[PATCH v2 5/7] bfq: fix potential kernel crash when print dev err info

[Yufen Yu <yuyufen@huawei.com>]

We use bdi_get_dev_name() to get device name, avoiding
use-after-free or NULL pointer reference for ->dev.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
---
 block/bfq-iosched.c         | 7 +++++--
 include/linux/backing-dev.h | 2 ++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 00904611b8e4..8d41783d8e77 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -123,6 +123,7 @@
 #include <linux/ioprio.h>
 #include <linux/sbitmap.h>
 #include <linux/delay.h>
+#include <linux/backing-dev.h>
 
 #include "blk.h"
 #include "blk-mq.h"
@@ -4971,6 +4972,7 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 	struct task_struct *tsk = current;
 	int ioprio_class;
 	struct bfq_data *bfqd = bfqq->bfqd;
+	char dname[BDI_DEV_NAME_LEN];
 
 	if (!bfqd)
 		return;
@@ -4978,8 +4980,9 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 	ioprio_class = IOPRIO_PRIO_CLASS(bic->ioprio);
 	switch (ioprio_class) {
 	default:
-		dev_err(&bfqq->bfqd->queue->backing_dev_info->rcu_dev->dev,
-			"bfq: bad prio class %d\n", ioprio_class);
+		bdi_get_dev_name(bfqq->bfqd->queue->backing_dev_info,
+				dname, BDI_DEV_NAME_LEN);
+		pr_err("bdi %s: bfq: bad prio class %d\n", dname, ioprio_class);
 		/* fall through */
 	case IOPRIO_CLASS_NONE:
 		/*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index 89d1cb7923f5..291db069f7da 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -19,6 +19,8 @@
 #include <linux/backing-dev-defs.h>
 #include <linux/slab.h>
 
+#define BDI_DEV_NAME_LEN       32
+
 static inline struct backing_dev_info *bdi_get(struct backing_dev_info *bdi)
 {
 	kref_get(&bdi->refcnt);
-- 
2.16.2.dirty