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Date:   Thu, 18 Oct 2018 08:46:59 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Daniel Colascione <dancol@google.com>,
        Joel Fernandes <joelaf@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        netdev <netdev@vger.kernel.org>,
        Lorenzo Colitti <lorenzo@google.com>,
        Chenbo Feng <fengc@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Alexei Starovoitov <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>, stable@vger.kernel.org
Subject: Re: [PATCH v4] Wait for running BPF programs when updating map-in-map
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On Tue, Oct 16, 2018 at 10:39:57AM -0700, Joel Fernandes wrote:
> On Fri, Oct 12, 2018 at 7:31 PM, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> > On Fri, Oct 12, 2018 at 03:54:27AM -0700, Daniel Colascione wrote:
> >> The map-in-map frequently serves as a mechanism for atomic
> >> snapshotting of state that a BPF program might record.  The current
> >> implementation is dangerous to use in this way, however, since
> >> userspace has no way of knowing when all programs that might have
> >> retrieved the "old" value of the map may have completed.
> >>
> >> This change ensures that map update operations on map-in-map map types
> >> always wait for all references to the old map to drop before returning
> >> to userspace.
> >>
> >> Signed-off-by: Daniel Colascione <dancol@google.com>
> >> ---
> >>  kernel/bpf/syscall.c | 14 ++++++++++++++
> >>  1 file changed, 14 insertions(+)
> >>
> >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> >> index 8339d81cba1d..d7c16ae1e85a 100644
> >> --- a/kernel/bpf/syscall.c
> >> +++ b/kernel/bpf/syscall.c
> >> @@ -741,6 +741,18 @@ static int map_lookup_elem(union bpf_attr *attr)
> >>       return err;
> >>  }
> >>
> >> +static void maybe_wait_bpf_programs(struct bpf_map *map)
> >> +{
> >> +     /* Wait for any running BPF programs to complete so that
> >> +      * userspace, when we return to it, knows that all programs
> >> +      * that could be running use the new map value.
> >> +      */
> >> +     if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS ||
> >> +         map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS) {
> >> +             synchronize_rcu();
> >> +     }
> >
> > extra {} were not necessary. I removed them while applying to bpf-next.
> > Please run checkpatch.pl next time.
> > Thanks
> 
> Thanks Alexei for taking it. Me and Lorenzo were discussing that not
> having this causes incorrect behavior for apps using map-in-map for
> this. So I CC'd stable as well.

It is too late in the release cycle.
We can submit it to stable releases after the merge window.