From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 110641 invoked by alias); 5 Jul 2018 12:32:27 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 109772 invoked by uid 89); 5 Jul 2018 12:32:26 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-2.5 required=5.0 tests=AWL,BAYES_00,SPF_PASS autolearn=ham version=3.3.2 spammy=H*r:15.1.1415.2, H*RU:15.1.1415.2, Hx-spam-relays-external:15.1.1415.2, formed X-HELO: 9pmail.ess.barracuda.com Received: from 9pmail.ess.barracuda.com (HELO 9pmail.ess.barracuda.com) (64.235.150.225) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 05 Jul 2018 12:32:24 +0000 Received: from mipsdag02.mipstec.com (mail2.mips.com [12.201.5.32]) by mx29.ess.sfj.cudaops.com (version=TLSv1.2 cipher=ECDHE-RSA-AES128-SHA256 bits=128 verify=NO); Thu, 05 Jul 2018 12:32:20 +0000 Received: from [10.20.78.49] (10.20.78.49) by mipsdag02.mipstec.com (10.20.40.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1415.2; Thu, 5 Jul 2018 05:32:23 -0700 Date: Thu, 05 Jul 2018 12:32:00 -0000 From: "Maciej W. Rozycki" To: Andrew Burgess CC: Sergey Korolev , Subject: Re: [PATCH] MIPS/GDB/linux-nat.c: Fix a child detach condition for uClibc-ng In-Reply-To: <20180703193753.GC2675@embecosm.com> Message-ID: References: <20180703193753.GC2675@embecosm.com> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Return-Path: macro@mips.com X-BESS-Apparent-Source-IP: 12.201.5.32 X-SW-Source: 2018-07/txt/msg00121.txt.bz2 On Tue, 3 Jul 2018, Andrew Burgess wrote: > > Current implementation expects that WIFSTOPPED (W_STOPCODE (0)) is true, > > but in the uClibc-ng it is false on MIPS. The patch adds a "detach" > > helper variable to avoid this corner case: WIFSTOPPED applied > > only to a status filled by a waitpid call that should never return > > the status with zero stop signal. > > I took a quick look through this patch, and have a little feedback. > > You might want to expand your commit message to explain _why_ MIPS is > different (having a signal 127), I didn't know this, and it puzzled me > for a while as to why your above text didn't just indicate a bug in > uClibc-ng :) I think it is a bug in uClibc-ng after all, because you can't receive signal #0 on Linux. This is what the kernel has (in kernel/signal.c): /* * The null signal is a permissions and process existence * probe. No signal is actually delivered. */ if (!error && sig) { error = do_send_sig_info(sig, info, p, false); /* * If lock_task_sighand() failed we pretend the task * dies after receiving the signal. The window is tiny, * and the signal is private anyway. */ if (unlikely(error == -ESRCH)) error = 0; } and also: if (!ret && sig) ret = do_send_sig_info(sig, info, p, true); and documentation for kill(2) agrees: "If sig is 0, then no signal is sent, but error checking is still per- formed; this can be used to check for the existence of a process ID or process group ID." > [PATCH] gdb: Avoid using W_STOPCODE(0) as this is ambiguous on MIPS > > The MIPS target supports 127 signals, and this can create an ambiguity > in process wait statuses. A status value of 0x007f could potentially > indicate a process that has exited with signal 127, or a process that > has stopped with signal 0. > > In uClibc-ng the interpretation of 0x007f is that the process has > exited with signal 127 rather than stopped with signal 0, and so, > WIFSTOPPED (W_STOPCODE (0)) will be false rather than true as it would > be on most other platforms. So there is no ambiguity, 0x007f means that the process has exited with signal 127 and nothing else, and while I agree the change might be a good workaround for uClibc-ng's oddity, I think uClibc-ng's implementation of WIFSTOPPED has also to be fixed to reflect reality. Which also means the commit description needs to be updated accordingly. FWIW, Maciej