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Subject[PATCH] [98/223] TTY: open/hangup race fixup
Date
2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
From: Jiri Slaby <jslaby@suse.cz>

commit acfa747baf73922021a047f2d87a2d866f5dbab5 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
drivers/char/tty_io.c | 10 +++++++++-
include/linux/tty.h | 1 +
2 files changed, 10 insertions(+), 1 deletion(-)

Index: linux/drivers/char/tty_io.c
===================================================================
--- linux.orig/drivers/char/tty_io.c
+++ linux/drivers/char/tty_io.c
@@ -514,7 +514,10 @@ static void do_tty_hangup(struct work_st
spin_unlock(&redirect_lock);

/* inuse_filps is protected by the single kernel lock */
+
lock_kernel();
+ /* some functions below drop BTM, so we need this bit */
+ set_bit(TTY_HUPPING, &tty->flags);
check_tty_count(tty, "do_tty_hangup");

file_list_lock();
@@ -530,6 +533,10 @@ static void do_tty_hangup(struct work_st
}
file_list_unlock();

+ /*
+ * it drops BTM and thus races with reopen
+ * we protect the race by TTY_HUPPING
+ */
tty_ldisc_hangup(tty);

read_lock(&tasklist_lock);
@@ -567,7 +574,6 @@ static void do_tty_hangup(struct work_st
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 */
@@ -593,6 +599,7 @@ static void do_tty_hangup(struct work_st
* can't yet guarantee all that.
*/
set_bit(TTY_HUPPED, &tty->flags);
+ clear_bit(TTY_HUPPING, &tty->flags);
tty_ldisc_enable(tty);
unlock_kernel();
if (f)
@@ -1258,6 +1265,7 @@ static int tty_reopen(struct tty_struct
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;

Index: linux/include/linux/tty.h
===================================================================
--- linux.orig/include/linux/tty.h
+++ linux/include/linux/tty.h
@@ -356,6 +356,7 @@ struct tty_struct {
#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))


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