From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757499AbcEEP2p (ORCPT ); Thu, 5 May 2016 11:28:45 -0400 Received: from mail-pf0-f181.google.com ([209.85.192.181]:34406 "EHLO mail-pf0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754807AbcEEP2n (ORCPT ); Thu, 5 May 2016 11:28:43 -0400 Subject: Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN > received >= buf To: One Thousand Gnomes , Julio Guerra References: <16504442-9e91-aaab-1f2b-9b4aa9c219f5@farjump.io> <572A80C0.4080704@hurleysoftware.com> <20160505110831.3548c1c7@lxorguk.ukuu.org.uk> Cc: "Peter Hurley ; Greg Kroah-Hartman" , linux-kernel@vger.kernel.org From: Peter Hurley Message-ID: <572B66A8.8080100@hurleysoftware.com> Date: Thu, 5 May 2016 08:28:40 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 MIME-Version: 1.0 In-Reply-To: <20160505110831.3548c1c7@lxorguk.ukuu.org.uk> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/05/2016 03:08 AM, One Thousand Gnomes wrote: > On Wed, 4 May 2016 16:07:44 -0700 > Peter Hurley wrote: > >> Hi Julio, >> >> On 05/04/2016 04:00 PM, Julio Guerra wrote: >>> Hi, >>> >>> When a tty (here a slave pty) is set in noncanonical input and blocking read modes, a read() randomly blocks when: >>> "VMIN > kernel received >= user buffer size > 0". >>> >>> The standard says that read() should block until VMIN bytes are received [1][2]. Whether this is an implementation defined case not really specified by POSIX or not, it should not behave randomly (otherwise it really should be documented in termios manpage). >> >> This is not a bug. >> >> >From the termios(3) man page: >> >> * MIN > 0; TIME == 0: read(2) blocks until the lesser of MIN bytes or the number of bytes requested are avail‐ >> able, and returns the lesser of these two values. > > The standard says > > Case B: MIN>0, TIME=0 > > In case B, since the value of TIME is zero, the timer plays no > role and only MIN is significant. A pending read shall not be > satisfied until MIN bytes are received (that is, the pending read > shall block until MIN bytes are received), or a signal is > received. A program that uses case B to read record-based > terminal I/O may block indefinitely in the read operation. > > That is if you do > > > read(fd, buf, 3) > > and MIN is 5, the read should not return until there are 5 bytes in the > queue. The following code is guaranteed to work reliably by the standard > with TIME 0 MIN 5 (ignoring signals for the moment) > > > read(fd, buf, 3); > fcntl(fd, F_SETFL, FNDELAY); > assert(read(fd, buf, 2) == 2); > > Historically this behaviour was useful for things like block transfer > protocols, especially with offloaded serial processing. > > So actually I think we do have a bug, the behaviuour is not standards > compliant, and the man page documents the erroneous behaviour. I disagree; I think SUSv4 fails to address this degenerate condition at all. For example, SUSv4 specifically states that there is no precedence of MIN/TIME with O_NONBLOCK. IOW, the standard does _not_ guarantee that your code fragment above won't block on the subsequent read anyway since it fails to meet the new MIN 5 watermark. But I have no problem fixing a bona fide regression; what's broken?