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

[Joel Fernandes]

On Thu, Oct 18, 2018 at 08:46:59AM -0700, Alexei Starovoitov wrote:
> 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@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.
> 
> It is too late in the release cycle.
> We can submit it to stable releases after the merge window.
> 

Sounds good, thanks.

- Joel