linux-kernel - Re: [PATCH 1/5] vt: Fix replacement console check when unbinding

Open Source and information security mailing list archives

Date:	Fri, 6 Jun 2014 09:16:13 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	Jiri Slaby <jslaby@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/5] vt: Fix replacement console check when unbinding

Hi

On Thu, Jun 5, 2014 at 4:58 PM, Daniel Vetter <daniel.vetter@...ll.ch> wrote:
> I don't fully understand the magic of the vt register/unregister
> logic, but apparently everything but the inital console (as set
> in the conswitchp pointer) is marked with FLAG_MODULE. Which means
> if something unregistered the boot vt driver (e.g. i915.ko kicking
> out vga_con) there's nothing left when trying to unbind e.g. fbcon
> through sysfs.
>
> But we always have the dummy console hanging around, so this test
> is fairly dubious. What we actually want is simply a different console
> than the one we want to unbind.

For unknown reasons, you can disable the dummy console via Kconfig, so
it's not guaranteed to be around. But your comment is still valid.

>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.cz>
> Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
>  drivers/tty/vt/vt.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 3ad0b61e35b4..ea600f482eeb 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3155,8 +3155,7 @@ int do_unbind_con_driver(const struct consw *csw, int first, int last, int deflt
>         for (i = 0; i < MAX_NR_CON_DRIVER; i++) {
>                 con_back = &registered_con_driver[i];
>
> -               if (con_back->con &&
> -                   !(con_back->flag & CON_DRIVER_FLAG_MODULE)) {
> +               if (con_back->con && con_back->con != csw) {

Funny thing is, if you run do_bind_con_driver() on the range first,
you kick out the existing driver and can then unload it regardless
whether the fallback was FLAG_MODULE or not. Therefore, I think that
change is safe. This is:

Reviewed-by: David Herrmann <dh.herrmann@...il.com>

Thanks
David

>                         defcsw = con_back->con;
>                         retval = 0;
>                         break;
> --
> 1.8.1.4
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/dri-devel
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