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loop: Fix mount(2) failure due to race with LOOP_SET_FD

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Commit 33ec3e53e7 ("loop: Don't change loop device under exclusive
opener") made LOOP_SET_FD ioctl acquire exclusive block device reference
while it updates loop device binding. However this can make perfectly
valid mount(2) fail with EBUSY due to racing LOOP_SET_FD holding
temporarily the exclusive bdev reference in cases like this:

for i in {a..z}{a..z}; do
        dd if=/dev/zero of=$i.image bs=1k count=0 seek=1024
        mkfs.ext2 $i.image
        mkdir mnt$i
done

echo "Run"
for i in {a..z}{a..z}; do
        mount -o loop -t ext2 $i.image mnt$i &
done

Fix the problem by not getting full exclusive bdev reference in
LOOP_SET_FD but instead just mark the bdev as being claimed while we
update the binding information. This just blocks new exclusive openers
instead of failing them with EBUSY thus fixing the problem.

Fixes: 33ec3e53e7 ("loop: Don't change loop device under exclusive opener")
Cc: stable@vger.kernel.org
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jan Kara
2019-07-30 13:10:14 +02:00
committed by Jens Axboe
parent 71d6c505b4
commit 89e524c04f
3 changed files with 73 additions and 32 deletions
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6
include/linux/fs.h
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@@ -2598,6 +2598,12 @@ extern struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
void *holder);
extern struct block_device *blkdev_get_by_dev(dev_t dev, fmode_t mode,
void *holder);
extern struct block_device *bd_start_claiming(struct block_device *bdev,
void *holder);
extern void bd_finish_claiming(struct block_device *bdev,
struct block_device *whole, void *holder);
extern void bd_abort_claiming(struct block_device *bdev,
struct block_device *whole, void *holder);
extern void blkdev_put(struct block_device *bdev, fmode_t mode);
extern int __blkdev_reread_part(struct block_device *bdev);
extern int blkdev_reread_part(struct block_device *bdev);