[RFC bpf-next 1/4] bpf: error handling when map_lookup_elem isn't supported

[Prashant Bhole]

The error value returned by map_lookup_elem doesn't differentiate
whether lookup was failed because of invalid key or lookup is not
supported.

Lets add handling for -EOPNOTSUPP return value of map_lookup_elem()
method of map, with expectation from map's implementation that it
should return -EOPNOTSUPP if lookup is not supported.

The errno for bpf syscall for BPF_MAP_LOOKUP_ELEM command will be set
to EOPNOTSUPP if map lookup is not supported.

Signed-off-by: Prashant Bhole <bhole_prashant...@lab.ntt.co.jp>
---
 kernel/bpf/syscall.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b3c2d09bcf7a..ecb06352b5a0 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -716,10 +716,15 @@ static int map_lookup_elem(union bpf_attr *attr)
        } else {
                rcu_read_lock();
                ptr = map->ops->map_lookup_elem(map, key);
-               if (ptr)
+               if (IS_ERR(ptr)) {
+                       err = PTR_ERR(ptr);
+               } else if (!ptr) {
+                       err = -ENOENT;
+               } else {
+                       err = 0;
                        memcpy(value, ptr, value_size);
+               }
                rcu_read_unlock();
-               err = ptr ? 0 : -ENOENT;
        }
 
        if (err)
-- 
2.17.1