linux-kernel - Re: Linux 2.6.38-rc6

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Message-ID: <AANLkTin7+AHAqKz5T-1_=N6tqPytyT4ooqyJuhHC6ET=@mail.gmail.com>
Date:	Thu, 24 Feb 2011 16:54:44 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anca Emanuel <anca.emanuel@...il.com>
Cc:	Dave Airlie <airlied@...hat.com>, linux-fbdev@...r.kernel.org,
	Ben Skeggs <bskeggs@...hat.com>,
	dri-devel@...ts.freedesktop.org, Borislav Petkov <bp@...64.org>,
	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.38-rc6

On Thu, Feb 24, 2011 at 4:48 PM, Anca Emanuel <anca.emanuel@...il.com> wrote:
>
> diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
> index e2bf953..e8f8925 100644
> --- a/drivers/video/fbmem.c
> +++ b/drivers/video/fbmem.c
> @@ -1511,6 +1511,7 @@ void remove_conflicting_framebuffers(struct
> apertures_struct *a,
>                               "%s vs %s - removing generic driver\n",
>                               name, registered_fb[i]->fix.id);
>                        unregister_framebuffer(registered_fb[i]);
> +                       registered_fb[i] = NULL;
>
> Tested the patch, and now I get this:
> dmesg: http://pastebin.com/ieMNrA7C
>
> [   12.252328] BUG: unable to handle kernel NULL pointer dereference
> at 00000000000003b8
> [   12.252342] IP: [<ffffffff81311178>] fb_mmap+0x58/0x1d0

Ok, goodie.

Or not so goodie, but it does make it clear that yeah, the fb code
seems to be using stale pointers from that registered_fb[] array, and
the whole unregistration process is just racing with people using it.

Herton had that much bigger patch, can you test it?

                         Linus
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