APPLIED/cmt: [B][PATCH] bcache: only set BCACHE_DEV_WB_RUNNING when cached device attached

[Khaled Elmously]

Marking thread as applied - this patch was already present in bionic as part of the August 2 bionic stable-update: 

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1838824


On 2019-07-29 16:17:24 , Mauricio Faria de Oliveira wrote:
> From: Coly Li <colyli at suse.de>
> 
> BugLink: https://bugs.launchpad.net/bugs/1837788
> 
> When people set a writeback percent via sysfs file,
>   /sys/block/bcache<N>/bcache/writeback_percent
> current code directly sets BCACHE_DEV_WB_RUNNING to dc->disk.flags
> and schedules kworker dc->writeback_rate_update.
> 
> If there is no cache set attached to, the writeback kernel thread is
> not running indeed, running dc->writeback_rate_update does not make
> sense and may cause NULL pointer deference when reference cache set
> pointer inside update_writeback_rate().
> 
> This patch checks whether the cache set point (dc->disk.c) is NULL in
> sysfs interface handler, and only set BCACHE_DEV_WB_RUNNING and
> schedule dc->writeback_rate_update when dc->disk.c is not NULL (it
> means the cache device is attached to a cache set).
> 
> This problem might be introduced from initial bcache commit, but
> commit 3fd47bfe55b0 ("bcache: stop dc->writeback_rate_update properly")
> changes part of the original code piece, so I add 'Fixes: 3fd47bfe55b0'
> to indicate from which commit this patch can be applied.
> 
> Fixes: 3fd47bfe55b0 ("bcache: stop dc->writeback_rate_update properly")
> Reported-by: Bjørn Forsman <bjorn.forsman at gmail.com>
> Signed-off-by: Coly Li <colyli at suse.de>
> Reviewed-by: Bjørn Forsman <bjorn.forsman at gmail.com>
> Cc: stable at vger.kernel.org
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> (backported from commit 1f0ffa67349c56ea54c03ccfd1e073c990e7411e)
> [mfo: backport: fix fuzz; applies with git cherry-pick, not git am]
> Signed-off-by: Mauricio Faria de Oliveira <mfo at canonical.com>
> ---
>  drivers/md/bcache/sysfs.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 17df47d085f1..6acbd38fa946 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -333,8 +333,13 @@ STORE(bch_cached_dev)
>  	if (attr == &sysfs_writeback_running)
>  		bch_writeback_queue(dc);
>  
> +	/*
> +	 * Only set BCACHE_DEV_WB_RUNNING when cached device attached to
> +	 * a cache set, otherwise it doesn't make sense.
> +	 */
>  	if (attr == &sysfs_writeback_percent)
> -		if (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags))
> +		if ((dc->disk.c != NULL) &&
> +		    (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)))
>  			schedule_delayed_work(&dc->writeback_rate_update,
>  				      dc->writeback_rate_update_seconds * HZ);
>  
> -- 
> 2.17.1
> 
> 
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