tty: n_gsm: fix missing corner cases in gsmld_poll()

stable inclusion
from stable-v5.10.137
commit 8fd063a6085abb0b4aa8682beed8158c3952d713
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I60PLB

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8fd063a6085abb0b4aa8682beed8158c3952d713

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

[ Upstream commit 7e5b4322 ]

gsmld_poll() currently fails to handle the following corner cases correctly:
- remote party closed the associated tty

Add the missing checks and map those to EPOLLHUP.
Reorder the checks to group them by their reaction.

Fixes: e1eaea46 ("tty: n_gsm line discipline")
Signed-off-by: NDaniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220707113223.3685-4-daniel.starke@siemens.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: NWei Li <liwei391@huawei.com>

drivers/tty/n_gsm.c

@@ -2747,12 +2747,15 @@ static __poll_t gsmld_poll(struct tty_struct *tty, struct file *file,
 
 	poll_wait(file, &tty->read_wait, wait);
 	poll_wait(file, &tty->write_wait, wait);
+
+	if (gsm->dead)
+		mask |= EPOLLHUP;
 	if (tty_hung_up_p(file))
 		mask |= EPOLLHUP;
+	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
+		mask |= EPOLLHUP;
 	if (!tty_is_writelocked(tty) && tty_write_room(tty) > 0)
 		mask |= EPOLLOUT | EPOLLWRNORM;
-	if (gsm->dead)
-		mask |= EPOLLHUP;
 	return mask;
 }