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Commit acfa747b authored by Jiri Slaby's avatar Jiri Slaby Committed by Greg Kroah-Hartman
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TTY: open/hangup race fixup 



Like in the "TTY: don't allow reopen when ldisc is changing" patch,
this one fixes a TTY WARNING as described in the option 1) there:
1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
section tty_lock is held. However tty_lock is temporarily dropped in
the middle of the function by tty_ldisc_hangup.

The fix is to introduce a new flag which we set during the unlocked
window and check it in tty_reopen too. The flag is TTY_HUPPING and is
cleared after TTY_HUPPED is set.

While at it, remove duplicate TTY_HUPPED set_bit. The one after
calling ops->hangup seems to be more correct. But anyway, we hold
tty_lock, so there should be no difference.

Also document the function it does that kind of crap.

Nicely reproducible with two forked children:
static void do_work(const char *tty)
{
	if (signal(SIGHUP, SIG_IGN) == SIG_ERR) exit(1);
	setsid();
	while (1) {
		int fd = open(tty, O_RDWR|O_NOCTTY);
		if (fd < 0) continue;
		if (ioctl(fd, TIOCSCTTY)) continue;
		if (vhangup()) continue;
		close(fd);
	}
	exit(0);
}

Signed-off-by: default avatarJiri Slaby <jslaby@suse.cz>
Reported-by: default avatar <Valdis.Kletnieks@vt.edu>
Reported-by: default avatarKyle McMartin <kyle@mcmartin.ca>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: stable <stable@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent e2efafbf

drivers/tty/tty_io.c

+9 −1
Original line number Diff line number Diff line
@@ -559,6 +559,9 @@ void __tty_hangup(struct tty_struct *tty) 


	tty_lock();



	/* some functions below drop BTM, so we need this bit */

	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 */
@@ -578,6 +581,10 @@ void __tty_hangup(struct tty_struct *tty) 
	}

	spin_unlock(&tty_files_lock);



	/*

	 * it drops BTM and thus races with reopen

	 * we protect the race by TTY_HUPPING

	 */

	tty_ldisc_hangup(tty);



	read_lock(&tasklist_lock);
@@ -615,7 +622,6 @@ void __tty_hangup(struct tty_struct *tty) 
	tty->session = NULL;

	tty->pgrp = NULL;

	tty->ctrl_status = 0;

	set_bit(TTY_HUPPED, &tty->flags);

	spin_unlock_irqrestore(&tty->ctrl_lock, flags);



	/* Account for the p->signal references we killed */
@@ -641,6 +647,7 @@ void __tty_hangup(struct tty_struct *tty) 
	 * can't yet guarantee all that.

	 */

	set_bit(TTY_HUPPED, &tty->flags);

	clear_bit(TTY_HUPPING, &tty->flags);

	tty_ldisc_enable(tty);



	tty_unlock();
@@ -1311,6 +1318,7 @@ static int tty_reopen(struct tty_struct *tty) 
	struct tty_driver *driver = tty->driver;



	if (test_bit(TTY_CLOSING, &tty->flags) ||

			test_bit(TTY_HUPPING, &tty->flags) ||

			test_bit(TTY_LDISC_CHANGING, &tty->flags))

		return -EIO;

include/linux/tty.h

+1 −0
Original line number Diff line number Diff line
@@ -367,6 +367,7 @@ struct tty_file_private { 
#define TTY_HUPPED 		18	/* Post driver->hangup() */

#define TTY_FLUSHING		19	/* Flushing to ldisc in progress */

#define TTY_FLUSHPENDING	20	/* Queued buffer flush pending */

#define TTY_HUPPING 		21	/* ->hangup() in progress */



#define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))