Re: general protection fault in wb_workfn (2)

[Jan Kara]

On Thu 31-05-18 22:19:44, Tetsuo Handa wrote:
> On 2018/05/31 20:42, Jan Kara wrote:
> > On Thu 31-05-18 01:00:08, Tetsuo Handa wrote:
> >> So, we have no idea what is happening...
> >> Then, what about starting from temporary debug printk() patch shown below?
> >>
> >> >From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001
> >> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> >> Date: Wed, 30 May 2018 09:57:10 +0900
> >> Subject: [PATCH] bdi: Add temporary config for debugging wb_workfn() versus
> >>  bdi_unregister() race bug.
> >>
> >> syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to
> >> limitations that syzbot cannot find reproducer for this bug (frequency is
> >> once or twice per a day) nor we can't capture vmcore in the environment
> >> which syzbot is using, for now we need to rely on printk() debugging.
> >>
> >> [1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206
> >>
> >> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > 
> > Hum a bit ugly solution but if others are fine with this, I can live with
> > it for a while as well. Or would it be possible for syzkaller to just test
> > some git tree where this patch is included? Then we would not even have to
> > have the extra config option...
> 
> If syzbot can reproduce this bug that way. While it is possible to add/remove
> git trees syzbot tests, frequently adding/removing trees is bothering.
> 
> syzbot can enable extra config option. Maybe the config name should be
> something like CONFIG_DEBUG_FOR_SYZBOT rather than individual topic.
> 
> I think that syzbot is using many VM instances. I don't know how many
> instances will be needed for reproducing this bug within reasonable period.
> More git trees syzbot tests, (I assume that) longer period will be needed
> for reproducing this bug. The most reliable way is to use the shared part
> of all trees (i.e. linux.git).

I understand this, I'd be just a bit reluctant to merge temporary debug
patches like this to Linus' tree only to revert them later just because
syzkaller... What do others think?

> From 3f3346d42b804e59d12caaa525365a8482505f08 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Thu, 31 May 2018 22:07:20 +0900
> Subject: [PATCH v2] bdi: Add temporary config for debugging wb_workfn() versus
>  bdi_unregister() race bug.
> 
> syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to
> limitations that syzbot cannot find reproducer for this bug (frequency is
> once or twice per a day) nor we can't capture vmcore in the environment
> which syzbot is using, for now we need to rely on printk() debugging.
> 
> [1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

The debug patch looks good to me now. Thanks for writing it.

								Honza

> ---
>  block/Kconfig     |  7 +++++++
>  fs/fs-writeback.c | 28 ++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/block/Kconfig b/block/Kconfig
> index 28ec557..fbce13e 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -139,6 +139,13 @@ config BLK_CMDLINE_PARSER
>  
>  	See Documentation/block/cmdline-partition.txt for more information.
>  
> +config BLK_DEBUG_WB_WORKFN_RACE
> +	bool "Dump upon hitting wb_workfn() versus bdi_unregister() race bug."
> +	default n
> +	---help---
> +	This is a temporary option used for obtaining information for
> +	specific bug. This option will be removed after the bug is fixed.
> +
>  config BLK_WBT
>  	bool "Enable support for block device writeback throttling"
>  	default n
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 471d863..14ab873 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1934,6 +1934,34 @@ void wb_workfn(struct work_struct *work)
>  						struct bdi_writeback, dwork);
>  	long pages_written;
>  
> +#ifdef CONFIG_BLK_DEBUG_WB_WORKFN_RACE
> +	if (!wb->bdi->dev) {
> +		pr_warn("WARNING: %s: device is NULL\n", __func__);
> +		pr_warn("wb->state=%lx\n", wb->state);
> +		pr_warn("(wb == &wb->bdi->wb)=%u\n", wb == &wb->bdi->wb);
> +		pr_warn("list_empty(&wb->work_list)=%u\n",
> +			list_empty(&wb->work_list));
> +		pr_warn("list_empty(&wb->bdi->bdi_list)=%u\n",
> +			list_empty(&wb->bdi->bdi_list));
> +		pr_warn("wb->bdi->wb.state=%lx\n", wb->bdi->wb.state);
> +		if (!wb->congested)
> +			pr_warn("wb->congested == NULL\n");
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +		else if (!wb->congested->__bdi)
> +			pr_warn("wb->congested->__bdi == NULL\n");
> +		else {
> +			pr_warn("(wb->congested->__bdi == wb->bdi)=%u\n",
> +				wb->congested->__bdi == wb->bdi);
> +			pr_warn("list_empty(&wb->congested->__bdi->bdi_list)=%u\n",
> +				list_empty(&wb->congested->__bdi->bdi_list));
> +			pr_warn("wb->congested->__bdi->wb.state=%lx\n",
> +				wb->congested->__bdi->wb.state);
> +		}
> +#endif
> +		/* Will halt shortly due to NULL pointer dereference... */
> +	}
> +#endif
> +
>  	set_worker_desc("flush-%s", dev_name(wb->bdi->dev));
>  	current->flags |= PF_SWAPWRITE;
>  
> -- 
> 1.8.3.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR