From: Daniel Colascione dancol@google.com
commit 1ae80cf31938c8f77c37a29bbe29e7f1cd492be8 upstream.
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 Reviewed-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Alexei Starovoitov ast@kernel.org
[fengc@google.com: 4.14 backport: adjust context] Signed-off-by: Chenbo Feng fengc@google.com --- kernel/bpf/syscall.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index ea22d0b6a9f0..5c9deed4524e 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -519,6 +519,17 @@ 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) @@ -592,6 +603,7 @@ static int map_update_elem(union bpf_attr *attr) } __this_cpu_dec(bpf_prog_active); preempt_enable(); + maybe_wait_bpf_programs(map);
if (!err) trace_bpf_map_update_elem(map, ufd, key, value); @@ -636,6 +648,7 @@ 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);
if (!err) trace_bpf_map_delete_elem(map, ufd, key);