From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by dcvr.yhbt.net (Postfix) with ESMTP id 51AB21F990 for ; Fri, 7 Aug 2020 07:58:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726584AbgHGH6k (ORCPT ); Fri, 7 Aug 2020 03:58:40 -0400 Received: from cloud.peff.net ([104.130.231.41]:51450 "EHLO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725805AbgHGH6i (ORCPT ); Fri, 7 Aug 2020 03:58:38 -0400 Received: (qmail 29518 invoked by uid 109); 7 Aug 2020 07:58:38 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with ESMTP; Fri, 07 Aug 2020 07:58:38 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 11725 invoked by uid 111); 7 Aug 2020 07:58:37 -0000 Received: from coredump.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.2) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Fri, 07 Aug 2020 03:58:37 -0400 Authentication-Results: peff.net; auth=none Date: Fri, 7 Aug 2020 03:58:37 -0400 From: Jeff King To: Patrick Steinhardt Cc: git@vger.kernel.org Subject: Re: [PATCH] refs: fix interleaving hook calls with reference-transaction hook Message-ID: <20200807075837.GA32344@coredump.intra.peff.net> References: <63fb363375b515b903ed1269d10124b727c1d1cc.1596783732.git.ps@pks.im> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <63fb363375b515b903ed1269d10124b727c1d1cc.1596783732.git.ps@pks.im> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Fri, Aug 07, 2020 at 09:05:58AM +0200, Patrick Steinhardt wrote: > In order to not repeatedly search for the reference-transaction hook in > case it's getting called multiple times, we use a caching mechanism to > only call `find_hook()` once. What was missed though is that the return > value of `find_hook()` actually comes from a static strbuf, which means > it will get overwritten when calling `find_hook()` again. As a result, > we may call the wrong hook with parameters of the reference-transaction > hook. > > This scenario was spotted in the wild when executing a git-push(1) with > multiple references, where there are interleaving calls to both the > update and the reference-transaction hook. While initial calls to the > reference-transaction hook work as expected, it will stop working after > the next invocation of the update hook. The result is that we now start > calling the update hook with parameters and stdin of the > reference-transaction hook. That makes sense. I think of push as a single transaction, but that's only if the caller sends the "atomic" capability. Otherwise get one per ref. > diff --git a/refs.c b/refs.c > index 2dd851fe81..17e515b288 100644 > --- a/refs.c > +++ b/refs.c > @@ -2044,7 +2044,7 @@ static int run_transaction_hook(struct ref_transaction *transaction, > if (hook == &hook_not_found) > return ret; > if (!hook) > - hook = find_hook("reference-transaction"); > + hook = xstrdup_or_null(find_hook("reference-transaction")); > if (!hook) { > hook = &hook_not_found; > return ret; The fix here looks obviously correct, though I have to wonder if the caching is even worth it. It's saving us an access() call, but we're about to exec a whole sub-process. It's perhaps more justifiable when there isn't a hook (we're still just paying that one access(), but it's proportionally bigger). I kind of doubt it's measurable, though, since a ref write requires a bunch of syscalls anyway. -Peff