On Mon, Oct 15, 2018 at 10:45:17AM +0800, Anand Jain wrote: > (Thanks for the comments on requiring to warn_on if we fail the device > change.) > (This fixes an ugly bug, I appreciate if you have any further comments). > > Its not that impossible to imagine that a device OR a btrfs image is > been copied just by using the dd or the cp command. Which in case both > the copies of the btrfs will have the same fsid. If on the system with > automount enabled, the copied FS gets scanned. > > We have a known bug in btrfs, that we let the device path be changed > after the device has been mounted. So using this loop hole the new > copied device would appears as if its mounted immediately after its > been copied. > > For example: > > Initially.. /dev/mmcblk0p4 is mounted as / > > lsblk > NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT > mmcblk0 179:0 0 29.2G 0 disk > |-mmcblk0p4 179:4 0 4G 0 part / > |-mmcblk0p2 179:2 0 500M 0 part /boot > |-mmcblk0p3 179:3 0 256M 0 part [SWAP] > `-mmcblk0p1 179:1 0 256M 0 part /boot/efi > > btrfs fi show > Label: none uuid: 07892354-ddaa-4443-90ea-f76a06accaba > Total devices 1 FS bytes used 1.40GiB > devid 1 size 4.00GiB used 3.00GiB path /dev/mmcblk0p4 > > Copy mmcblk0 to sda > dd if=/dev/mmcblk0 of=/dev/sda > > And immediately after the copy completes the change in the device > superblock is notified which the automount scans using > btrfs device scan and the new device sda becomes the mounted root > device. > > lsblk > NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT > sda 8:0 1 14.9G 0 disk > |-sda4 8:4 1 4G 0 part / > |-sda2 8:2 1 500M 0 part > |-sda3 8:3 1 256M 0 part > `-sda1 8:1 1 256M 0 part > mmcblk0 179:0 0 29.2G 0 disk > |-mmcblk0p4 179:4 0 4G 0 part > |-mmcblk0p2 179:2 0 500M 0 part /boot > |-mmcblk0p3 179:3 0 256M 0 part [SWAP] > `-mmcblk0p1 179:1 0 256M 0 part /boot/efi > > btrfs fi show / > Label: none uuid: 07892354-ddaa-4443-90ea-f76a06accaba > Total devices 1 FS bytes used 1.40GiB > devid 1 size 4.00GiB used 3.00GiB path /dev/sda4 > > The bug is quite nasty that you can't either unmount /dev/sda4 or > /dev/mmcblk0p4. And the problem does not get solved until you take > sda out of the system on to another system to change its fsid > using the 'btrfstune -u' command. > > Signed-off-by: Anand Jain <anand.j...@oracle.com>
I'm adding the patch to misc-next now, with an update message that matches the format when a device is scanned. "BTRFS: device fsid %pU devid %llu moved old:%s new:%s\n", That way it should be possible to grep for all messages that are related to the scanning ioctl.
