bpf: Check IS_ERR for the bpf_map_get() return value

This patch fixes a mistake in checking NULL instead of
checking IS_ERR for the bpf_map_get() return value.

It also fixes the return value in link_update_map() from -EINVAL
to PTR_ERR(*_map).

Reported-by: syzbot+71ccc0fe37abb458406b@syzkaller.appspotmail.com
Fixes: 68b04864 ("bpf: Create links for BPF struct_ops maps.")
Fixes: aef56f2e ("bpf: Update the struct_ops of a bpf_link.")
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Kui-Feng Lee <kuifeng@meta.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230324184241.1387437-1-martin.lau@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/bpf_struct_ops.c

@@ -871,8 +871,8 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
 	int err;
 
 	map = bpf_map_get(attr->link_create.map_fd);
-	if (!map)
-		return -EINVAL;
+	if (IS_ERR(map))
+		return PTR_ERR(map);
 
 	st_map = (struct bpf_struct_ops_map *)map;
 

kernel/bpf/syscall.c

@@ -4689,12 +4689,12 @@ static int link_update_map(struct bpf_link *link, union bpf_attr *attr)
 
 	new_map = bpf_map_get(attr->link_update.new_map_fd);
 	if (IS_ERR(new_map))
-		return -EINVAL;
+		return PTR_ERR(new_map);
 
 	if (attr->link_update.flags & BPF_F_REPLACE) {
 		old_map = bpf_map_get(attr->link_update.old_map_fd);
 		if (IS_ERR(old_map)) {
-			ret = -EINVAL;
+			ret = PTR_ERR(old_map);
 			goto out_put;
 		}
 	} else if (attr->link_update.old_map_fd) {