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=-2.3 required=3.0 tests=BAYES_00,DATE_IN_PAST_03_06, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 C3F75C4363A for ; Thu, 29 Oct 2020 02:14:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6BA2A20720 for ; Thu, 29 Oct 2020 02:14:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=sargun.me header.i=@sargun.me header.b="tF7de7mb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388290AbgJ2CMn (ORCPT ); Wed, 28 Oct 2020 22:12:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46258 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726803AbgJ1VgS (ORCPT ); Wed, 28 Oct 2020 17:36:18 -0400 Received: from mail-ej1-x644.google.com (mail-ej1-x644.google.com [IPv6:2a00:1450:4864:20::644]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 49DFEC0613D2 for ; Wed, 28 Oct 2020 14:36:16 -0700 (PDT) Received: by mail-ej1-x644.google.com with SMTP id h24so952661ejg.9 for ; Wed, 28 Oct 2020 14:36:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sargun.me; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=1KKfnGnBEdd12X89nwYRLUfbpvsSCoekdcU0DHREQhE=; b=tF7de7mbM6VeEXWhVjMrNC5cvKMpl0WDoIIp7fDbiFMyA9PP4g3gTmf4LaBzwarT14 4HlQUlmQiDgdPcyNEbZCIirlXnui4R0jL648Tx95iRiVKLtDrIrgQ3s56LWy8BP1Jh4m e5D2GJEtVeWX660AowCOJZdoycU2F+72CGe2Y= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=1KKfnGnBEdd12X89nwYRLUfbpvsSCoekdcU0DHREQhE=; b=bZN/EcylUO8s38kDM7j54jGUoZ8wVlsfBaN3XBOjBLI7+oVvS6MJHCFn8v9ZsKBlXC HP9Kw1iDesg/QE93mHd5J9vqvTnCT4ZKxBUKiX4HeizHn5QWGEzk2f1e9VP1vIevf8Gi x/kAA3SsB+cacOTnWMFgXDImh0EkuMRnAsWPtclLPQYP5ExEqWWzWxwatnieaFbh3CUO 8M7++GxY9jYLaVgBsoGQRhHwItslbEVvV4NJ31WFSkN32eTGaxS3NZna+Yibhczc0c0a lrCKgMBnoivUwKm4f7cwAfMFLnhAMtrlQUfLB08Qi9+NAWYtbb38Zq4prKEimtiOEogT Y6Pw== X-Gm-Message-State: AOAM532VuMxuiP6gGIQQ9u7/Cv9o+pMklSS/YwOOts6Uj92wMKLEjDnM tUwvcgtMyWE7hA4VRV61XjU+5G6qkXFpwMKdaEQYfkG63za+kOFn X-Google-Smtp-Source: ABdhPJxusKQfLH7ldNK0ZejYey8JceBXqtdks06+e2oAEK1AFi5yniwjLl5sysSu+nsrzKiTHTK4yg881qNZWDn/8tc= X-Received: by 2002:a17:906:bc42:: with SMTP id s2mr173094ejv.251.1603907070187; Wed, 28 Oct 2020 10:44:30 -0700 (PDT) MIME-Version: 1.0 References: <45f07f17-18b6-d187-0914-6f341fe90857@gmail.com> <20200930150330.GC284424@cisco> <8bcd956f-58d2-d2f0-ca7c-0a30f3fcd5b8@gmail.com> <20200930230327.GA1260245@cisco> <20200930232456.GB1260245@cisco> <656a37b5-75e3-0ded-6ba8-3bb57b537b24@gmail.com> In-Reply-To: From: Sargun Dhillon Date: Wed, 28 Oct 2020 10:43:54 -0700 Message-ID: Subject: Re: For review: seccomp_user_notif(2) manual page To: Jann Horn Cc: "Michael Kerrisk (man-pages)" , Tycho Andersen , Kees Cook , Christian Brauner , linux-man , lkml , Aleksa Sarai , Alexei Starovoitov , Will Drewry , bpf , Song Liu , Daniel Borkmann , Andy Lutomirski , Linux Containers , Giuseppe Scrivano , Robert Sesek Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Wed, Oct 28, 2020 at 2:43 AM Jann Horn wrote: > > On Wed, Oct 28, 2020 at 7:32 AM Sargun Dhillon wrote: > > On Tue, Oct 27, 2020 at 3:28 AM Jann Horn wrote: > > > On Tue, Oct 27, 2020 at 7:14 AM Michael Kerrisk (man-pages) > > > wrote: > > > > On 10/26/20 4:54 PM, Jann Horn wrote: > > > > > I'm a bit on the fence now on whether non-blocking mode should use > > > > > ENOTCONN or not... I guess if we returned ENOENT even when there are > > > > > no more listeners, you'd have to disambiguate through the poll() > > > > > revents, which would be kinda ugly? > > > > > > > > I must confess, I'm not quite clear on which two cases you > > > > are trying to distinguish. Can you elaborate? > > > > > > Let's say someone writes a program whose responsibilities are just to > > > handle seccomp events and to listen on some other fd for commands. And > > > this is implemented with an event loop. Then once all the target > > > processes are gone (including zombie reaping), we'll start getting > > > EPOLLERR. > > > > > > If NOTIF_RECV starts returning -ENOTCONN at this point, the event loop > > > can just call into the seccomp logic without any arguments; it can > > > just call NOTIF_RECV one more time, see the -ENOTCONN, and terminate. > > > The downside is that there's one more error code userspace has to > > > special-case. > > > This would be more consistent with what we'd be doing in the blocking case. > > > > > > If NOTIF_RECV keeps returning -ENOENT, the event loop has to also tell > > > the seccomp logic what the revents are. > > > > > > I guess it probably doesn't really matter much. > > > > So, in practice, if you're emulating a blocking syscall (such as open, > > perf_event_open, or any of a number of other syscalls), you probably > > have to do it on a separate thread in the supervisor because you want > > to continue to be able to receive new notifications if any other process > > generates a seccomp notification event that you need to handle. > > > > In addition to that, some of these syscalls are preemptible, so you need > > to poll SECCOMP_IOCTL_NOTIF_ID_VALID to make sure that the program > > under supervision hasn't left the syscall. > > > > If we're to implement a mechanism that makes the seccomp ioctl receive > > non-blocking, it would be valuable to address this problem as well (getting > > a notification when the supervisor is processing a syscall and needs to > > preempt it). In the best case, this can be a minor inconvenience, and > > in the worst case this can result in weird errors where you're keeping > > resources open that the container expects to be closed. > > Does "a notification" mean signals? Or would you want to have a second > thread in userspace that poll()s for cancellation events on the > seccomp fd and then somehow takes care of interrupting the first > thread, or something like that? I would be reluctant to be prescriptive in that it be a signal. Right now, it's implemented as a second thread in userspace that does a ioctl(...) and checks if the notification is valid / alive, and does what's required if the notification has died (interrupting the first thread). > > Either way, I think your proposal goes beyond the scope of patching > the existing weirdness, and should be a separate patch. I agree it should be a separate patch, but I think that it'd be nice if there was a way to do something like: * opt-in to getting another message after receiving the notification that indicates the program has left the syscall * when you do the RECV, you can specify a flag or some such asking that you get signaled / notified about the program leaving the syscall * a multiplexed receive that can say if an existing notification in progress has left the valid state. --- The reason I bring this up as part of this current thread / discussion is that I think that they may be related in terms of how we want the behaviour to act. I would love to hear how people think this should work, or better suggestions than the second thread approach above, or the alternative approach of polling all the notifications in progress on some interval [and relying on epoll timeout to trigger that interval].