linux-kernel - Re: [patch] fbcon: don't use vc

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Message-Id: <20090730161149.657e8e9a.akpm@linux-foundation.org>
Date:	Thu, 30 Jul 2009 16:11:49 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	catalin.marinas@....com, penberg@...helsinki.fi,
	linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net
Subject: Re: [patch] fbcon: don't use vc_resize() on initialization

On Wed, 29 Jul 2009 19:21:23 +0200
Johannes Weiner <hannes@...xchg.org> wrote:

> Catalin and kmemleak spotted a leak of a VC screen buffer in
> vc_allocate() due to the following chain of events:
> 
> 	vc_allocate()
> 	  visual_init(init=1)
> 	    vc->vc_sw->con_init(init=1)
>               fbcon_init()
> 	        vc_resize()
> 	          vc->screen_buf = kmalloc()
> 	  vc->screen_buf = kmalloc()
> 
> The common way for the VC drivers is to set the screen dimension
> parameters manually in the init case and only call vc_resize() for
> !init - which allocates a screen buffer according to the new
> dimensions.
> 
> fbcon instead would do vc_resize() unconditionally and afterwards set
> the dimensions manually (again) for !init - i.e. completely upside
> down.  The vc_resize() allocated buffer would then get lost by
> vc_allocate() allocating a fresh one.
> 
> Use vc_resize() only for actual resizing to close the leak.
> 
> Set the dimensions manually only in initialization mode to remove the
> redundant setting in resize mode.
> 
> The kmemleak trace from Catalin:
> 
> unreferenced object 0xde158000 (size 12288):

That's a big leak!

What sequence of user actions would cause it to occur?

>   comm "Xorg", pid 1439, jiffies 4294961016
>   hex dump (first 32 bytes):
>     20 00 20 00 20 00 20 00 20 00 20 00 20 00 20 00   . . . . . . . .
>     20 00 20 00 20 00 20 00 20 00 20 00 20 00 20 00   . . . . . . . .
>   backtrace:
>     [<c006f74b>] __save_stack_trace+0x17/0x1c
>     [<c006f81d>] create_object+0xcd/0x188
>     [<c01f5457>] kmemleak_alloc+0x1b/0x3c
>     [<c006e303>] __kmalloc+0xdb/0xe8
>     [<c012cc4b>] vc_do_resize+0x73/0x1e0
>     [<c012cdf1>] vc_resize+0x15/0x18
>     [<c011afc1>] fbcon_init+0x1f9/0x2b8
>     [<c0129e87>] visual_init+0x9f/0xdc
>     [<c012aff3>] vc_allocate+0x7f/0xfc
>     [<c012b087>] con_open+0x17/0x80
>     [<c0120e43>] tty_open+0x1f7/0x2e4
>     [<c0072fa1>] chrdev_open+0x101/0x118
>     [<c006ffad>] __dentry_open+0x105/0x1cc
>     [<c00700fd>] nameidata_to_filp+0x2d/0x38
>     [<c00788cd>] do_filp_open+0x2c1/0x54c
>     [<c006fdff>] do_sys_open+0x3b/0xb4
> 
> Reported-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Tested-by: Catalin Marinas <catalin.marinas@....com>
> Cc: Pekka Enberg <penberg@...helsinki.fi>
> ---
>  drivers/video/console/fbcon.c |    6 +++---
>  1 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 471a9a6..3a44695 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -1082,7 +1082,6 @@ static void fbcon_init(struct vc_data *vc, int init)
>  	new_rows = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
>  	new_cols /= vc->vc_font.width;
>  	new_rows /= vc->vc_font.height;
> -	vc_resize(vc, new_cols, new_rows);
>  
>  	/*
>  	 * We must always set the mode. The mode of the previous console
> @@ -1111,10 +1110,11 @@ static void fbcon_init(struct vc_data *vc, int init)
>  	 *  vc_{cols,rows}, but we must not set those if we are only
>  	 *  resizing the console.
>  	 */
> -	if (!init) {
> +	if (init) {
>  		vc->vc_cols = new_cols;
>  		vc->vc_rows = new_rows;
> -	}
> +	} else
> +		vc_resize(vc, new_cols, new_rows);
>  
>  	if (logo)
>  		fbcon_prepare_logo(vc, info, cols, rows, new_cols, new_rows);

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