Re: [patch] fbcon: don't use vc_resize() on initialization

Andrew Morton
2009-07-30T23:12:27Z

On Wed, 29 Jul 2009 19:21:23 +0200
Johannes Weiner <hannes@cmpxchg.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@arm.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Tested-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Pekka Enberg <penberg@cs.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);