block: fix overlapping multiwrite requests
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>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
diff --git a/block.c b/block.c
index 9c5566b..cb670fd 100644
--- a/block.c
+++ b/block.c
@@ -4553,6 +4553,12 @@
// 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;
