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

[Joel Fernandes]

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();
> +	}
> +}
> +
>  #define BPF_MAP_UPDATE_ELEM_LAST_FIELD flags
>  
>  static int map_update_elem(union bpf_attr *attr)
> @@ -831,6 +843,7 @@ static int map_update_elem(union bpf_attr *attr)
>  	}
>  	__this_cpu_dec(bpf_prog_active);
>  	preempt_enable();
> +	maybe_wait_bpf_programs(map);
>  out:
>  free_value:
>  	kfree(value);
> @@ -883,6 +896,7 @@ static int map_delete_elem(union bpf_attr *attr)
>  	rcu_read_unlock();
>  	__this_cpu_dec(bpf_prog_active);
>  	preempt_enable();
> +	maybe_wait_bpf_programs(map);

Looks good to me,

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Also I believe that those rcu_read_lock() and rcu_read_unlock() calls in the
existing code are useless. preempt_disable()d code is already an RCU
read-side section, and synchronize_rcu and friends work on those type of
read-side sections as well (as of recent kernel releases) however removing it
may make lockdep unhappy, unless we also replace all rcu_dereference() usages
with rcu_dereference_sched(), so lets leave that alone for now I guess.

thanks,

- Joel