[PATCH] loop: Add sanity check for read/write_iter

[Lizhi Xu]

Some file systems do not support read_iter or write_iter, such as selinuxfs
in this issue.
So before calling them, first confirm that the interface is supported and
then call it.

Reported-by: syzbot+6af973a3b8dfd2faefdc@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=6af973a3b8dfd2faefdc
Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
---
 drivers/block/loop.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 674527d770dc..4f968e3071ed 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -449,10 +449,15 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	cmd->iocb.ki_flags = IOCB_DIRECT;
 	cmd->iocb.ki_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0);
 
-	if (rw == ITER_SOURCE)
-		ret = file->f_op->write_iter(&cmd->iocb, &iter);
-	else
-		ret = file->f_op->read_iter(&cmd->iocb, &iter);
+	ret = 0;
+	if (rw == ITER_SOURCE) {
+		if (likely(file->f_op->write_iter))
+			ret = file->f_op->write_iter(&cmd->iocb, &iter);
+	}
+	else {
+		if (likely(file->f_op->read_iter))
+			ret = file->f_op->read_iter(&cmd->iocb, &iter);
+	}
 
 	lo_rw_aio_do_completion(cmd);
 
-- 
2.43.0