[Qemu-devel] [PATCH v2 1/2] block: fix overlapping multiwrite requests

[Stefan Hajnoczi <stefanha@redhat.com>]

When request A is a strict superset of request B:

  AAAAAAAA
    BBBB

multiwrite_merge() merges them as follows:

  AABBBB

The tail of request A should have been included:

  AABBBBAA

This patch fixes data loss but this code path is probably rare.  Since
guests cannot assume ordering between in-flight requests, few
applications submit overlapping write requests.

Reported-by: Slava Pestov <sviatoslav.pestov@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block.c b/block.c
index 8cf519b..0a3ac43 100644
--- a/block.c
+++ b/block.c
@@ -4498,6 +4498,12 @@ static int multiwrite_merge(BlockDriverState *bs, BlockRequest *reqs,
             // Add the second request
             qemu_iovec_concat(qiov, reqs[i].qiov, 0, reqs[i].qiov->size);
 
+            // Add tail of first request, if necessary
+            if (qiov->size < reqs[outidx].qiov->size) {
+                qemu_iovec_concat(qiov, reqs[outidx].qiov, qiov->size,
+                                  reqs[outidx].qiov->size - qiov->size);
+            }
+
             reqs[outidx].nb_sectors = qiov->size >> 9;
             reqs[outidx].qiov = qiov;
 
-- 
1.9.3