linux-kernel - Re: [PATCH] tty: fix tty->ldisc leak on ENODEV from driver install

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Date:	Wed, 23 Mar 2011 09:12:51 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Julian Anastasov <ja@....bg>
CC:	Greg Kroah-Hartman <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH] tty: fix tty->ldisc leak on ENODEV from driver install

Ccing Alan.

On 03/23/2011 01:45 AM, Julian Anastasov wrote:
> 	When a USB serial device is not attached I see
> open() to leak 8-byte structures:
> 
> modprobe usbserial
> [ -e /dev/ttyUSB0 ] && cat /dev/ttyUSB0
> cat: /dev/ttyUSB0: No such device
> 
> 	Note that error must be ENODEV with usbserial
> loaded and when name exists, not ENXIO.
> 
> 	kmemleak shows such output:
> 
> unreferenced object 0xc0e19860 (size 8):
>   comm cat, pid 1226, jiffies 4294919464 (age 287.476s)
>   hex dump (first 8 bytes):
>     44 de 2d c1 01 00 00 00                          D.-.....
>   backtrace:
>     [<c1065a74>] create_object+0x109/0x1ad
>     [<c1063d2b>] kmem_cache_alloc+0x60/0x68
>     [<c113505c>] tty_ldisc_get+0x54/0x76
>     [<c11358c9>] tty_ldisc_init+0xa/0x20
>     [<c1130ab4>] initialize_tty_struct+0x2d/0x1ac
>     [<c1130c8c>] tty_init_dev+0x59/0x10d
>     [<c113136d>] tty_open+0x24a/0x3a2
>     [<c1069516>] chrdev_open+0xd1/0xef
>     [<c106d2d5>] nameidata_drop_rcu_last+0x3b/0x49
>     [<c1069445>] chrdev_open+0x0/0xef
>     [<c1065d42>] __dentry_open.clone.15+0xec/0x1c3
>     [<c10669a5>] nameidata_to_filp+0x2a/0x33
>     [<c106f7c7>] finish_open+0x6e/0xfc
>     [<c106fbda>] do_filp_open+0x144/0x4af
>     [<c1076f55>] alloc_fd+0x41/0xa5
>     [<c10669ef>] do_sys_open+0x41/0xc3
> 
> 	Looking at tty_init_dev() it seems initialize_tty_struct()
> attaches tty->ldisc via tty_ldisc_init() but on
> tty_driver_install_tty() failure (-ENODEV) we call free_tty_struct()
> which does nothing with tty->ldisc.
> 
> 	The appended patch fixes the leak but I'm not
> sure what tty->ldisc value we can see in release_one_tty(),
> I assume if ldisc is freed tty->ldisc should be NULL, so
> free_tty_struct() will not try to double-free the ldisc.
> 
> Signed-off-by: Julian Anastasov <ja@....bg>
> ---
>  drivers/tty/tty_io.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> --- linux-2.6.38/drivers/tty/tty_io.c
> +++ linux-2.6.38/drivers/tty/tty_io.c
> @@ -188,6 +188,7 @@ void free_tty_struct(struct tty_struct *
>  		put_device(tty->dev);
>  	kfree(tty->write_buf);
>  	tty_buffer_free_all(tty);
> +	kfree(tty->ldisc);

We should not mess up with ldisc here. We should call something like
tty_ldisc_deinit from tty_init_dev on fail path. The deinit should drop
the reference (call put_ldisc). Or maybe even deinitialize_tty_struct ->
tty_ldisc_deinit -> put_ldisc).

Note that your way you do not drop the module refcount of ld ops.

>  	kfree(tty);
>  }

regards,
-- 
js
suse labs
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