From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933066AbeAXVaA (ORCPT ); Wed, 24 Jan 2018 16:30:00 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:49161 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932612AbeAXV34 (ORCPT ); Wed, 24 Jan 2018 16:29:56 -0500 From: ebiederm@xmission.com (Eric W. Biederman) To: Russell King - ARM Linux Cc: Dave Martin , linux-arch@vger.kernel.org, Arnd Bergmann , Nicolas Pitre , Tony Lindgren , Catalin Marinas , Tyler Baicar , Will Deacon , linux-kernel@vger.kernel.org, Oleg Nesterov , James Morse , Al Viro , Olof Johansson , Santosh Shilimkar , linux-arm-kernel@lists.infradead.org References: <87373b6ghs.fsf@xmission.com> <20180112005940.23279-7-ebiederm@xmission.com> <20180115163028.GU22781@e103592.cambridge.arm.com> <87h8rnox3c.fsf@xmission.com> <20180116172407.GA22781@e103592.cambridge.arm.com> <871sipl9p9.fsf@xmission.com> <20180117115708.GM17719@n2100.armlinux.org.uk> <20180117121505.GD22781@e103592.cambridge.arm.com> <20180117123752.GN17719@n2100.armlinux.org.uk> <87y3kwh1t5.fsf@xmission.com> <20180117171425.GQ17719@n2100.armlinux.org.uk> Date: Wed, 24 Jan 2018 15:28:51 -0600 In-Reply-To: <20180117171425.GQ17719@n2100.armlinux.org.uk> (Russell King's message of "Wed, 17 Jan 2018 17:14:25 +0000") Message-ID: <87shavt08c.fsf@xmission.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1eeScA-00016o-L2;;;mid=<87shavt08c.fsf@xmission.com>;;;hst=in02.mta.xmission.com;;;ip=97.121.88.104;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/iRWbR3CZhGRuyMfpgK7s1Ev+0/OWdMQE= X-SA-Exim-Connect-IP: 97.121.88.104 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 0.0 TVD_RCVD_IP Message was received from an IP address * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.7 XMSubLong Long Subject * 0.0 T_TM2_M_HEADER_IN_MSG BODY: No description available. * 0.8 BAYES_50 BODY: Bayes spam probability is 40 to 60% * [score: 0.5000] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa02 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 T_TooManySym_01 4+ unique symbols in subject * 0.0 T_TooManySym_02 5+ unique symbols in subject * 0.2 T_XMDrugObfuBody_14 obfuscated drug references X-Spam-DCC: XMission; sa02 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: **;Russell King - ARM Linux X-Spam-Relay-Country: X-Spam-Timing: total 1594 ms - load_scoreonly_sql: 0.05 (0.0%), signal_user_changed: 3.1 (0.2%), b_tie_ro: 2.1 (0.1%), parse: 1.62 (0.1%), extract_message_metadata: 29 (1.8%), get_uri_detail_list: 6 (0.4%), tests_pri_-1000: 10 (0.6%), tests_pri_-950: 2.2 (0.1%), tests_pri_-900: 1.85 (0.1%), tests_pri_-400: 49 (3.1%), check_bayes: 47 (2.9%), b_tokenize: 20 (1.3%), b_tok_get_all: 12 (0.7%), b_comp_prob: 7 (0.5%), b_tok_touch_all: 3.0 (0.2%), b_finish: 0.89 (0.1%), tests_pri_0: 1045 (65.6%), check_dkim_signature: 0.98 (0.1%), check_dkim_adsp: 4.7 (0.3%), tests_pri_500: 447 (28.0%), poll_dns_idle: 437 (27.4%), rewrite_mail: 0.00 (0.0%) Subject: Re: [PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE, SIGTRAP, SIGBUS X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Russell King - ARM Linux writes: > On Wed, Jan 17, 2018 at 10:45:10AM -0600, Eric W. Biederman wrote: >> Russell King - ARM Linux writes: >> >From your description there still seems to be an association with an >> instruction so I don't know if I would really call the signal >> asynchronous. It sounds like the exception is delayed and not >> asynchronous. > > Traps can only be passed from ARM coprocessors by a coprocessor refusing > to execute an instruction. That's what happens in this case - the VFP > gets offered an instruction to execute. It accepts it, and the CPU > continues, leaving the VFP to execute its instruction independently. If > this instruction generates an error, then nothing happens at this point. > > That error remains pending until the CPU offers the coprocessor the next > VFP instruction, which it refuses. That causes an undefined instruction > exception, and we trap into the kernel VFP code which reads the VFP > status and works out what needs to be done. > > What this means is that if you execute a VFP instruction, wait 10 minutes > and then execute another VFP instruction, if the first VFP instruction > raised an exception, you'll get to hear about it 10 minutes later. > > You can use whatever weasel words you want to describe that situation, > my choice is "asynchronous", your choice is "delayed". However, it is > clearly not "synchronous", and arguing that we should report something > synchronously that is not reported to _us_ synchronously (where > synchronous means "at the same time") is IMHO daft. > > So, let's take an example: > > installs SIGFPE handler > ..fp instructions.. one of which raises an exception > returns to main loop > main loop blocks all signals while it sets stuff up > calls ppoll() > > In the synchronous SIGFPE delivery case, the SIGFPE handler will be > called when the exception is generated in the FP code, and delivered > at that time. The fact that the main loop blocks all signals happens > later, so the users handler gets called as one expects. > > In the VFP case, however, the FP instructions towards the end may not > end up causing the exception to be signalled until sometime later, > and as I've already explained, that may be the result of a C library > function accessing the VFP registers. This could well end up trying > to deliver the SIGFPE while signals are blocked, and we get > drastically different behaviour if force_sig_info() is used. > > In the VFP case, if force_sig_info() is used, the program gets killed > at this point. In the non-VFP case, the program's signal handler was > called. > > Using send_sig_info() results in the already delayed or asynchronous > signal being held off until ppoll() drops the blocking, at which point > the signal is delivered, the program handles it in its handler, and > the program continues to run. > > So > 1. non-VFP case, program doesn't get killed but gets the opportunity > to handle the signal. > 2. VFP case with send_sig_info, program doesn't get killed but gets > the opportunity to handle the signal. > 3. VFP case with force_sig_info, the program gets killed and dumps > core. > > Which one of these results in a big change of behaviour in your > opinion? I want to apologize for the disagreement. In part of my due diligence for cleaning up the signal handling I am introducing some helpers for generating siginfo. I decided to ask which kind of helpers should I introduce. Very basic generic helpers that just wrap the current functionality today. Or some slightly smarter helpers that solve some other problems as well. After consideration I am shelving the smarter helpers for now, as the need to introduce the helpers universally is strong, so that I can guarantee struct siginfo is always fully initialized before being passed to userspace. Given the choice between force_sig_info and send_sig_info I agree that send_sig_info is the right choice for signals that can be ignored. The problem I was focusing on is the problem where force_sig_info and send_sig_info can be tricked into causing the instruction pointer to point to the wrong instruction (even when the signal is not blocked), due to the delivery of another signal. So I was wondering if in practice we could introduce a singal delivery function that would operation synchronously and would solve the instruction pointer problem. It looks to me like this location on arm where we are using send_sig_info is a clear candidate for such a function as long as it has a mode where you can say deliverly the signal like send_sig_info if the signal is blocked. Still like I said such a smarter helper is not the priority and I don't intend any semantic changes when I introduce helpers into the signal deliver path. Just fewer places initializing struct siginfo. Eric