From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 54ECCC5CFEB for ; Mon, 9 Jul 2018 21:35:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ED9E9208DB for ; Mon, 9 Jul 2018 21:35:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="ZGimXEMA" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ED9E9208DB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=efficios.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933412AbeGIVfh (ORCPT ); Mon, 9 Jul 2018 17:35:37 -0400 Received: from mail.efficios.com ([167.114.142.138]:39964 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933060AbeGIVfg (ORCPT ); Mon, 9 Jul 2018 17:35:36 -0400 Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 34326232AD1; Mon, 9 Jul 2018 17:35:35 -0400 (EDT) Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10032) with ESMTP id fZXA1iYc0EI8; Mon, 9 Jul 2018 17:35:34 -0400 (EDT) Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 778AA232ACA; Mon, 9 Jul 2018 17:35:34 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com 778AA232ACA DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1531172134; bh=qRun2pRhBv+s/bzI2lJZOhz8DSBjQXe+Fa5Zhn5Rz+k=; h=Date:From:To:Message-ID:MIME-Version; b=ZGimXEMAdn3gDSXRqPiHS6vZ5i7AF7lasVg58ffZunAKfXWExfkcQX2fr7uoSf7ru d6T2ONFWINfRyeo2pvewW7t/sCr442VFvVzNio9vMVWXcNB1FHqNCG/vczSGLx8Ahg xZpJtJ6NXOiN1fJg1nxpx7HPHDlTmHJptjb8cxdu25hzbjv+zY1nvWf0oVa9T/YNLA 8+cwJ7zDTXb4oavV6Xsyix4D+HktjQ+arh2wlBna3KiiPw7tyF8sT4Azozlz6lS6h/ 7wmkdnWkHDg5/V0jIEaBDVvo2fFExqMvJ64psyVf5NHPxj+wxhI6x4bX8P31V/cGWf l36B/pRtpWTOA== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10026) with ESMTP id s6V6nCzrzRRW; Mon, 9 Jul 2018 17:35:34 -0400 (EDT) Received: from mail02.efficios.com (mail02.efficios.com [167.114.142.138]) by mail.efficios.com (Postfix) with ESMTP id 60726232AC3; Mon, 9 Jul 2018 17:35:34 -0400 (EDT) Date: Mon, 9 Jul 2018 17:35:34 -0400 (EDT) From: Mathieu Desnoyers To: Alexei Starovoitov Cc: Joel Fernandes , Daniel Colascione , Alexei Starovoitov , linux-kernel , Tim Murray , Daniel Borkmann , netdev , fengc , "Paul E. McKenney" Message-ID: <566257859.2699.1531172134285.JavaMail.zimbra@efficios.com> In-Reply-To: <20180709210944.quulirpmv3ydytk7@ast-mbp.dhcp.thefacebook.com> References: <20180707015616.25988-1-dancol@google.com> <20180707025426.ssxipi7hsehoiuyo@ast-mbp.dhcp.thefacebook.com> <20180707203340.GA74719@joelaf.mtv.corp.google.com> <951478560.1636.1531083278064.JavaMail.zimbra@efficios.com> <20180709210944.quulirpmv3ydytk7@ast-mbp.dhcp.thefacebook.com> Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.8_GA_2096 (ZimbraWebClient - FF52 (Linux)/8.8.8_GA_1703) Thread-Topic: Add BPF_SYNCHRONIZE bpf(2) command Thread-Index: +KIGmNyUw1xOtPrYtR2vfnWh1xjChA== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Jul 9, 2018, at 5:09 PM, Alexei Starovoitov alexei.starovoitov@gmail.com wrote: > On Sun, Jul 08, 2018 at 04:54:38PM -0400, Mathieu Desnoyers wrote: >> ----- On Jul 7, 2018, at 4:33 PM, Joel Fernandes joelaf@google.com wrote: >> >> > On Fri, Jul 06, 2018 at 07:54:28PM -0700, Alexei Starovoitov wrote: >> >> On Fri, Jul 06, 2018 at 06:56:16PM -0700, Daniel Colascione wrote: >> >> > BPF_SYNCHRONIZE waits for any BPF programs active at the time of >> >> > BPF_SYNCHRONIZE to complete, allowing userspace to ensure atomicity of >> >> > RCU data structure operations with respect to active programs. For >> >> > example, userspace can update a map->map entry to point to a new map, >> >> > use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to >> >> > complete, and then drain the old map without fear that BPF programs >> >> > may still be updating it. >> >> > >> >> > Signed-off-by: Daniel Colascione >> >> > --- >> >> > include/uapi/linux/bpf.h | 1 + >> >> > kernel/bpf/syscall.c | 14 ++++++++++++++ >> >> > 2 files changed, 15 insertions(+) >> >> > >> >> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h >> >> > index b7db3261c62d..4365c50e8055 100644 >> >> > --- a/include/uapi/linux/bpf.h >> >> > +++ b/include/uapi/linux/bpf.h >> >> > @@ -98,6 +98,7 @@ enum bpf_cmd { >> >> > BPF_BTF_LOAD, >> >> > BPF_BTF_GET_FD_BY_ID, >> >> > BPF_TASK_FD_QUERY, >> >> > + BPF_SYNCHRONIZE, >> >> > }; >> >> > >> >> > enum