bpf: error handling when map_lookup_elem isn't supported

mainline inclusion
from mainline-4.20
commit 509db2833e0d
category: bugfix
bugzilla: 9355
CVE: NA

-------------------------------------------------

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: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>
Reviewed-by: NHanjun Guo <guohanjun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/bpf/syscall.c

@@ -717,10 +717,15 @@ 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)