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-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_NONE shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 85CEB1F4C0 for ; Tue, 29 Oct 2019 20:09:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726695AbfJ2UJh (ORCPT ); Tue, 29 Oct 2019 16:09:37 -0400 Received: from cloud.peff.net ([104.130.231.41]:33244 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1726104AbfJ2UJh (ORCPT ); Tue, 29 Oct 2019 16:09:37 -0400 Received: (qmail 2365 invoked by uid 109); 29 Oct 2019 20:09:37 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Tue, 29 Oct 2019 20:09:37 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 12502 invoked by uid 111); 29 Oct 2019 20:12:45 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Tue, 29 Oct 2019 16:12:45 -0400 Authentication-Results: peff.net; auth=none Date: Tue, 29 Oct 2019 16:09:36 -0400 From: Jeff King To: Johannes Schindelin Cc: Alexandr Miloslavskiy , Johannes Schindelin via GitGitGadget , git@vger.kernel.org, Junio C Hamano Subject: Re: [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering Message-ID: <20191029200936.GA12337@sigill.intra.peff.net> References: <1a6119b4-b44f-0edd-da18-46b7d31a274f@syntevo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Tue, Oct 29, 2019 at 08:57:33PM +0100, Johannes Schindelin wrote: > > On 29.10.2019 14:37, Johannes Schindelin via GitGitGadget wrote: > > > > > - vsnprintf(msg, sizeof(msg), err, params); > > > - for (p = msg; *p; p++) { > > > + p = msg + off < pend ? msg + off : pend - 1; > > > > According to my understanding, this is undefined behavior: > > It is not entirely obvious to me what exactly you mean by "this", > assuming that you refer to comparing two pointers via `<`, I agree that > this is not the best idea, I changed it to `off < pend - msg`. If my > assumption is correct, however, we are still talking about C, so I > wonder how this C++ document you linked could bear any relevance: I think the issue is not the comparison, but rather that forming the pointer "msg + off" is undefined, since its point outside of any object (including the "one past" rule). And this is illegal in both C and C++, though of course it works fine in practice most of the time. Doing "off < pend - msg" is legal. Or note that "pend" is just "msg + sizeof(msg)", subtract out the common term and end up with "off < sizeof(msg)". :) > > Can you please preserve the research text about fprintf() behavior on > > different platforms that I provided before? Change formatting to what you > > think fits best. > > Quite honestly, I purposefully refrained from copying that information. > While even the following paragraphs are incomplete by necessity (there > are many more standard C libraries we try to work against, e.g. musl and > newlibc, and it would be insanity to try to cover them all in an > analysis of stdio buffering), the only finding that is relevant to the > patch under discussion is that MSVC's runtime outputs to `stderr` byte > by byte (or more correctly, character by character, I guess), and I did > mention this in the commit message (in the part you quoted). One other case is impacted, which is: > > 1) If `stderr` is fully buffered: > > the ordering of `stdout` and `stderr` messages could be wrong, > > because `stderr` output waits for the buffer to become full. We'll now output the stderr message closer to its time-of-printing, which is a good thing (it may still be in a weird place with respect to a buffered stdout, but getting the error message out ASAP is the best we can do there). I agree that the rest of the research is not especially relevant to the code change (though I'm glad it is available in the list archive). -Peff