Re: [PATCH v2 3/4] backup: Make sure that source and target size match

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

05.05.2020 13:03, Kevin Wolf wrote:
> Am 30.04.2020 um 20:21 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 30.04.2020 17:27, Kevin Wolf wrote:
>>> Since the introduction of a backup filter node in commit 00e30f05d, the
>>> backup block job crashes when the target image is smaller than the
>>> source image because it will try to write after the end of the target
>>> node without having BLK_PERM_RESIZE. (Previously, the BlockBackend layer
>>> would have caught this and errored out gracefully.)
>>>
>>> We can fix this and even do better than the old behaviour: Check that
>>> source and target have the same image size at the start of the block job
>>> and unshare BLK_PERM_RESIZE. (This permission was already unshared
>>> before the same commit 00e30f05d, but the BlockBackend that was used to
>>> make the restriction was removed without a replacement.) This will
>>> immediately error out when starting the job instead of only when writing
>>> to a block that doesn't exist in the target.
>>>
>>> Longer target than source would technically work because we would never
>>> write to blocks that don't exist, but semantically these are invalid,
>>> too, because a backup is supposed to create a copy, not just an image
>>> that starts with a copy.
>>>
>>> Fixes: 00e30f05de1d19586345ec373970ef4c192c6270
>>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1778593
>>> Cc: qemu-stable@nongnu.org
>>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
>>
>> I'm OK with it as is, as it fixes bug:
>>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>
>> still, some notes below
>>
>>
>>> ---
>>>    block/backup-top.c | 14 +++++++++-----
>>>    block/backup.c     | 14 +++++++++++++-
>>>    2 files changed, 22 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/block/backup-top.c b/block/backup-top.c
>>> index 3b50c06e2c..79b268e6dc 100644
>>> --- a/block/backup-top.c
>>> +++ b/block/backup-top.c
>>> @@ -148,8 +148,10 @@ static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
>>>             *
>>>             * Share write to target (child_file), to not interfere
>>>             * with guest writes to its disk which may be in target backing chain.
>>> +         * Can't resize during a backup block job because we check the size
>>> +         * only upfront.
>>>             */
>>> -        *nshared = BLK_PERM_ALL;
>>> +        *nshared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
>>>            *nperm = BLK_PERM_WRITE;
>>>        } else {
>>>            /* Source child */
>>> @@ -159,7 +161,7 @@ static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
>>>            if (perm & BLK_PERM_WRITE) {
>>>                *nperm = *nperm | BLK_PERM_CONSISTENT_READ;
>>>            }
>>> -        *nshared &= ~BLK_PERM_WRITE;
>>> +        *nshared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
>>>        }
>>>    }
>>> @@ -192,11 +194,13 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
>>>    {
>>>        Error *local_err = NULL;
>>>        BDRVBackupTopState *state;
>>> -    BlockDriverState *top = bdrv_new_open_driver(&bdrv_backup_top_filter,
>>> -                                                 filter_node_name,
>>> -                                                 BDRV_O_RDWR, errp);
>>> +    BlockDriverState *top;
>>>        bool appended = false;
>>> +    assert(source->total_sectors == target->total_sectors);
>>
>> May be better to error-out, just to keep backup-top independent. Still, now it's not
>> really needed, as we have only one caller. And this function have to be refactored
>> anyway, when publishing this filter (open() and close() should appear, so this code
>> will be rewritten anyway.)
> 
> Yes, the whole function only works because it's used in this restricted
> context today. For example, we only know that total_sectors is up to
> date because the caller has called bdrv_getlength() just a moment ago.
> 
> I think fixing this would be beyond the scope of this patch, but
> certainly a good idea anyway.
> 
>> And the other thought: the permissions we declared above, will be activated only after
>> successful bdrv_child_refresh_perms(). I think some kind of race is possible, so that
>> size is changed actual permission activation. So, may be good to double check sizes after
>> bdrv_child_refresh_perms().. But it's a kind of paranoia.
> 
> We're not in coroutine context, so we can't yield. I don't see who could
> change the size in parallel (apart from an external process, but an
> external process can mess up anything).
> 
> When we make backup-top an independent driver, instead of double
> checking (what would you do on error?), maybe we could move the size
> initialisation (then with bdrv_getlength()) to after
> bdrv_child_refresh_perms().
> 
>> Also, third thought: the restricted permissions doesn't save us from resizing
>> of the source through exactly this node, does it? Hmm, but your test works somehow. But
>> (I assume) it worked in a previous patch version without unsharing on source..
>>
>> Ha, but bdrv_co_truncate just can't work on backup-top, because it doesn't have file child.
>> But, if we fix bdrv_co_truncate to skip filters, we'll need to define .bdrv_co_truncate in
>> backup_top, which will return something like -EBUSY.. Or just -ENOTSUP, doesn't matter.
> 
> Maybe this is a sign that bdrv_co_truncate shouldn't automatically skip
> filters because filters might depend on a fixed size?
> 
> Or we could make the automatic skipping depend on having BLK_PERM_RESIZE
> for the child. If the filter doesn't have the permission, we must not
> call truncate for its child (it would crash). Then backup-top and
> similar filters must just be careful not to take RESIZE permissions.
> 

Hmm this should work.. Still it's a workaround, seems out of the concept of permission system..

I think, that the problem is that .bdrv_top_child_perm can't return an error.
The handler answers the question:

- Hi, we are your owners and we want the following cumulative permissions on you. Then, which permissions do you want on your child?

And the handler can't answer: "Hi, you guys want too much, I refuse to play by your rules"..


-- 
Best regards,
Vladimir