From mboxrd@z Thu Jan  1 00:00:00 1970
From: Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH bpf] bpf: fix a rcu usage warning in
 bpf_prog_array_copy_core()
Date: Wed, 15 Aug 2018 14:37:52 -0700
Message-ID: <20180815213749.GA5409@castle.DHCP.thefacebook.com>
References: <20180814180112.293058-1-yhs@fb.com>
 <20180814235944.5nufzmj6d2rkknvs@ast-mbp>
 <20180815000841.GA25304@castle.DHCP.thefacebook.com>
 <20180815213009.t2nhtoxlqpd2sgk4@ast-mbp>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Cc: Yonghong Song <yhs@fb.com>, <ast@fb.com>, <daniel@iogearbox.net>,
        <netdev@vger.kernel.org>, <kernel-team@fb.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mx0a-00082601.pphosted.com ([67.231.145.42]:34876 "EHLO
        mx0a-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK)
        by vger.kernel.org with ESMTP id S1727327AbeHPAcY (ORCPT
        <rfc822;netdev@vger.kernel.org>); Wed, 15 Aug 2018 20:32:24 -0400
Content-Disposition: inline
In-Reply-To: <20180815213009.t2nhtoxlqpd2sgk4@ast-mbp>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Wed, Aug 15, 2018 at 02:30:11PM -0700, Alexei Starovoitov wrote:
> On Tue, Aug 14, 2018 at 05:08:44PM -0700, Roman Gushchin wrote:
> > On Tue, Aug 14, 2018 at 04:59:45PM -0700, Alexei Starovoitov wrote:
> > > On Tue, Aug 14, 2018 at 11:01:12AM -0700, Yonghong Song wrote:
> > > > Commit 394e40a29788 ("bpf: extend bpf_prog_array to store pointers
> > > > to the cgroup storage") refactored the bpf_prog_array_copy_core()
> > > > to accommodate new structure bpf_prog_array_item which contains
> > > > bpf_prog array itself.
> > > > 
> > > > In the old code, we had
> > > >    perf_event_query_prog_array():
> > > >      mutex_lock(...)
> > > >      bpf_prog_array_copy_call():
> > > >        prog = rcu_dereference_check(array, 1)->progs
> > > >        bpf_prog_array_copy_core(prog, ...)
> > > >      mutex_unlock(...)
> > > > 
> > > > With the above commit, we had
> > > >    perf_event_query_prog_array():
> > > >      mutex_lock(...)
> > > >      bpf_prog_array_copy_call():
> > > >        bpf_prog_array_copy_core(array, ...):
> > > >          item = rcu_dereference(array)->items;
> > > >          ...
> > > >      mutex_unlock(...)
> > > > 
> > > > The new code will trigger a lockdep rcu checking warning.
> > > > The fix is to change rcu_dereference() to rcu_dereference_check()
> > > > to prevent such a warning.
> > > > 
> > > > Reported-by: syzbot+6e72317008eef84a216b@syzkaller.appspotmail.com
> > > > Fixes: 394e40a29788 ("bpf: extend bpf_prog_array to store pointers to the cgroup storage")
> > > > Cc: Roman Gushchin <guro@fb.com>
> > > > Signed-off-by: Yonghong Song <yhs@fb.com>
> > > 
> > > makes sense to me
> > > Acked-by: Alexei Starovoitov <ast@kernel.org>
> > > 
> > > Roman, would you agree?
> > > 
> > 
> > rcu_dereference_check(<>, 1) always looks a bit strange to me,
> > but if it's the only reasonable way to silence the warning,
> > of course I'm fine with it.
> 
> do you have better suggestion?
> This patch is a fix for the regression introduced in your earlier patch,
> so I think the only fair path forward is either to Ack it or
> to send an alternative patch asap.
> 

As I said, I've nothing against.

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!