Hi,

On Friday 27 May 2016 12:59:35 Vegard Nossum wrote:
> Quentin ran into this bug:
> 
> WARNING: CPU: 64 PID: 10085 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x65/0x80
> sysfs: cannot create duplicate filename '/devices/virtual/block/nbd3/pid'
> Modules linked in: nbd
> CPU: 64 PID: 10085 Comm: qemu-nbd Tainted: G      D         4.6.0+ #7
>  0000000000000000 ffff8820330bba68 ffffffff814b8791 ffff8820330bbac8
>  0000000000000000 ffff8820330bbab8 ffffffff810d04ab ffff8820330bbaa8
>  0000001f00000296 0000000000017681 ffff8810380bf000 ffffffffa0001790
> Call Trace:
>  [<ffffffff814b8791>] dump_stack+0x4d/0x6c
>  [<ffffffff810d04ab>] __warn+0xdb/0x100
>  [<ffffffff810d0574>] warn_slowpath_fmt+0x44/0x50
>  [<ffffffff81218c65>] sysfs_warn_dup+0x65/0x80
>  [<ffffffff81218a02>] sysfs_add_file_mode_ns+0x172/0x180
>  [<ffffffff81218a35>] sysfs_create_file_ns+0x25/0x30
>  [<ffffffff81594a76>] device_create_file+0x36/0x90
>  [<ffffffffa0000e8d>] __nbd_ioctl+0x32d/0x9b0 [nbd]
>  [<ffffffff814cc8e8>] ? find_next_bit+0x18/0x20
>  [<ffffffff810f7c29>] ? select_idle_sibling+0xe9/0x120
>  [<ffffffff810f6cd7>] ? __enqueue_entity+0x67/0x70
>  [<ffffffff810f9bf0>] ? enqueue_task_fair+0x630/0xe20
>  [<ffffffff810efa76>] ? resched_curr+0x36/0x70
>  [<ffffffff810f0078>] ? check_preempt_curr+0x78/0x90
>  [<ffffffff810f00a2>] ? ttwu_do_wakeup+0x12/0x80
>  [<ffffffff810f01b1>] ? ttwu_do_activate.constprop.86+0x61/0x70
>  [<ffffffff810f0c15>] ? try_to_wake_up+0x185/0x2d0
>  [<ffffffff810f0d6d>] ? default_wake_function+0xd/0x10
>  [<ffffffff81105471>] ? autoremove_wake_function+0x11/0x40
>  [<ffffffffa0001577>] nbd_ioctl+0x67/0x94 [nbd]
>  [<ffffffff814ac0fd>] blkdev_ioctl+0x14d/0x940
>  [<ffffffff811b0da2>] ? put_pipe_info+0x22/0x60
>  [<ffffffff811d96cc>] block_ioctl+0x3c/0x40
>  [<ffffffff811ba08d>] do_vfs_ioctl+0x8d/0x5e0
>  [<ffffffff811aa329>] ? ____fput+0x9/0x10
>  [<ffffffff810e9092>] ? task_work_run+0x72/0x90
>  [<ffffffff811ba627>] SyS_ioctl+0x47/0x80
>  [<ffffffff8185f5df>] entry_SYSCALL_64_fastpath+0x17/0x93
> ---[ end trace 7899b295e4f850c8 ]---
> 
> It seems fairly obvious that device_create_file() is not being protected
> from being run concurrently on the same nbd.
> 
> Quentin found the following relevant commits:
> 
> 1a2ad21 nbd: add locking to nbd_ioctl
> 90b8f28 [PATCH] end of methods switch: remove the old ones
> d4430d6 [PATCH] beginning of methods conversion
> 08f8585 [PATCH] move block_device_operations to blkdev.h
> 
> It would seem that the race was introduced in the process of moving nbd
> from BKL to unlocked ioctls.
> 
> By setting nbd->task_recv while the mutex is held, we can prevent other
> processes from running concurrently (since nbd->task_recv is also checked
> while the mutex is held).
> 
> Reported-and-tested-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
> Cc: Markus Pargmann <mpa@pengutronix.de>
> Cc: Paul Clements <paul.clements@steeleye.com>
> Cc: Pavel Machek <pavel@suse.cz>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>

Thanks, applied.

Best Regards,

Markus

> ---
>  drivers/block/nbd.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 31e73a7..a831f2b 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -451,14 +451,9 @@ static int nbd_thread_recv(struct nbd_device *nbd, struct block_device *bdev)
>  
>  	sk_set_memalloc(nbd->sock->sk);
>  
> -	nbd->task_recv = current;
> -
>  	ret = device_create_file(disk_to_dev(nbd->disk), &pid_attr);
>  	if (ret) {
>  		dev_err(disk_to_dev(nbd->disk), "device_create_file failed!\n");
> -
> -		nbd->task_recv = NULL;
> -
>  		return ret;
>  	}
>  
> @@ -477,9 +472,6 @@ static int nbd_thread_recv(struct nbd_device *nbd, struct block_device *bdev)
>  	nbd_size_clear(nbd, bdev);
>  
>  	device_remove_file(disk_to_dev(nbd->disk), &pid_attr);
> -
> -	nbd->task_recv = NULL;
> -
>  	return ret;
>  }
>  
> @@ -788,6 +780,8 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		if (!nbd->sock)
>  			return -EINVAL;
>  
> +		/* We have to claim the device under the lock */
> +		nbd->task_recv = current;
>  		mutex_unlock(&nbd->tx_lock);
>  
>  		nbd_parse_flags(nbd, bdev);
> @@ -796,6 +790,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  				     nbd_name(nbd));
>  		if (IS_ERR(thread)) {
>  			mutex_lock(&nbd->tx_lock);
> +			nbd->task_recv = NULL;
>  			return PTR_ERR(thread);
>  		}
>  
> @@ -805,6 +800,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		kthread_stop(thread);
>  
>  		mutex_lock(&nbd->tx_lock);
> +		nbd->task_recv = NULL;
>  
>  		sock_shutdown(nbd);
>  		nbd_clear_que(nbd);
> 

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