Re: [PATCH v2 6/7] loopfs: start attaching correct namespace during loop_add()

["Serge E. Hallyn" <serge@hallyn.com>]

On Wed, Apr 22, 2020 at 04:54:36PM +0200, Christian Brauner wrote:
> Tag loop devices with the namespace the loopfs instance was mounted in.
> This has the consequence that loopfs devices carry the correct sysfs
> permissions for all their core files. All other devices files will
> continue to be correctly owned by the initial namespaces. Here is sample
> output:
> 
> root@b1:~# mount -t loop loop /mnt
> root@b1:~# ln -sf /mnt/loop-control /dev/loop-control
> root@b1:~# losetup -f
> /dev/loop8
> root@b1:~# ln -sf /mnt/loop8 /dev/loop8
> root@b1:~# ls -al /sys/class/block/loop8
> lrwxrwxrwx 1 root root 0 Apr  7 13:06 /sys/class/block/loop8 -> ../../devices/virtual/block/loop8
> root@b1:~# ls -al /sys/class/block/loop8/
> total 0
> drwxr-xr-x  9 root   root       0 Apr  7 13:06 .
> drwxr-xr-x 18 nobody nogroup    0 Apr  7 13:07 ..
> -r--r--r--  1 root   root    4096 Apr  7 13:06 alignment_offset
> lrwxrwxrwx  1 nobody nogroup    0 Apr  7 13:07 bdi -> ../../bdi/7:8
> -r--r--r--  1 root   root    4096 Apr  7 13:06 capability
> -r--r--r--  1 root   root    4096 Apr  7 13:06 dev
> -r--r--r--  1 root   root    4096 Apr  7 13:06 discard_alignment
> -r--r--r--  1 root   root    4096 Apr  7 13:06 events
> -r--r--r--  1 root   root    4096 Apr  7 13:06 events_async
> -rw-r--r--  1 root   root    4096 Apr  7 13:06 events_poll_msecs
> -r--r--r--  1 root   root    4096 Apr  7 13:06 ext_range
> -r--r--r--  1 root   root    4096 Apr  7 13:06 hidden
> drwxr-xr-x  2 nobody nogroup    0 Apr  7 13:07 holders
> -r--r--r--  1 root   root    4096 Apr  7 13:06 inflight
> drwxr-xr-x  2 nobody nogroup    0 Apr  7 13:07 integrity
> drwxr-xr-x  3 nobody nogroup    0 Apr  7 13:07 mq
> drwxr-xr-x  2 root   root       0 Apr  7 13:06 power
> drwxr-xr-x  3 nobody nogroup    0 Apr  7 13:07 queue
> -r--r--r--  1 root   root    4096 Apr  7 13:06 range
> -r--r--r--  1 root   root    4096 Apr  7 13:06 removable
> -r--r--r--  1 root   root    4096 Apr  7 13:06 ro
> -r--r--r--  1 root   root    4096 Apr  7 13:06 size
> drwxr-xr-x  2 nobody nogroup    0 Apr  7 13:07 slaves
> -r--r--r--  1 root   root    4096 Apr  7 13:06 stat
> lrwxrwxrwx  1 nobody nogroup    0 Apr  7 13:07 subsystem -> ../../../../class/block
> drwxr-xr-x  2 root   root       0 Apr  7 13:06 trace
> -rw-r--r--  1 root   root    4096 Apr  7 13:06 uevent
> root@b1:~#
> 
> Cc: Jens Axboe <axboe@kernel.dk>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

I was a *bit* worried about not taking a reference to the
user namespace, but it doesn't look like the chain of
loop_remove() -> del_gendisk() -> device_del() will allow any later
access through sysfs, so I guess it's fine.

Reviewed-by: Serge Hallyn <serge@hallyn.com>

> ---
> /* v2 */
> unchanged
> - Christian Brauner <christian.brauner@ubuntu.com>:
>   - Adapted commit message otherwise unchanged.
> ---
>  drivers/block/loop.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 8e21d4b33e01..2dc53bad4b48 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -2212,6 +2212,10 @@ static int loop_add(struct loop_device **l, int i, struct inode *inode)
>  	disk->private_data	= lo;
>  	disk->queue		= lo->lo_queue;
>  	sprintf(disk->disk_name, "loop%d", i);
> +#ifdef CONFIG_BLK_DEV_LOOPFS
> +	if (loopfs_i_sb(inode))
> +		disk->user_ns = loopfs_i_sb(inode)->s_user_ns;
> +#endif
>  
>  	add_disk(disk);
>  
> -- 
> 2.26.1