tun: honor IOCB_NOWAIT flag

tun only checks the file O_NONBLOCK flag, but it should also be checking
the iocb IOCB_NOWAIT flag. Any fops using ->read/write_iter() should check
both, otherwise it breaks users that correctly expect O_NONBLOCK semantics
if IOCB_NOWAIT is set.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/e9451860-96cc-c7c7-47b8-fe42cadd5f4c@kernel.dkSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/tun.c

@@ -1961,12 +1961,15 @@ static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct tun_file *tfile = file->private_data;
 	struct tun_struct *tun = tun_get(tfile);
 	ssize_t result;
+	int noblock = 0;
 
 	if (!tun)
 		return -EBADFD;
 
-	result = tun_get_user(tun, tfile, NULL, from,
-			      file->f_flags & O_NONBLOCK, false);
+	if ((file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT))
+		noblock = 1;
+
+	result = tun_get_user(tun, tfile, NULL, from, noblock, false);
 
 	tun_put(tun);
 	return result;
@@ -2185,10 +2188,15 @@ static ssize_t tun_chr_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	struct tun_file *tfile = file->private_data;
 	struct tun_struct *tun = tun_get(tfile);
 	ssize_t len = iov_iter_count(to), ret;
+	int noblock = 0;
 
 	if (!tun)
 		return -EBADFD;
-	ret = tun_do_read(tun, tfile, to, file->f_flags & O_NONBLOCK, NULL);
+
+	if ((file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT))
+		noblock = 1;
+
+	ret = tun_do_read(tun, tfile, to, noblock, NULL);
 	ret = min_t(ssize_t, ret, len);
 	if (ret > 0)
 		iocb->ki_pos = ret;