Re: [PATCH] btrfs-progs: do not allow setting seed flag on fs with dirty log

[Qu Wenruo <wqu@suse.com>]

On 2022/4/18 23:30, Josef Bacik wrote:
> On Fri, Apr 15, 2022 at 07:37:43PM +0800, Qu Wenruo wrote:
>> [BUG]
>> The following sequence operation can lead to a seed fs rejected by
>> kernel:
>>
>>   # Generate a fs with dirty log
>>   mkfs.btrfs -f $file
>>   mount $dev $mnt
>>   xfs_io -f -c "pwrite 0 16k" -c fsync $mnt/file
>>   cp $file $file.backup
>>   umount $mnt
>>   mv $file.backup $file
>>
>>   # now $file has dirty log, set seed flag on it
>>   btrfstune -S1 $file
>>
>>   # mount will fail
>>   mount $file $mnt
>>
>> The mount failure with the following dmesg:
>>
>> [  980.363667] loop0: detected capacity change from 0 to 262144
>> [  980.371177] BTRFS info (device loop0): flagging fs with big metadata feature
>> [  980.372229] BTRFS info (device loop0): using free space tree
>> [  980.372639] BTRFS info (device loop0): has skinny extents
>> [  980.375075] BTRFS info (device loop0): start tree-log replay
>> [  980.375513] BTRFS warning (device loop0): log replay required on RO media
>> [  980.381652] BTRFS error (device loop0): open_ctree failed
>>
>> [CAUSE]
>> Although btrfs will replay its dirty log even with RO mount, but kernel
>> will treat seed device as RO device, and dirty log can not be replayed
>> on RO device.
>>
>> This rejection is already the better end, just imagine if we don't treat
>> seed device as RO, and replayed the dirty log.
>> The filesystem relying on the seed device will be completely screwed up.
>>
>> [FIX]
>> Just add extra check on log tree in btrfstune to reject setting seed
>> flag on filesystems with dirty log.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
> 
> Can we get a progs test for this as well?

Thanks to the suggestion from Nik, I finally remember we can use raw 
image for fs with dirty journal.

Test case will come soon.

Thanks,
Qu
> 
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> 
> Thanks,
> 
> Josef
>