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TTY: open/hangup race fixup

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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: Jiri Slaby <jslaby@suse.cz>
Reported-by: <Valdis.Kletnieks@vt.edu>
Reported-by: Kyle McMartin <kyle@mcmartin.ca>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Jiri Slaby 2010-11-29 10:16:54 +01:00 • committed by Greg Kroah-Hartman
commit acfa747baf
2 changed files with 10 additions and 1 deletions
Download patch file Download diff file Expand all files Collapse all files

10
drivers/tty/tty_io.c
View file

@ -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;

1
include/linux/tty.h
View file

@ -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))