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Subject[PATCH] Remove race between con_open and con_close
I have a laptop (G3 powerbook) which will pretty reliably hit a race
between con_open and con_close late in the boot process and oops in
vt_ioctl due to tty->driver_data being NULL.

What happens is this: process A opens /dev/tty6; it comes into
con_open() (drivers/char/vt.c) and assign a non-NULL value to
tty->driver_data. Then process A closes that and concurrently process
B opens /dev/tty6. Process A gets through con_close() and clears
tty->driver_data, since tty->count == 1. However, before process A
can decrement tty->count, we switch to process B (e.g. at the
down(&tty_sem) call at drivers/char/tty_io.c line 1626).

So process B gets to run and comes into con_open with tty->count == 2,
as tty->count is incremented (in init_dev) before con_open is called.
Because tty->count != 1, we don't set tty->driver_data. Then when the
process tries to do anything with that fd, it oopses.

The simple and effective fix for this is to test tty->driver_data
rather than tty->count in con_open. The testing and setting of
tty->driver_data is serialized with respect to the clearing of
tty->driver_data in con_close by the console_sem. We can't get a
situation where con_open sees tty->driver_data != NULL and then
con_close on a different fd clears tty->driver_data, because
tty->count is incremented before con_open is called. Thus this patch
eliminates the race, and in fact with this patch my laptop doesn't
oops.

Could this go into 2.6.13 please?

Signed-off-by: Paul Mackerras <paulus@samba.org>

diff -urN linux-2.6/drivers/char/vt.c pmac-2.6/drivers/char/vt.c
--- linux-2.6/drivers/char/vt.c 2005-07-17 10:59:52.000000000 +1000
+++ pmac-2.6/drivers/char/vt.c 2005-08-27 22:59:36.000000000 +1000
@@ -2433,7 +2433,7 @@
int ret = 0;

acquire_console_sem();
- if (tty->count == 1) {
+ if (tty->driver_data == NULL) {
ret = vc_allocate(currcons);
if (ret == 0) {
struct vc_data *vc = vc_cons[currcons].d;
-
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