linux-kernel - Re: [PATCH] writeback: fix dereferencing NULL bdi->dev on trace_writeback

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Message-ID: <CAKYAXd_t4+ea=qVHETiuSZn4PExYTCboV0_upnoOTUrvsjuSTA@mail.gmail.com>
Date:	Mon, 6 Feb 2012 11:13:57 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Rabin Vincent <rabin@....in>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue

2012/2/6 Wu Fengguang <fengguang.wu@...el.com>:
> On Thu, Jan 19, 2012 at 01:39:21AM +0530, Rabin Vincent wrote:
>> On Tue, Jan 17, 2012 at 09:02, Wu Fengguang <fengguang.wu@...el.com> wrote:
>> > On Sun, Jan 15, 2012 at 08:58:06PM +0530, Rabin Vincent wrote:
>> >>  Unable to handle kernel NULL pointer dereference at virtual address 0000002c
>> >>  pgd = c0004000
>> >>  [0000002c] *pgd=00000000
>> >>  Internal error: Oops: 17 [#1] PREEMPT SMP
>> >>  PC is at ftrace_raw_event_writeback_single_inode_template+0x60/0xe4
>> >>  LR is at ftrace_raw_event_writeback_single_inode_template+0x50/0xe4
>> >>
>> >> The full trace+log is attached.  My kernel (current linus) has a delay
>> >> inserted in __mark_inode_dirty, to easily trigger the condition:
>> >
>> > Rabin, thanks for showing the helpful details! It should be fixable by
>> > the use of inode_to_bdi():
>>
>> Thanks, this fixes that one.
>>
>> However, I've found one more race condition leading to a crash when
>> tracing is enabled, this time from the writeback:queue trace point from
>> bdi_queue_work().  The cause is the same, i.e.  bdi->dev is NULL.  This
>> was produced with the help of the following delay patch.  trace+log is
>> attached.
>
> Rabin, this should fix the bug. Note that I take no efforts to remove
> the to-be-queued and already-queued works. I'm also a bit afraid if
> the traces in the balance_dirty_pages() path (trace_balance_dirty_pages,
> trace_bdi_dirty_ratelimit and writeback_wake_background) will have
> similar NULL dereference bug. Do you test it by physically hot
> removing a SD card, or with some detach command or emulation?
>
> Thanks,
> Fengguang

Hi. Wu.
I can reproduce this problem too. And I know this problem is fixed
with your patch.
Thanks.
Tested-by: Namjae Jeon <linkinjeon@...il.com>

>
> ---
> Subject: writeback: fix dereferencing NULL bdi->dev on trace_writeback_queue
> Date: Sat Feb 04 20:54:03 CST 2012
>
> When the SD card is hot removed without umount, del_gendisk() will call
> bdi_unregister() but not destroy/free it. This leaves the bdi in the
> bdi->dev = NULL, bdi->wb.task = NULL, bdi->bdi_list removed state.
>
> If someone gets the bdi before bdi_unregister() and calls
> bdi_queue_work() after the unregister, trace_writeback_queue will be
> dereferencing the NULL bdi->dev. Fix it with a simple test for NULL.
>
> LKML-reference: http://lkml.org/lkml/2012/1/18/346
> Reported-by: Rabin Vincent <rabin@....in>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  include/trace/events/writeback.h |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> --- linux-next.orig/include/trace/events/writeback.h    2012-02-04 20:51:01.000000000 +0800
> +++ linux-next/include/trace/events/writeback.h 2012-02-04 20:54:00.000000000 +0800
> @@ -47,7 +47,10 @@ DECLARE_EVENT_CLASS(writeback_work_class
>                __field(int, reason)
>        ),
>        TP_fast_assign(
> -               strncpy(__entry->name, dev_name(bdi->dev), 32);
> +               struct device *dev = bdi->dev;
> +               if (!dev)
> +                       dev = default_backing_dev_info.dev;
> +               strncpy(__entry->name, dev_name(dev), 32);
>                __entry->nr_pages = work->nr_pages;
>                __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
>                __entry->sync_mode = work->sync_mode;
> --
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