bpf_map_type { >> >> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c >> >> > index d10ecd78105f..60ec7811846e 100644 >> >> > --- a/kernel/bpf/syscall.c >> >> > +++ b/kernel/bpf/syscall.c >> >> > @@ -2272,6 +2272,20 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, >> >> > uattr, unsigned int, siz >> >> > if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN)) >> >> > return -EPERM; >> >> > >> >> > + if (cmd == BPF_SYNCHRONIZE) { >> >> > + if (uattr != NULL || size != 0) >> >> > + return -EINVAL; >> >> > + err = security_bpf(cmd, NULL, 0); >> >> > + if (err < 0) >> >> > + return err; >> >> > + /* BPF programs are run with preempt disabled, so >> >> > + * synchronize_sched is sufficient even with >> >> > + * RCU_PREEMPT. >> >> > + */ >> >> > + synchronize_sched(); >> >> > + return 0; >> >> >> >> I don't think it's necessary. sys_membarrier() can do this already >> >> and some folks use it exactly for this use case. >> > >> > Alexei, the use of sys_membarrier for this purpose seems kind of weird to me >> > though. No where does the manpage say membarrier should be implemented this >> > way so what happens if the implementation changes? >> > >> > Further, membarrier manpage says that a memory barrier should be matched with >> > a matching barrier. In this use case there is no matching barrier, so it >> > makes it weirder. >> > >> > Lastly, sys_membarrier seems will not work on nohz-full systems, so its a bit >> > fragile to depend on it for this? >> > >> > case MEMBARRIER_CMD_GLOBAL: >> > /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */ >> > if (tick_nohz_full_enabled()) >> > return -EINVAL; >> > if (num_online_cpus() > 1) >> > synchronize_sched(); >> > return 0; >> > >> > >> > Adding Mathieu as well who I believe is author/maintainer of membarrier. >> >> See commit 907565337 >> "Fix: Disable sys_membarrier when nohz_full is enabled" >> >> "Userspace applications should be allowed to expect the membarrier system >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on >> nohz_full CPUs, but synchronize_sched() does not take those into >> account." >> >> So AFAIU you'd want to re-use membarrier to issue synchronize_sched, and you >> only care about kernel preempt off critical sections. >> >> Clearly bpf code does not run in user-space, so it would "work". >> >> But the guarantees provided by membarrier are not to synchronize against >> preempt off per se. It's just that the current implementation happens to >> do that. The point of membarrier is to turn user-space memory barriers >> into compiler barriers. >> >> If what you need is to wait for a RCU grace period for whatever RCU flavor >> ebpf is using, I would against using membarrier for this. I would rather >> recommend adding a dedicated BPF_SYNCHRONIZE so you won't leak >> implementation details to user-space, *and* you can eventually change you >> RCU implementation for e.g. SRCU in the future if needed. > > The point about future changes to underlying bpf mechanisms is valid. > There is work already on the way to reduce the scope of preempt_off+rcu_lock > that currently lasts the whole prog. We will have new prog types that won't > have such wrappers and will do rcu_lock/unlock and preempt on/off only > when necessary. > So something like BPF_SYNCHRONIZE will break soon, since the kernel cannot have > guarantees on when programs finish. Calling this command BPF_SYNCHRONIZE_PROG > also won't make sense for the same reason. > What we can do it instead is to define synchronization barrier for > programs accessing maps. May be call it something like: > BPF_SYNC_MAP_ACCESS ? > uapi/bpf.h would need to have extensive comment what this barrier is doing. > Implementation should probably call synchronize_rcu() and not play games > with synchronize_sched(), since that's going too much into implementation. > Also should such sys_bpf command be root only? > I'm not sure whether dos attack can be made by spamming synchronize_rcu() > and synchronize_sched() for that matter. Adding Paul E. McKenney in CC. He may want to share his thoughts on the matter. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com