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

[John Snow <jsnow@redhat.com>]

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.

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?

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?

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?

>       bdrv_reset_dirty_bitmap(job->copy_bitmap, start,
>                               job->cluster_size * nr_clusters);
>