Hello,
I'm experiencing unexpected behaviour with the ioctl LOOP_CLR_FD on a
*partition* of a loop device: it disassociates the whole (parent) loop
*device*, rather than (trying to) remove the mapping of the addressed
partition.
== Scenario / Environment / Use
In my scenario, I `partprobe` a loop device for having convenient access
to its partitions (I think it uses ioctl(BLKRRPART), but I'm not totally
sure..).
When now calling ioctl(LOOP_CLR_FD) on the loop *partition*, the mapping
of its parent - the overall loop *device* - is removed.
A `strace` reveals, that umount indeed opens the loop partition, so the
ioctl is called on the right inode (I first assumed it being a bug in
umount calling the parent device).
I would not expect ioctl(LOOP_CLR_FD) being called on a loop partition
would result in disassociation of the overall device holding that partition.
Long story short (userland is busybox which behaves slightly different
than the typical tools from util-linux, e.g. umount *by default* gets
rid of the loop device mapping when being called on a dir mounting a
loop device[1]):
root@LEDE:/# loop=$(losetup -f)
root@LEDE:/# echo ${loop}
/dev/loop2
root@LEDE:/# losetup ${loop} /IMAGE
root@LEDE:/# ls -l ${loop}*
brw------- 1 root root 7, 2 Mar 6 20:09 /dev/loop2
root@LEDE:/# partprobe ${loop}
root@LEDE:/# ls -l ${loop}*
brw------- 1 root root 7, 2 Mar 6 20:09 /dev/loop2
brw------- 1 root root 259, 8 Mar 6 21:59 /dev/loop2p1
brw------- 1 root root 259, 9 Mar 6 21:59 /dev/loop2p2
brw------- 1 root root 259, 10 Mar 6 21:59 /dev/loop2p3
brw------- 1 root root 259, 11 Mar 6 21:59 /dev/loop2p4
brw------- 1 root root 259, 12 Mar 6 21:59 /dev/loop2p5
brw------- 1 root root 259, 13 Mar 6 21:59 /dev/loop2p6
brw------- 1 root root 259, 14 Mar 6 21:59 /dev/loop2p7
brw------- 1 root root 259, 15 Mar 6 21:59 /dev/loop2p8
root@LEDE:/# mount ${loop}p8 /MOUNT # mount loop partition
root@LEDE:/# losetup -a | grep $loop # loop dev mapping still there
/dev/loop2: 0 /mnt/IMAGE
root@LEDE:/# strace umount /MOUNT 2> /log # unmount loop partition
root@LEDE:/# losetup -a | grep ${loop} # loop device mapping is gone
root@LEDE:/# grep -i loop /log
open("/dev/loop2p7", O_RDONLY|O_LARGEFILE) = 3
ioctl(3, LOOP_CLR_FD) = 0
root@LEDE:/#
Is this intended behaviour or a bug?
Cheers
mirko
http://lists.busybox.net/pipermail/busybox/2012-April/077619.html
