bpf: Acquire map uref in .init_seq_private for hash map iterator

bpf_iter_attach_map() acquires a map uref, and the uref may be released
before or in the middle of iterating map elements. For example, the uref
could be released in bpf_iter_detach_map() as part of
bpf_link_release(), or could be released in bpf_map_put_with_uref() as
part of bpf_map_release().

So acquiring an extra map uref in bpf_iter_init_hash_map() and
releasing it in bpf_iter_fini_hash_map().

Fixes: d6c4503c ("bpf: Implement bpf iterator for hash maps")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220810080538.1845898-3-houtao@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/hashtab.c

@@ -2060,6 +2060,7 @@ static int bpf_iter_init_hash_map(void *priv_data,
 		seq_info->percpu_value_buf = value_buf;
 	}
 
+	bpf_map_inc_with_uref(map);
 	seq_info->map = map;
 	seq_info->htab = container_of(map, struct bpf_htab, map);
 	return 0;
@@ -2069,6 +2070,7 @@ static void bpf_iter_fini_hash_map(void *priv_data)
 {
 	struct bpf_iter_seq_hash_map_info *seq_info = priv_data;
 
+	bpf_map_put_with_uref(seq_info->map);
 	kfree(seq_info->percpu_value_buf);
 }