Re: [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_cow_with_offload for last cluster

[John Snow <jsnow@redhat.com>]

On 9/19/19 3:02 AM, Vladimir Sementsov-Ogievskiy wrote:
> 18.09.2019 23:14, John Snow wrote:
>>
>>
>> On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
>>> We shouldn't try to copy bytes beyond EOF. Fix it.
>>>
>>> Fixes: 9ded4a0114968e
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> Reviewed-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>>   block/backup.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/block/backup.c b/block/backup.c
>>> index d8fdbfadfe..89f7f89200 100644
>>> --- a/block/backup.c
>>> +++ b/block/backup.c
>>> @@ -161,7 +161,7 @@ static int coroutine_fn backup_cow_with_offload(BackupBlockJob *job,
>>>       assert(QEMU_IS_ALIGNED(job->copy_range_size, job->cluster_size));
>>>       assert(QEMU_IS_ALIGNED(start, job->cluster_size));
>>> -    nbytes = MIN(job->copy_range_size, end - start);
>>> +    nbytes = MIN(job->copy_range_size, MIN(end, job->len) - start);
>>
>> I'm a little confused. I think the patch as written is correct, but I don't know what problem it solves.
> 
> last cluster may exceed EOF. And backup_do_cow (who calls  backup_cow_with_offload) rounds all to clusters.
> It's not bad, but we need to crop nbytes before calling actual io functions. backup_cow_with_bounce_buffer does the same thing.
> 
>>
>> If we're going to allow the caller to pass in an end that's beyond EOF, does that mean we are *requiring* the caller to pass in a value that's aligned?
> 
> Actually yes, as we are resetting dirty bitmap.
> 
>>
>> We should probably assert what kind of a value we're accepted here, right? We do for start, but should we for 'end' as well?
> 
> Yes assertion may be added.
> 
>>
>> Then ...
>>
>>>       nr_clusters = DIV_ROUND_UP(nbytes, job->cluster_size);
>>
>> Don't we just round this right back up immediately anyway? Does that mean we have callers that are passing in an 'end' that's more than 1 job-cluster beyond EOF? That seems like something that should be fixed in the caller, surely?
> 
> nr_clusters are used to set/reset dirty bitmap. It's OK. Still, for last cluster we can drop it and use nbytes directly. No there should not be such callers.
> nbytes is used to call blk_co_copy_range, and must be cropped to not exceed EOF.
> 

Ah, right, right ... I *was* confused. We don't use nr_clusters for the
IO itself, just the bitmap. So we effectively re-calculate aligned and
unaligned values for use in different places.

> Also, backup_cow_with_bounce_buffer behave in similar way: it crops nbytes.
> 
> Of course, there is a place for good refactoring, but I think not in this patch, it's a small bug fix.
> 
>>
>>>       bdrv_reset_dirty_bitmap(job->copy_bitmap, start,
>>>                               job->cluster_size * nr_clusters);
>>>
>>
> 
> 

We should make the interface here a little more clear I think, but what
you wrote is correct.

Reviewed-by: John Snow <jsnow@redhat.com>