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From:   Daniel Colascione <dancol@google.com>
Date:   Fri, 27 Jul 2018 12:17:37 -0700
Message-ID: <CAKOZueu37C49TGny85_LZ_yp+NgQtWfjpBU0CB7OS6HFF-CeHA@mail.gmail.com>
Subject: Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Lorenzo Colitti <lorenzo@google.com>,
        Chenbo Feng <fengc@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Joel Fernandes <joelaf@google.com>,
        Alexei Starovoitov <ast@fb.com>,
        lkml <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        netdev <netdev@vger.kernel.org>
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On Sat, Jul 14, 2018 at 11:18 AM, Joel Fernandes <joel@joelfernandes.org>
wrote:

> By the way just curious I was briefly going through kernel/bpf/arraymap.c.
> How are you protecting against load-store tearing of values of array map
> updates/lookups?
>
> For example, if userspace reads an array map at a particular index, while
> another CPU is updating it, then userspace can read partial values /
> half-updated values right? Since rcu_read_lock is in use, I was hoping to
> find something like rcu_assign_pointer there to protect readers against
> concurrent updates.  Thanks for any clarification.


I'm also curious about the answer to this question.

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On S=
at, Jul 14, 2018 at 11:18 AM, Joel Fernandes <span dir=3D"ltr">&lt;<a href=
=3D"mailto:joel@joelfernandes.org" target=3D"_blank">joel@joelfernandes.org=
</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin=
:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">By the way just cu=
rious I was briefly going through kernel/bpf/arraymap.c.<br>
How are you protecting against load-store tearing of values of array map<br=
>
updates/lookups?<br>
<br>
For example, if userspace reads an array map at a particular index, while<b=
r>
another CPU is updating it, then userspace can read partial values /<br>
half-updated values right? Since rcu_read_lock is in use, I was hoping to<b=
r>
find something like rcu_assign_pointer there to protect readers against<br>
concurrent updates.=C2=A0 Thanks for any clarification.</blockquote><div><b=
r></div><div>I&#39;m also curious about the answer to this question.=C2=A0<=
/div></div></div></div>

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