Re: [PATCH] Protect tty drivers list a little

[Alexey Dobriyan <adobriyan@sw.ru>]

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@sw.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@sw.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 = {