Re: [PATCH] btrfs: revert fs_devices state on error of btrfs_init_new_device()

[Filipe Manana <fdmanana@gmail.com>]

On Fri, Aug 3, 2018 at 8:29 AM, Anand Jain <anand.jain@oracle.com> wrote:
>
>
> On 08/03/2018 02:36 PM, Anand Jain wrote:
>>
>>
>>
>>
>> On 07/31/2018 07:47 PM, Filipe Manana wrote:
>>>
>>> On Tue, Jul 31, 2018 at 11:12 AM, Anand Jain <anand.jain@oracle.com>
>>> wrote:
>>>>
>>>>
>>>>
>>>> On 07/27/2018 08:04 AM, Naohiro Aota wrote:
>>>>>
>>>>>
>>>>> When btrfs hits error after modifying fs_devices in
>>>>> btrfs_init_new_device() (such as btrfs_add_dev_item() returns error),
>>>>> it
>>>>> leaves everything as is, but frees allocated btrfs_device. As a result,
>>>>> fs_devices->devices and fs_devices->alloc_list contain already freed
>>>>> btrfs_device, leading to later use-after-free bug.
>>>>
>>>>
>>>>
>>>>   the undo part of the btrfs_init_new_device() is broken for a while
>>>> now.
>>>>   Thanks for the fix, but..
>>>>
>>>>    - this patch does not fix the seed device context, its ok to fix that
>>>>      in a separate patch though.
>>>>    - and does not undo the effect of
>>>>
>>>> -----
>>>>          if (!blk_queue_nonrot(q))
>>>>                  fs_info->fs_devices->rotating = 1
>>>> ::
>>>>          btrfs_clear_space_info_full(fs_info);
>>>> ----
>>>>       which I think should be handled as part of this patch.
>>>
>>>
>>> Doesn't matter, the filesystem was turned to RO mode (transaction
>>> aborted).
>>
>>
>> . That's not true in all cases. Filesystem can still be in the RW

Yes, if nothing was done yet in the transaction (exactly what happens
in your test), in which case there's no risk of leaving inconsistent
metadata on disk.

Space info being full is rather rare, further setting it to full only
makes the next allocation attempt to do some work looking for space
instead of returning enospc immediately.
Settting the rotating flag has currently no effect for a mounted filesystem.

That is, there are no problems.

