FAILED: patch "[PATCH] exit: hang up the tty before closing the files" failed to apply to 5.15-stable tree

PATCH New [email protected] 2 participants 2 messages Oct 28 Stable branch

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[email protected]

author maintainer

2026-10-28 15:36 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x ad016f0131ebfec250a3360ea75d35128b3f4aa8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026102828-impotency-harddisk-0e94@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ad016f0131ebfec250a3360ea75d35128b3f4aa8 Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Mon, 21 Sep 2026 15:44:57 +0200
Subject: [PATCH] exit: hang up the tty before closing the files

do_exit() closes the task's files in exit_files() and hangs up the controlling tty of a session leader in disassociate_ctty(1) after that. Since commit d99d38540bf0 ("fs: make close_files() synchronous") the final __fput() of every file runs inside exit_files(). So when the session leader holds the last open of its tty the tty is released before disassociate_ctty() runs. tty_release() clears signal->tty for the whole session in session_clear_tty() once the count drops to zero and sends no signal doing so. disassociate_ctty(1) then finds neither a tty nor a tty_old_pgrp and does nothing. The foreground process group loses its SIGHUP:

    do_exit()
      exit_files()
        close_files()
          tty_release()             tty->count == 0
            session_clear_tty()     signal->tty = NULL, no signal
      disassociate_ctty(1)
        get_current_tty()           NULL
        signal->tty_old_pgrp        NULL, nothing sent

That only affects real ttys. For a pty the master's open keeps the slave's count above zero. And it only affects a foreground job that holds no descriptor to the tty anymore while its session leader exits. Everything else is unchanged. The DTR drop on the last close happens in tty_port_shutdown() regardless, stopped jobs get their SIGHUP from kill_orphaned_pgrp() and signal->tty is cleared either way.

Before that commit the final __fput() ran from exit_task_work() which comes after disassociate_ctty(). That order isn't old. Until v3.14 exit_task_work() came right after exit_files() and before v3.6 fput() was synchronous, so the tty was always released first. Commit c39df5fa37b0 ("exit: call disassociate_ctty() before exit_task_namespaces()") moved disassociate_ctty() up to fix a pppd crash and in front of exit_task_work() as a side effect. The hangup in this case has worked since then and that's eleven years of userspace being able to rely on it.

Hang the tty up before closing the files. This is the ordinary hangup with the file still open: __tty_hangup() swaps in hung_up_tty_fops and tty_release() runs from the close afterwards as it does when a modem drops the line. disassociate_ctty() stays in front of exit_task_namespaces() which the pppd fix needs.

Reported-by Chris Mason <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Acked-by Oleg Nesterov <[email protected]>

Signed-off-by Christian Brauner (Amutable) <[email protected]>

kernel/exit.c +3 -2

index 55dbea3b242e..9ed5eb03d0e1 100644
@@ -1003,10 +1003,11 @@ void __noreturn do_exit(long code)
 
 	exit_sem(tsk);
 	exit_shm(tsk);
-	exit_files(tsk);
-	exit_fs(tsk);
+	/* Hang the tty up before the last close of it can clear the session. */
 	if (group_dead)
 		disassociate_ctty(1);
+	exit_files(tsk);
+	exit_fs(tsk);
 	exit_nsproxy_namespaces(tsk);
 	exit_task_work(tsk);
 	exit_thread(tsk);

Sasha Levin

maintainer

2026-10-28 22:03 UTC #1 lore raw

PATCH 1/1 exit: hang up the tty before closing the files

From: Christian Brauner <[email protected]>

[ Upstream commit ad016f0131ebfec250a3360ea75d35128b3f4aa8 ]

    do_exit()
      exit_files()
        close_files()
          tty_release()             tty->count == 0
            session_clear_tty()     signal->tty = NULL, no signal
      disassociate_ctty(1)
        get_current_tty()           NULL
        signal->tty_old_pgrp        NULL, nothing sent

Reported-by Chris Mason <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Acked-by Oleg Nesterov <[email protected]>

Signed-off-by Christian Brauner (Amutable) <[email protected]>

[ sashal: adjusted context for exit_task_namespaces() instead of exit_nsproxy_namespaces() ]

Signed-off-by Sasha Levin <[email protected]>

---
 kernel/exit.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

kernel/exit.c +3 -2

index 776994c28887a..be36344282182 100644
@@ -882,10 +882,11 @@ void __noreturn do_exit(long code)
 
 	exit_sem(tsk);
 	exit_shm(tsk);
-	exit_files(tsk);
-	exit_fs(tsk);
+	/* Hang the tty up before the last close of it can clear the session. */
 	if (group_dead)
 		disassociate_ctty(1);
+	exit_files(tsk);
+	exit_fs(tsk);
 	exit_task_namespaces(tsk);
 	exit_task_work(tsk);
 	exit_thread(tsk);

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