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Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_c

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v12 2/2] block/backup: fix backup_cow_with_offload for last cluster
Date: Wed, 18 Sep 2019 16:14:22 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0

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 <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
  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);

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