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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 894FBC433DF for ; Thu, 28 May 2020 10:35:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 66A23208A7 for ; Thu, 28 May 2020 10:35:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=cloudflare.com header.i=@cloudflare.com header.b="x5nn/Lqv" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387921AbgE1KfD (ORCPT ); Thu, 28 May 2020 06:35:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33782 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387800AbgE1KfB (ORCPT ); Thu, 28 May 2020 06:35:01 -0400 Received: from mail-ej1-x643.google.com (mail-ej1-x643.google.com [IPv6:2a00:1450:4864:20::643]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 02E69C08C5C5 for ; Thu, 28 May 2020 03:34:59 -0700 (PDT) Received: by mail-ej1-x643.google.com with SMTP id d7so31521392eja.7 for ; Thu, 28 May 2020 03:34:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cloudflare.com; s=google; h=references:user-agent:from:to:cc:subject:in-reply-to:date :message-id:mime-version; bh=ih4qu3nrhFJBc0dVExbD6xtuqEnyjY/tH3t/Hv4k5ro=; b=x5nn/LqvtgJ5DSHhWhBqvQv1Qnl6jcIUUfUWyNuo97b2Gk+aArjYpOyTV6vS1FHMyH OntsRbK8DDDOmCKp8IVIAdQLr6f+49xp0cfEhYtwd9uns2MyT0PlfIoxIZgBv2KgbTIl Z0ySk4sdhgpNk55y2BmqqW+R3VX7W+Te1aIyE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:references:user-agent:from:to:cc:subject :in-reply-to:date:message-id:mime-version; bh=ih4qu3nrhFJBc0dVExbD6xtuqEnyjY/tH3t/Hv4k5ro=; b=mLSSs2TSVhXShN39uyD7BNtiRupPT6uw/cLzXvdPggmIVbj7gSble0xn60PYpJpCrZ OvJYTBPln7tg7icdZO7ywQvr2P26ZvpXNJ23fQlByaEr9cj0NUhafoehF64vaPzGJkxF lgIPfGSiAtMM31Wd5Lh4JxqvLcXhzCc643tJfSuA8zy7F3SI1OlPmnNwEBNCM5eF26Ce /YuEcCrO6PBtv/9tTHRBaSVhGwnf+vKgARJDaCiBrE0bZCIOB7polXf3aWzbWXwfbu8A XxJLeAR8JbTxMnkjsQoR1MVejM1c1W4VqTcbLJ75FhgI57m3UmYKTuXc//YDQYK4jN76 6NOg== X-Gm-Message-State: AOAM5301LyLSdYUiMvGMWAGoNpMlCoSoAEigHfw5DkIHWBwZSdLevSq0 r/HtvpNl90nJwjbJbG0mQRPUeg== X-Google-Smtp-Source: ABdhPJzpnzBbPeSN+Y8mIp++rpAKKbpG9TXa6AAMyAIM/sgSqjJeztb+dZGRbjgWSao5Q+gQTRDvtg== X-Received: by 2002:a17:906:16d3:: with SMTP id t19mr2316230ejd.36.1590662098490; Thu, 28 May 2020 03:34:58 -0700 (PDT) Received: from cloudflare.com ([2a02:a310:c262:aa00:b35e:8938:2c2a:ba8b]) by smtp.gmail.com with ESMTPSA id q19sm4906881ejb.88.2020.05.28.03.34.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 May 2020 03:34:57 -0700 (PDT) References: <20200527170840.1768178-1-jakub@cloudflare.com> <20200527170840.1768178-6-jakub@cloudflare.com> <20200527174805.GG49942@google.com> <87tv012lxs.fsf@cloudflare.com> <20200527203823.GB57268@google.com> User-agent: mu4e 1.1.0; emacs 26.3 From: Jakub Sitnicki To: sdf@google.com Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, kernel-team@cloudflare.com Subject: Re: [PATCH bpf-next 5/8] bpf: Add link-based BPF program attachment to network namespace In-reply-to: <20200527203823.GB57268@google.com> Date: Thu, 28 May 2020 12:34:56 +0200 Message-ID: <87sgfk2vqn.fsf@cloudflare.com> MIME-Version: 1.0 Content-Type: text/plain Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, May 27, 2020 at 10:38 PM CEST, sdf@google.com wrote: > On 05/27, Jakub Sitnicki wrote: >> On Wed, May 27, 2020 at 07:48 PM CEST, sdf@google.com wrote: >> > On 05/27, Jakub Sitnicki wrote: >> >> Add support for bpf() syscall subcommands that operate on >> >> bpf_link (LINK_CREATE, LINK_UPDATE, OBJ_GET_INFO) for attach points tied to >> >> network namespaces (that is flow dissector at the moment). >> > >> >> Link-based and prog-based attachment can be used interchangeably, but only >> >> one can be in use at a time. Attempts to attach a link when a prog is >> >> already attached directly, and the other way around, will be met with >> >> -EBUSY. >> > >> >> Attachment of multiple links of same attach type to one netns is not >> >> supported, with the intention to lift it when a use-case presents >> >> itself. Because of that attempts to create a netns link, when one already >> >> exists result in -E2BIG error, signifying that there is no space left for >> >> another attachment. >> > >> >> Link-based attachments to netns don't keep a netns alive by holding a ref >> >> to it. Instead links get auto-detached from netns when the latter is being >> >> destroyed by a pernet pre_exit callback. >> > >> >> When auto-detached, link lives in defunct state as long there are open FDs >> >> for it. -ENOLINK is returned if a user tries to update a