>
>
>    typo I mean not true in some cases and FS can still be RW able
>    after the transaction abort, below is a test case and results.
>
> Thanks. Aannd
>
>
>>    mode after the transaction aborted. Tested with the following
>>    simulation.
>>
>> --------------
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index f46af7928963..5609d70b4372 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -2458,6 +2458,10 @@ int btrfs_init_new_device(struct btrfs_fs_info
>> *fs_info, const char *device_path
>>                  }
>>          }
>>
>> +       ret = -ENOMEM;
>> +       btrfs_abort_transaction(trans, ret);
>> +       goto error_sysfs;
>> +
>>          ret = btrfs_add_dev_item(trans, device);
>>          if (ret) {
>>                  btrfs_abort_transaction(trans, ret);
>> -------------------
>>
>>
>> # mount /dev/sdb /btrfs
>>
>> # btrfs dev add /dev/sdc /btrfs
>> ERROR: error adding device '/dev/sdc': Cannot allocate memory
>>
>> # cat /proc/self/mounts | grep btrfs
>> /dev/sdb /btrfs btrfs rw,relatime,space_cache,subvolid=5,subvol=/ 0 0
>>
>> # echo "test" > /btrfs/tf; echo $?
>> 0
>>
>> . In any case, I would rather put the things right even if it just
>>   theoretical. A core dump taken after this would indicate a wrong
>>   state of the space and fs_devices::rotating.
>>
>>
>> Thanks, Anand
>>
>>>>
>>>> Thanks, Anand
>>>>
>>>>
>>>>
>>>>> Error path also messes the things like ->num_devices. While they go
>>>>> backs
>>>>> to the original value by unscanning btrfs devices, it is safe to revert
>>>>> them here.
>>>>>
>>>>> Fixes: 79787eaab461 ("btrfs: replace many BUG_ONs with proper error
>>>>> handling")
>>>>> Signed-off-by: Naohiro Aota <naota@elisp.net>
>>>>> ---
>>>>>    fs/btrfs/volumes.c | 28 +++++++++++++++++++++++-----
>>>>>    1 file changed, 23 insertions(+), 5 deletions(-)
>>>>>
>>>>>    This patch applies on master, but not on kdave/for-next because of
>>>>>    74b9f4e186eb ("btrfs: declare fs_devices in
>>>>> btrfs_init_new_device()")
>>>>>
>>>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>>>> index 1da162928d1a..5f0512fffa52 100644
>>>>> --- a/fs/btrfs/volumes.c
>>>>> +++ b/fs/btrfs/volumes.c
>>>>> @@ -2410,7 +2410,7 @@ int btrfs_init_new_device(struct btrfs_fs_info
>>>>> *fs_info, const char *device_path
>>>>>          struct list_head *devices;
>>>>>          struct super_block *sb = fs_info->sb;
>>>>>          struct rcu_string *name;
>>>>> -       u64 tmp;
>>>>> +       u64 orig_super_total_bytes, orig_super_num_devices;
>>>>>          int seeding_dev = 0;
>>>>>          int ret = 0;
>>>>>          bool unlocked = false;
>>>>> @@ -2509,12 +2509,14 @@ int btrfs_init_new_device(struct btrfs_fs_info
>>>>> *fs_info, const char *device_path
>>>>>          if (!blk_queue_nonrot(q))
>>>>>                  fs_info->fs_devices->rotating = 1;
>>>>>    -     tmp = btrfs_super_total_bytes(fs_info->super_copy);
>>>>> +       orig_super_total_bytes =
>>>>> btrfs_super_total_bytes(fs_info->super_copy);
>>>>>          btrfs_set_super_total_bytes(fs_info->super_copy,
>>>>> -               round_down(tmp + device->total_bytes,
>>>>> fs_info->sectorsize));
>>>>> +               round_down(orig_super_total_bytes +
>>>>> device->total_bytes,
>>>>> +                          fs_info->sectorsize));
>>>>>    -     tmp = btrfs_super_num_devices(fs_info->super_copy);
>>>>> -       btrfs_set_super_num_devices(fs_info->super_copy, tmp + 1);
>>>>> +       orig_super_num_devices =
>>>>> btrfs_super_num_devices(fs_info->super_copy);
>>>>> +       btrfs_set_super_num_devices(fs_info->super_copy,
>>>>> +                                   orig_super_num_devices + 1);
>>>>>          /* add sysfs device entry */
>>>>>          btrfs_sysfs_add_device_link(fs_info->fs_devices, device);
>>>>> @@ -2594,6 +2596,22 @@ int btrfs_init_new_device(struct btrfs_fs_info
>>>>> *fs_info, const char *device_path
>>>>>      error_sysfs:
>>>>>          btrfs_sysfs_rm_device_link(fs_info->fs_devices, device);
>>>>> +       mutex_lock(&fs_info->fs_devices->device_list_mutex);
>>>>> +       mutex_lock(&fs_info->chunk_mutex);
>>>>> +       list_del_rcu(&device->dev_list);
>>>>> +       list_del(&device->dev_alloc_list);
>>>>> +       fs_info->fs_devices->num_devices--;
>>>>> +       fs_info->fs_devices->open_devices--;
>>>>> +       fs_info->fs_devices->rw_devices--;
>>>>> +       fs_info->fs_devices->total_devices--;
>>>>> +       fs_info->fs_devices->total_rw_bytes -= device->total_bytes;
>>>>> +       atomic64_sub(device->total_bytes, &fs_info->free_chunk_space);
>>>>> +       btrfs_set_super_total_bytes(fs_info->super_copy,
>>>>> +                                   orig_super_total_bytes);
>>>>> +       btrfs_set_super_num_devices(fs_info->super_copy,
>>>>> +                                   orig_super_num_devices);
>>>>> +       mutex_unlock(&fs_info->chunk_mutex);
>>>>> +       mutex_unlock(&fs_info->fs_devices->device_list_mutex);
>>>>>    error_trans:
>>>>>          if (seeding_dev)
>>>>>                  sb->s_flags |= SB_RDONLY;
>>>>>
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>>>
>>>
>>>
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-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”