004 19:14:53 -0000 Mailing-List: contact gdb-help@sources.redhat.com; run by ezmlm Precedence: bulk List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sources.redhat.com Received: (qmail 8725 invoked from network); 19 Feb 2004 19:14:52 -0000 Received: from unknown (HELO coyote.egenera.com) (63.160.166.46) by sources.redhat.com with SMTP; 19 Feb 2004 19:14:52 -0000 Received: from localhost (localhost.localdomain [127.0.0.1]) by coyote.egenera.com (Postfix) with ESMTP id 1D205A0514; Thu, 19 Feb 2004 14:01:16 -0500 (EST) Received: from coyote.egenera.com ([127.0.0.1]) by localhost (coyote.egenera.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 31928-05; Thu, 19 Feb 2004 14:01:12 -0500 (EST) Received: from hasufel.egenera.com (southeast.egenera.com [63.160.166.4]) by coyote.egenera.com (Postfix) with ESMTP id 9CCEFA0517; Thu, 19 Feb 2004 14:01:11 -0500 (EST) Received: from localhost (localhost.localdomain [127.0.0.1]) by hasufel.egenera.com (8.11.6/8.11.6) with ESMTP id i1JJAae07648; Thu, 19 Feb 2004 14:10:36 -0500 Subject: Re: execute_control_command may not remove its cleanups From: Dave Allan Reply-To: da_gdb@egenera.com To: Daniel Jacobowitz Cc: gdb@sources.redhat.com In-Reply-To: <20040219184730.GA26281@nevyn.them.org> References: <1077204518.1305.1192.camel@hasufel.egenera.com> <20040219154016.GA24829@nevyn.them.org> <1077214912.1304.1351.camel@hasufel.egenera.com> <20040219184730.GA26281@nevyn.them.org> Content-Type: text/plain Organization: Egenera, Inc. Message-Id: <1077217836.1304.1361.camel@hasufel.egenera.com> Mime-Version: 1.0 Date: Thu, 19 Feb 2004 19:14:00 -0000 Content-Transfer-Encoding: 7bit X-Virus-Scanned: by amavisd-new at egenera.com X-SW-Source: 2004-02/txt/msg00253.txt.bz2 Ok. I see your point. How about setting old_chain to cleanup_chain unconditionally at the beginning of the function and doing the cleanups unconditionally at the end? That way, we're safe against both scenarios: against doing cleanups prematurely, but also safe against getting into the function with cleanup_chain null and then freeing something random at a later point. Dave On Thu, 2004-02-19 at 13:47, Daniel Jacobowitz wrote: > On Thu, Feb 19, 2004 at 01:21:52PM -0500, Dave Allan wrote: > > > > However, it seems from code inspection and the gdb internals > > > > documentation that the call to do_cleanups ought to be unconditional. > > > > Does that seem right? > > > > > > No, instead, the cleanup chain should always have an item on it. If > > > make_cleanup is not called then old_chain will remain NULL, and > > > do_cleanups (NULL) means "do all cleanups", not "do nothing". It looks > > > to me like command_handler is responsible for there always being a > > > cleanup on the chain: > > > old_chain = make_cleanup (null_cleanup, 0); > > > but maybe I'm mistaken about that; it's a bit far down the tree. > > > > I definitely understand that do_cleanups(NULL) will do all cleanups > > which is not what's wanted here. The call is do_cleanups(old_chain), > > though, so if there are cleanups on the chain already, they are > > preserved. The problem isn't the do_cleanups call, it's the fact that > > the do_cleanups call is conditional. The solution is to remove the if > > (old_chain) statement and always do the cleanup. > > > > Given what's stated in the docs, that a function must always remove the > > cleanups it creates, it would seem to me that regardless of the state of > > cleanup_chain at the beginning of execute_control_command, whether it's > > NULL or contains cleanups, we want to get back to that state before we > > return. > > > > Looking at what cleanups execute_control_command puts on cleanup_chain, > > that is correct. Either one or two cleanups are put on the chain where > > arg is an automatic variable and function is free_current_contents. If > > these cleanups aren't done before the stack frame is destroyed, > > something undefined will later be freed when the cleanups are done. > > Think about this again. Both of those cleanups are conditionally > created. If neither of them is created, old_chain will still be NULL. > This will lead to running cleanups prematurely. If the cleanup chain > is non-empty, things work OK. > > The alternative is null_cleanup.