Re: [PATCH v4] Wait for running BPF programs when updating map-in-map

[Joel Fernandes]

On Fri, Oct 12, 2018 at 7:31 PM, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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