On 2018-10-01 04:56, Anand Jain wrote:
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>
---
Hi,
There was previous attempt to fix this bug ref:
www.spinics.net/lists/linux-btrfs/msg37466.html
which broke the Ubuntu subvol mount at boot. The reason
for that is, Ubuntu changes the device path in the boot
process, and the earlier fix checked for the device-path
instead of block_device as in here and so we failed the
subvol mount request and thus the bootup process.
I have tested this with Oracle Linux with btrfs as boot device
with a subvol to be mounted at boot. And also have verified
with new test case btrfs/173.
It will be good if someone run this through Ubuntu boot test case.
fs/btrfs/volumes.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f4405e430da6..62173a3abcc4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -850,6 +850,29 @@ static noinline struct btrfs_device *device_list_add(const
char *path,
return ERR_PTR(-EEXIST);
}
+ /*
+ * we are going to replace the device path, make sure its the
+ * same device if the device mounted
+ */
+ if (device->bdev) {
+ struct block_device *path_bdev;
+
+ path_bdev = lookup_bdev(path);
+ if (IS_ERR(path_bdev)) {
+ mutex_unlock(&fs_devices->device_list_mutex);
+ return ERR_CAST(path_bdev);
+ }
+
+ if (device->bdev != path_bdev) {
+ bdput(path_bdev);
+ mutex_unlock(&fs_devices->device_list_mutex);
+ return ERR_PTR(-EEXIST);
It would be _really_ nice to have an informative error message printed
here. Aside from the possibility of an admin accidentally making a
block-level copy of the volume, this code triggering could represent an
attempted attack against the system, so it's arguably something that
should be reported as happening. Personally, I think a WARN_ON_ONCE for
this would make sense, ideally per-volume if possible.
+ }
+ bdput(path_bdev);
+ pr_info("BTRFS: device fsid:devid %pU:%llu old path:%s new
path:%s\n",
+ disk_super->fsid, devid,
rcu_str_deref(device->name), path);
+ }
+
name = rcu_string_strdup(path, GFP_NOFS);
if (!name) {
mutex_unlock(&fs_devices->device_list_mutex);
