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Message-ID: <20070323085858.GA11221@localhost.sw.ru>
Date: Fri, 23 Mar 2007 11:58:58 +0300
From: Alexey Dobriyan <adobriyan@...ru>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: alan@...hat.com, linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [PATCH] Protect tty drivers list a little
On Thu, Mar 22, 2007 at 09:29:05AM -0800, Andrew Morton wrote:
> On Thu, 22 Mar 2007 14:25:42 +0300 Alexey Dobriyan <adobriyan@...ru> wrote:
> > Additions and removal from tty_drivers list were just done as well as
> > iterating on it for /proc/tty/drivers generation.
> >
> > --- a/drivers/char/tty_io.c
> > +++ b/drivers/char/tty_io.c
> > @@ -127,6 +127,7 @@ EXPORT_SYMBOL(tty_std_termios);
> > into this file */
> >
> > LIST_HEAD(tty_drivers); /* linked list of tty drivers */
> > +DEFINE_SPINLOCK(tty_drivers_lock);
> >
> > /* Mutex to protect creating and releasing a tty. This is shared with
> > vt.c for deeply disgusting hack reasons */
> > @@ -1086,13 +1087,16 @@ static struct tty_driver *get_tty_driver
> > {
> > struct tty_driver *p;
> >
> > + spin_lock(&tty_drivers_lock);
> > list_for_each_entry(p, &tty_drivers, tty_drivers) {
> > dev_t base = MKDEV(p->major, p->minor_start);
> > if (device < base || device >= base + p->num)
> > continue;
> > + spin_unlock(&tty_drivers_lock);
> > *index = device - base;
> > return p;
> > }
> > + spin_unlock(&tty_drivers_lock);
> > return NULL;
> > }
>
> The locking in here is kinda meaningless: we drop the lock and return an
> unrefcounted something which really should have been covered by that lock.
> Or refcounted.
Ahh, indeed. I stated at this place too long choosing spin_unlock
placement, but completely missed pointer :-\
> The reason is that get_tty_driver() and its return value are already covered
> by tty_mutex.
>
> So can we use tty_mutex to fix this race rather than adding a new lock?
I don't see why we can't do it. So here goes version 2 which also
survives some beating described in changelog.
[PATCH] Protect tty drivers list with tty_mutex
Additions and removal from tty_drivers list were just done as well as
iterating on it for /proc/tty/drivers generation.
testing: modprobe/rmmod loop of simple module which does nothing but
tty_register_driver() vs cat /proc/tty/drivers loop
BUG: unable to handle kernel paging request at virtual address 6b6b6b6b
printing eip:
c01cefa7
*pde = 00000000
Oops: 0000 [#1]
PREEMPT
last sysfs file: devices/pci0000:00/0000:00:1d.7/usb5/5-0:1.0/bInterfaceProtocol
Modules linked in: ohci_hcd af_packet e1000 ehci_hcd uhci_hcd usbcore xfs
CPU: 0
EIP: 0060:[<c01cefa7>] Not tainted VLI
EFLAGS: 00010297 (2.6.21-rc4-mm1 #4)
EIP is at vsnprintf+0x3a4/0x5fc
eax: 6b6b6b6b ebx: f6cb50f2 ecx: 6b6b6b6b edx: fffffffe
esi: c0354700 edi: f6cb6000 ebp: 6b6b6b6b esp: f31f5e68
ds: 007b es: 007b fs: 00d8 gs: 0033 ss: 0068
Process cat (pid: 31864, ti=f31f4000 task=c1998030 task.ti=f31f4000)
Stack: 00000000 c0103f20 c013003a c0103f20 00000000 f6cb50da 0000000a 00000f0e
f6cb50f2 00000010 00000014 ffffffff ffffffff 00000007 c0354753 f6cb50f2
f73e39dc f73e39dc 00000001 c0175416 f31f5ed8 f31f5ed4 0ee00000 f32090bc
Call Trace:
[<c0103f20>] restore_nocheck+0x12/0x15
[<c013003a>] mark_held_locks+0x6d/0x86
[<c0103f20>] restore_nocheck+0x12/0x15
[<c0175416>] seq_printf+0x2e/0x52
[<c0192895>] show_tty_range+0x35/0x1f3
[<c0175416>] seq_printf+0x2e/0x52
[<c0192add>] show_tty_driver+0x8a/0x1d9
[<c01758f6>] seq_read+0x70/0x2ba
[<c0175886>] seq_read+0x0/0x2ba
[<c018d8e6>] proc_reg_read+0x63/0x9f
[<c015e764>] vfs_read+0x7d/0xb5
[<c018d883>] proc_reg_read+0x0/0x9f
[<c015eab1>] sys_read+0x41/0x6a
[<c0103e4e>] sysenter_past_esp+0x5f/0x99
=======================
Code: 00 8b 4d 04 e9 44 ff ff ff 8d 4d 04 89 4c 24 50 8b 6d 00 81 fd ff 0f 00 00 b8 a4 c1 35 c0 0f 46 e8 8b 54 24 2c 89 e9 89 c8 eb 06 <80> 38 00 74 07 40 4a 83 fa ff 75 f4 29 c8 89 c6 8b 44 24 28 89
EIP: [<c01cefa7>] vsnprintf+0x3a4/0x5fc SS:ESP 0068:f31f5e68
Signed-off-by: Alexey Dobriyan <adobriyan@...ru>
---
drivers/char/tty_io.c | 5 ++++-
fs/proc/proc_tty.c | 3 +++
2 files changed, 7 insertions(+), 1 deletion(-)
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -3769,7 +3769,9 @@ int tty_register_driver(struct tty_drive
if (!driver->put_char)
driver->put_char = tty_default_put_char;
+ mutex_lock(&tty_mutex);
list_add(&driver->tty_drivers, &tty_drivers);
+ mutex_unlock(&tty_mutex);
if ( !(driver->flags & TTY_DRIVER_DYNAMIC_DEV) ) {
for(i = 0; i < driver->num; i++)
@@ -3795,8 +3797,9 @@ int tty_unregister_driver(struct tty_dri
unregister_chrdev_region(MKDEV(driver->major, driver->minor_start),
driver->num);
-
+ mutex_lock(&tty_mutex);
list_del(&driver->tty_drivers);
+ mutex_unlock(&tty_mutex);
/*
* Free the termios and termios_locked structures because
--- a/fs/proc/proc_tty.c
+++ b/fs/proc/proc_tty.c
@@ -108,6 +108,8 @@ static void *t_start(struct seq_file *m,
{
struct list_head *p;
loff_t l = *pos;
+
+ mutex_lock(&tty_mutex);
list_for_each(p, &tty_drivers)
if (!l--)
return list_entry(p, struct tty_driver, tty_drivers);
@@ -124,6 +126,7 @@ static void *t_next(struct seq_file *m,
static void t_stop(struct seq_file *m, void *v)
{
+ mutex_unlock(&tty_mutex);
}
static struct seq_operations tty_drivers_op = {
-
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