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

[Mauricio Faria de Oliveira]

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