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From d84b9b93755ece6618ed98fa84386beeb1a0e40b Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Mon, 8 Jun 2020 15:01:36 +0100
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Subject: [PATCH 08/17] block: truncate: Don't make backing file data visible
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20200608150140.38218-8-kwolf@redhat.com>
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Patchwork-id: 97454
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O-Subject: [RHEL-AV-8.2.1 qemu-kvm PATCH 07/11] block: truncate: Don't make backing file data visible
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Bugzilla: 1780574
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RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
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RH-Acked-by: Eric Blake <eblake@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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When extending the size of an image that has a backing file larger than
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its old size, make sure that the backing file data doesn't become
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visible in the guest, but the added area is properly zeroed out.
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Consider the following scenario where the overlay is shorter than its
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backing file:
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base.qcow2: AAAAAAAA
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overlay.qcow2: BBBB
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When resizing (extending) overlay.qcow2, the new blocks should not stay
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unallocated and make the additional As from base.qcow2 visible like
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before this patch, but zeros should be read.
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A similar case happens with the various variants of a commit job when an
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intermediate file is short (- for unallocated):
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base.qcow2: A-A-AAAA
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mid.qcow2: BB-B
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top.qcow2: C--C--C-
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After commit top.qcow2 to mid.qcow2, the following happens:
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mid.qcow2: CB-C00C0 (correct result)
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mid.qcow2: CB-C--C- (before this fix)
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Without the fix, blocks that previously read as zeros on top.qcow2
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suddenly turn into A.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Message-Id: <20200424125448.63318-8-kwolf@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit 955c7d6687fefcd903900a1e597fcbc896c661cd)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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block/io.c | 25 +++++++++++++++++++++++++
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1 file changed, 25 insertions(+)
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diff --git a/block/io.c b/block/io.c
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index 3235ce5..6c70b56 100644
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--- a/block/io.c
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+++ b/block/io.c
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@@ -3370,6 +3370,31 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
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goto out;
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}
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+ /*
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+ * If the image has a backing file that is large enough that it would
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+ * provide data for the new area, we cannot leave it unallocated because
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+ * then the backing file content would become visible. Instead, zero-fill
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+ * the new area.
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+ *
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+ * Note that if the image has a backing file, but was opened without the
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+ * backing file, taking care of keeping things consistent with that backing
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+ * file is the user's responsibility.
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+ */
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+ if (new_bytes && bs->backing) {
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+ int64_t backing_len;
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+
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+ backing_len = bdrv_getlength(backing_bs(bs));
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+ if (backing_len < 0) {
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+ ret = backing_len;
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+ error_setg_errno(errp, -ret, "Could not get backing file size");
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+ goto out;
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+ }
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+
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+ if (backing_len > old_size) {
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+ flags |= BDRV_REQ_ZERO_WRITE;
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+ }
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+ }
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+
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if (drv->bdrv_co_truncate) {
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if (flags & ~bs->supported_truncate_flags) {
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error_setg(errp, "Block driver does not support requested flags");
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--
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1.8.3.1
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