defunct link. >> > >> >> Because bpf_link to netns doesn't hold a ref to struct net, special care is >> >> taken when releasing the link. The netns might be getting torn down when >> >> the release function tries to access it to detach the link. >> > >> >> To ensure the struct net object is alive when release function accesses it >> >> we rely on the fact that cleanup_net(), struct net destructor, calls >> >> synchronize_rcu() after invoking pre_exit callbacks. If auto-detach from >> >> pre_exit happens first, link release will not attempt to access struct net. >> > >> >> Same applies the other way around, network namespace doesn't keep an >> >> attached link alive because by not holding a ref to it. Instead bpf_links >> >> to netns are RCU-freed, so that pernet pre_exit callback can safely access >> >> and auto-detach the link when racing with link release/free. >> > >> > [..] >> >> + rcu_read_lock(); >> >> for (type = 0; type < MAX_NETNS_BPF_ATTACH_TYPE; type++) { >> >> - if (rcu_access_pointer(net->bpf.progs[type])) >> >> + if (rcu_access_pointer(net->bpf.links[type])) >> >> + bpf_netns_link_auto_detach(net, type); >> >> + else if (rcu_access_pointer(net->bpf.progs[type])) >> >> __netns_bpf_prog_detach(net, type); >> >> } >> >> + rcu_read_unlock(); >> > Aren't you doing RCU_INIT_POINTER in __netns_bpf_prog_detach? >> > Is it allowed under rcu_read_load? > >> Yes, that's true. __netns_bpf_prog_detach does > >> RCU_INIT_POINTER(net->bpf.progs[type], NULL); > >> RCU read lock is here for the rcu_dereference() that happens in >> bpf_netns_link_auto_detach (netns doesn't hold a ref to bpf_link): > >> /* Called with RCU read lock. */ >> static void __net_exit >> bpf_netns_link_auto_detach(struct net *net, enum netns_bpf_attach_type type) >> { >> struct bpf_netns_link *net_link; >> struct bpf_link *link; > >> link = rcu_dereference(net->bpf.links[type]); >> if (!link) >> return; >> net_link = to_bpf_netns_link(link); >> RCU_INIT_POINTER(net_link->net, NULL); >> } > >> I've pulled it up, out of the loop, perhaps too eagerly and just made it >> confusing, considering we're iterating over a 1-item array :-) > >> Now, I'm also doing RCU_INIT_POINTER on the *contents of bpf_link* in >> bpf_netns_link_auto_detach. Is that allowed? I'm not sure, that bit is >> hazy to me. > >> There are no concurrent writers to net_link->net, just readers, i.e. >> bpf_netns_link_release(). And I know bpf_link won't be freed until the >> grace period elapses. > >> sparse and CONFIG_PROVE_RCU are not shouting at me, but please do if it >> doesn't hold up or make sense. > >> I certainly can push the rcu_read_lock() down into >> bpf_netns_link_auto_detach(). > I think it would be much nicer if you push them down to preserve the > assumption that nothing is modified under read lock and you flip > the pointers only when holding the mutexes. I certainly see how that would save some head-scratching. Might be doable with grabbing a temporary reference to struct net/struct bpf_link. Please see the code snippet below. > > I'll do another pass on this patch, I think I don't understand a bunch > of bits where you do: > > mutex_lock > rcu_read_lock -> why? you're already in the update section, can use > rcu_dereference_protected > ... > rcu_read_unlock > mutex_unlock The rcu_read_lock is to get the grace-period guarantee for the value (struct net) we access by dereferencing RCU protected pointer (bpf_netns_link.net). While mutex_lock is to serialize updates to values within struct net. That is net->bpf.progs or net->bpf.links. The locking is done in reverse order because I cannot grab the mutex while in RCU read-side critical section. If I was holding a reference to struct net, I would not need to be inside an RCU read-side critical section to access it. (This is how bpf_cgroup_link does it when accessing cgroup->bpf.) One thought I had is that I could rearrange sychnronization so that we try to grab a reference to struct net when we need to modify it: rcu_read_lock(); net = rcu_dereference(to_bpf_netns_link(link)->net); if (net) net = maybe_get_net(net); rcu_read_unlock(); if (!net) return; /* netns is dead */ mutex_lock(&netns_bpf_mutex); rcu_assign_pointer(net->bpf.progs[type], link->prog); rcu_assign_pointer(net->bpf.links[type], link); mutex_unlock(&netns_bpf_mutex); put_net(net); That seems be easier to reason about, no? > But I'll post those comments inline shortly. Thanks. Will be better to discuss in context.