[PATCH] bcache: not access dc->bdev when backing device is offline

[Coly Li <colyli@xxxxxxx>]

When backing device is offline, memory object pointed by dc->bdev might be
released in some condititions. I have bug report that bdevname() triggers
a NULL pointer deference panic inside bch_cached_dev_error(), where
dc->bdev is NULL.

This patch adds char backing_dev_name[BDEVNAME_SIZE] in struct cached_dev,
and stored backing device name in backing_dev_nmae during backing device
registration. Then when backing device is offline, pr_err() can still
print message with backing device name without calling bdevname(). And
backing_dev_name is also helpful when debugging crash code.

Fixes: c7b7bd07404c ("bcache: add io_disable to struct cached_dev")
Signed-off-by: Coly Li <colyli@xxxxxxx>
---
 drivers/md/bcache/bcache.h |  2 ++
 drivers/md/bcache/super.c  | 10 ++++------
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index d338b7086013..9af0ad7f6243 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -392,6 +392,8 @@ struct cached_dev {
 #define DEFAULT_CACHED_DEV_ERROR_LIMIT	64
 	atomic_t		io_errors;
 	unsigned		error_limit;
+
+	char			backing_dev_name[BDEVNAME_SIZE];
 };
 
 enum alloc_reserve {
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index d90d9e59ca00..f2512e3e67e6 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1225,13 +1225,13 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 				 struct block_device *bdev,
 				 struct cached_dev *dc)
 {
-	char name[BDEVNAME_SIZE];
 	const char *err = "cannot allocate memory";
 	struct cache_set *c;
 
 	memcpy(&dc->sb, sb, sizeof(struct cache_sb));
 	dc->bdev = bdev;
 	dc->bdev->bd_holder = dc;
+	bdevname(bdev, dc->backing_dev_name);
 
 	bio_init(&dc->sb_bio, dc->sb_bio.bi_inline_vecs, 1);
 	bio_first_bvec_all(&dc->sb_bio)->bv_page = sb_page;
@@ -1247,7 +1247,7 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 	if (bch_cache_accounting_add_kobjs(&dc->accounting, &dc->disk.kobj))
 		goto err;
 
-	pr_info("registered backing device %s", bdevname(bdev, name));
+	pr_info("registered backing device %s", dc->backing_dev_name);
 
 	list_add(&dc->list, &uncached_devices);
 	list_for_each_entry(c, &bch_cache_sets, list)
@@ -1259,7 +1259,7 @@ static void register_bdev(struct cache_sb *sb, struct page *sb_page,
 
 	return;
 err:
-	pr_notice("error %s: %s", bdevname(bdev, name), err);
+	pr_notice("error %s: %s", dc->backing_dev_name, err);
 	bcache_device_stop(&dc->disk);
 }
 
@@ -1367,8 +1367,6 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)
 
 bool bch_cached_dev_error(struct cached_dev *dc)
 {
-	char name[BDEVNAME_SIZE];
-
 	if (!dc || test_bit(BCACHE_DEV_CLOSING, &dc->disk.flags))
 		return false;
 
@@ -1377,7 +1375,7 @@ bool bch_cached_dev_error(struct cached_dev *dc)
 	smp_mb();
 
 	pr_err("stop %s: too many IO errors on backing device %s\n",
-		dc->disk.disk->disk_name, bdevname(dc->bdev, name));
+		dc->disk.disk->disk_name, dc->backing_dev_name);
 
 	bcache_device_stop(&dc->disk);
 	return true;
-- 
2.16.2