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Subject[PATCH] tty: make n_tty_read() always abort if hangup is in progress
A tty is hung up by __tty_hangup() setting file->f_op to
hung_up_tty_fops, which is skipped on ttys whose write operation isn't
tty_write(). This means that, for example, /dev/console whose write
op is redirected_tty_write() is never actually marked hung up.

Because n_tty_read() uses the hung up status to decide whether to
abort the waiting readers, the lack of hung-up marking can lead to the
following scenario.

1. A session contains two processes. The leader and its child. The
child ignores SIGHUP.

2. The leader exits and starts disassociating from the controlling
terminal (/dev/console).

3. __tty_hangup() skips setting f_op to hung_up_tty_fops.

4. SIGHUP is delivered and ignored.

5. tty_ldisc_hangup() is invoked. It wakes up the waits which should
clear the read lockers of tty->ldisc_sem.

6. The reader wakes up but because tty_hung_up_p() is false, it
doesn't abort and goes back to sleep while read-holding
tty->ldisc_sem.

7. The leader progresses to tty_ldisc_lock() in tty_ldisc_hangup()
and is now stuck in D sleep indefinitely waiting for
tty->ldisc_sem.

The following is Alan's explanation on why some ttys aren't hung up.

http://lkml.kernel.org/r/20171101170908.6ad08580@alans-desktop

1. It broke the serial consoles because they would hang up and close
down the hardware. With tty_port that *should* be fixable properly
for any cases remaining.

2. The console layer was (and still is) completely broken and doens't
refcount properly. So if you turn on console hangups it breaks (as
indeed does freeing consoles and half a dozen other things).

As neither can be fixed quickly, this patch works around the problem
by introducing a new flag, TTY_HUPPING, which is used solely to tell
n_tty_read() that hang-up is in progress for the console and the
readers should be aborted regardless of the hung-up status of the
device.


The following is a sample hung task warning caused by this issue.

INFO: task agetty:2662 blocked for more than 120 seconds.
Not tainted 4.11.3-dbg-tty-lockup-02478-gfd6c7ee-dirty #28
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
0 2662 1 0x00000086
Call Trace:
__schedule+0x267/0x890
schedule+0x36/0x80
schedule_timeout+0x23c/0x2e0
ldsem_down_write+0xce/0x1f6
tty_ldisc_lock+0x16/0x30
tty_ldisc_hangup+0xb3/0x1b0
__tty_hangup+0x300/0x410
disassociate_ctty+0x6c/0x290
do_exit+0x7ef/0xb00
do_group_exit+0x3f/0xa0
get_signal+0x1b3/0x5d0
do_signal+0x28/0x660
exit_to_usermode_loop+0x46/0x86
do_syscall_64+0x9c/0xb0
entry_SYSCALL64_slow_path+0x25/0x25

The following is the repro. Run "$PROG /dev/console". The parent
process hangs in D state.

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <signal.h>
#include <time.h>
#include <termios.h>

int main(int argc, char **argv)
{
struct sigaction sact = { .sa_handler = SIG_IGN };
struct timespec ts1s = { .tv_sec = 1 };
pid_t pid;
int fd;

if (argc < 2) {
fprintf(stderr, "test-hung-tty /dev/$TTY\n");
return 1;
}

/* fork a child to ensure that it isn't already the session leader */
pid = fork();
if (pid < 0) {
perror("fork");
return 1;
}

if (pid > 0) {
/* top parent, wait for everyone */
while (waitpid(-1, NULL, 0) >= 0)
;
if (errno != ECHILD)
perror("waitpid");
return 0;
}

/* new session, start a new session and set the controlling tty */
if (setsid() < 0) {
perror("setsid");
return 1;
}

fd = open(argv[1], O_RDWR);
if (fd < 0) {
perror("open");
return 1;
}

if (ioctl(fd, TIOCSCTTY, 1) < 0) {
perror("ioctl");
return 1;
}

/* fork a child, sleep a bit and exit */
pid = fork();
if (pid < 0) {
perror("fork");
return 1;
}

if (pid > 0) {
nanosleep(&ts1s, NULL);
printf("Session leader exiting\n");
exit(0);
}

/*
* The child ignores SIGHUP and keeps reading from the controlling
* tty. Because SIGHUP is ignored, the child doesn't get killed on
* parent exit and the bug in n_tty makes the read(2) block the
* parent's control terminal hangup attempt. The parent ends up in
* D sleep until the child is explicitly killed.
*/
sigaction(SIGHUP, &sact, NULL);
printf("Child reading tty\n");
while (1) {
char buf[1024];

if (read(fd, buf, sizeof(buf)) < 0) {
perror("read");
return 1;
}
}

return 0;
}


Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alan Cox <alan@llwyncelyn.cymru>
Cc: stable@vger.kernel.org
---
Hello,

This patch was initially posted several months ago as a sort of RFC.

http://lkml.kernel.org/r/20171101020651.GK3252168@devbig577.frc2.facebook.com

IIUC Alan's explanation correctly, this workaround actually seems like
the right thing to do, so I refreshed the patch, added some comments
and updated the patch description accordingly.

We've been running this in the fleet for the past couple months to
avoid the process hang issues and it hasn't caused any issues. Greg,
can you please pick it up?

Thanks.

drivers/tty/n_tty.c | 6 ++++++
drivers/tty/tty_io.c | 9 +++++++++
include/linux/tty.h | 1 +
3 files changed, 16 insertions(+)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 5c0e59e..cbe98bc 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2180,6 +2180,12 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
}
if (tty_hung_up_p(file))
break;
+ /*
+ * Abort readers for ttys which never actually
+ * get hung up. See __tty_hangup().
+ */
+ if (test_bit(TTY_HUPPING, &tty->flags))
+ break;
if (!timeout)
break;
if (file->f_flags & O_NONBLOCK) {
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index eb9133b..63114ea 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -586,6 +586,14 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
return;
}

+ /*
+ * Some console devices aren't actually hung up for technical and
+ * historical reasons, which can lead to indefinite interruptible
+ * sleep in n_tty_read(). The following explicitly tells
+ * n_tty_read() to abort readers.
+ */
+ set_bit(TTY_HUPPING, &tty->flags);
+
/* inuse_filps is protected by the single tty lock,
this really needs to change if we want to flush the
workqueue with the lock held */
@@ -640,6 +648,7 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
* from the ldisc side, which is now guaranteed.
*/
set_bit(TTY_HUPPED, &tty->flags);
+ clear_bit(TTY_HUPPING, &tty->flags);
tty_unlock(tty);

if (f)
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 0a6c71e..47f8af2 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -364,6 +364,7 @@ struct tty_file_private {
#define TTY_PTY_LOCK 16 /* pty private */
#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
#define TTY_HUPPED 18 /* Post driver->hangup() */
+#define TTY_HUPPING 19 /* Hangup in progress */
#define TTY_LDISC_HALTED 22 /* Line discipline is halted */

/* Values for tty->flow_change */
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