Re: [RFC bpf-next 2/4] bpf: return EOPNOTSUPP when map lookup isn't supported

[Prashant Bhole]

On 9/20/2018 12:14 AM, Alexei Starovoitov wrote:
On Wed, Sep 19, 2018 at 04:51:41PM +0900, Prashant Bhole wrote:
Return ERR_PTR(-EOPNOTSUPP) from map_lookup_elem() methods of below
map types:
- BPF_MAP_TYPE_PROG_ARRAY
- BPF_MAP_TYPE_STACK_TRACE
- BPF_MAP_TYPE_XSKMAP
- BPF_MAP_TYPE_SOCKMAP/BPF_MAP_TYPE_SOCKHASH

Signed-off-by: Prashant Bhole <bhole_prashant...@lab.ntt.co.jp>
---
  kernel/bpf/arraymap.c | 2 +-
  kernel/bpf/sockmap.c  | 2 +-
  kernel/bpf/stackmap.c | 2 +-
  kernel/bpf/xskmap.c   | 2 +-
  4 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index dded84cbe814..24583da9ffd1 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -449,7 +449,7 @@ static void fd_array_map_free(struct bpf_map *map)
  
  static void *fd_array_map_lookup_elem(struct bpf_map *map, void *key)
  {
-       return NULL;
+       return ERR_PTR(-EOPNOTSUPP);
  }

conceptually the set looks good to me.
Please add a test to test_verifier.c to make sure
that these lookup helpers cannot be called from BPF program.
Otherwise this diff may cause crashes.

Thanks for reviewing.
Is the verifier change below sufficient?

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2128,10 +2128,18 @@ static int check_map_func_compatibility(struct 
bpf_verifier_env *env,
                if (env->subprog_cnt > 1) {
 			verbose(env, "tail_calls are not allowed in programs with 
bpf-to-bpf calls\n");
                        return -EINVAL;
                }
                break;
+       case BPF_FUNC_map_lookup_elem:
+               if (map->map_type == BPF_MAP_TYPE_PROG_ARRAY ||
+                   map->map_type == BPF_MAP_TYPE_STACK_TRACE ||
+                   map->map_type == BPF_MAP_TYPE_XSKMAP ||
+                   map->map_type == BPF_MAP_TYPE_SOCKMAP ||
+                   map->map_type == BPF_MAP_TYPE_SOCKHASH)
+                       goto error;
+               break;
        case BPF_FUNC_perf_event_read:
        case BPF_FUNC_perf_event_output:
        case BPF_FUNC_perf_event_read_value:
                if (map->map_type != BPF_MAP_TYPE_PERF_EVENT_ARRAY)
                        goto error;

-Prashant