linux-kernel - Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop detach and loop open

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Date: Tue, 21 May 2024 19:39:35 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Gulam Mohamed <gulam.mohamed@...cle.com>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: shinichiro.kawasaki@....com, chaitanyak@...dia.com, hch@....de
Subject: Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop
 detach and loop open

On 5/21/24 4:42 PM, Gulam Mohamed wrote:
> Description
> ===========
> 
> 1. Userspace sends the command "losetup -d" which uses the open() call
>    to open the device
> 2. Kernel receives the ioctl command "LOOP_CLR_FD" which calls the
>    function loop_clr_fd()
> 3. If LOOP_CLR_FD is the first command received at the time, then the
>    AUTOCLEAR flag is not set and deletion of the
>    loop device proceeds ahead and scans the partitions (drop/add
>    partitions)
> 
> 	if (disk_openers(lo->lo_disk) > 1) {
> 		lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
> 		loop_global_unlock(lo, true);
> 		return 0;
> 	}
> 
>  4. Before scanning partitions, it will check to see if any partition of
>     the loop device is currently opened
>  5. If any partition is opened, then it will return EBUSY:
> 
>     if (disk->open_partitions)
> 		return -EBUSY;
>  6. So, after receiving the "LOOP_CLR_FD" command and just before the above
>     check for open_partitions, if any other command
>     (like blkid) opens any partition of the loop device, then the partition
>     scan will not proceed and EBUSY is returned as shown in above code
>  7. But in "__loop_clr_fd()", this EBUSY error is not propagated
>  8. We have noticed that this is causing the partitions of the loop to
>     remain stale even after the loop device is detached resulting in the
>     IO errors on the partitions
> 
> Fix
> ---
> Re-introduce the lo_open() call to restrict any process to open the loop
> device when its being detached
> 
> Test case
> =========
> Test case involves the following two scripts:
> 
> script1.sh
> ----------
> while [ 1 ];
> do
> 	losetup -P -f /home/opt/looptest/test10.img
> 	blkid /dev/loop0p1
> done
> 
> script2.sh
> ----------
> while [ 1 ];
> do
> 	losetup -d /dev/loop0
> done
> 
> Without fix, the following IO errors have been observed:
> 
> kernel: __loop_clr_fd: partition scan of loop0 failed (rc=-16)
> kernel: I/O error, dev loop0, sector 20971392 op 0x0:(READ) flags 0x80700
>         phys_seg 1 prio class 0
> kernel: I/O error, dev loop0, sector 108868 op 0x0:(READ) flags 0x0
>         phys_seg 1 prio class 0
> kernel: Buffer I/O error on dev loop0p1, logical block 27201, async page
>         read
> 
> V1->V2:
> 	Added a test case, 010, in blktests in tests/loop/
> Signed-off-by: Gulam Mohamed <gulam.mohamed@...cle.com>
> ---
>  drivers/block/loop.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 28a95fd366fe..9a235d8c062d 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1717,6 +1717,24 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
>  }
>  #endif
>  
> +static int lo_open(struct gendisk *disk, blk_mode_t mode)
> +{
> +        struct loop_device *lo = disk->private_data;
> +        int err;
> +
> +        if (!lo)
> +                return -ENXIO;
> +
> +        err = mutex_lock_killable(&lo->lo_mutex);
> +        if (err)
> +                return err;
> +
> +        if (lo->lo_state == Lo_rundown)
> +                err = -ENXIO;
> +        mutex_unlock(&lo->lo_mutex);
> +	return err;
> +}

Most of this function uses spaces rather than tabs.

-- 
Jens Axboe