Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <20181013023138.m445q6itdmyxdtoc@ast-mbp.dhcp.thefacebook.com>
References: <20180816040145.gqzqicab4s6kcbye@ast-mbp.dhcp.thefacebook.com>
 <20181012105427.243779-1-dancol@google.com> <20181013023138.m445q6itdmyxdtoc@ast-mbp.dhcp.thefacebook.com>
From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Tue, 16 Oct 2018 10:39:57 -0700
Message-ID: <CAEXW_YQFs9mcw5Qv04ZRv=ZAK7YeBLJKByQxXYLj34iigS-Q=Q@mail.gmail.com>
Subject: Re: [PATCH v4] Wait for running BPF programs when updating map-in-map
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
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
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

-Joel