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block: fix overlapping multiwrite requests

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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>
This commit is contained in:
Stefan Hajnoczi 2014-07-30 09:53:30 +01:00
parent f21492817b
commit 391827eb10
1 changed files with 6 additions and 0 deletions
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6
block.c
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@ -4553,6 +4553,12 @@ 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;