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[PATCH v6 07/10] block: truncate: Don't make backing file data visible
From: |
Kevin Wolf |
Subject: |
[PATCH v6 07/10] block: truncate: Don't make backing file data visible |
Date: |
Thu, 23 Apr 2020 17:01:24 +0200 |
When extending the size of an image that has a backing file larger than
its old size, make sure that the backing file data doesn't become
visible in the guest, but the added area is properly zeroed out.
Consider the following scenario where the overlay is shorter than its
backing file:
base.qcow2: AAAAAAAA
overlay.qcow2: BBBB
When resizing (extending) overlay.qcow2, the new blocks should not stay
unallocated and make the additional As from base.qcow2 visible like
before this patch, but zeros should be read.
A similar case happens with the various variants of a commit job when an
intermediate file is short (- for unallocated):
base.qcow2: A-A-AAAA
mid.qcow2: BB-B
top.qcow2: C--C--C-
After commit top.qcow2 to mid.qcow2, the following happens:
mid.qcow2: CB-C00C0 (correct result)
mid.qcow2: CB-C--C- (before this fix)
Without the fix, blocks that previously read as zeros on top.qcow2
suddenly turn into A.
Signed-off-by: Kevin Wolf <address@hidden>
Reviewed-by: Alberto Garcia <address@hidden>
---
block/io.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
diff --git a/block/io.c b/block/io.c
index 795075954e..f618db3499 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3394,6 +3394,30 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child,
int64_t offset, bool exact,
goto out;
}
+ /*
+ * If the image has a backing file that is large enough that it would
+ * provide data for the new area, we cannot leave it unallocated because
+ * then the backing file content would become visible. Instead, zero-fill
+ * the new area.
+ *
+ * Note that if the image has a backing file, but was opened without the
+ * backing file, taking care of keeping things consistent with that backing
+ * file is the user's responsibility.
+ */
+ if (new_bytes && bs->backing) {
+ int64_t backing_len;
+
+ backing_len = bdrv_getlength(backing_bs(bs));
+ if (backing_len < 0) {
+ ret = backing_len;
+ goto out;
+ }
+
+ if (backing_len > old_size) {
+ flags |= BDRV_REQ_ZERO_WRITE;
+ }
+ }
+
if (drv->bdrv_co_truncate) {
if (flags & ~bs->supported_truncate_flags) {
error_setg(errp, "Block driver does not support requested flags");
--
2.25.3
- [PATCH v6 02/10] block: Add flags to bdrv(_co)_truncate(), (continued)
- [PATCH v6 02/10] block: Add flags to bdrv(_co)_truncate(), Kevin Wolf, 2020/04/23
- [PATCH v6 01/10] block: Add flags to BlockDriver.bdrv_co_truncate(), Kevin Wolf, 2020/04/23
- [PATCH v6 04/10] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate, Kevin Wolf, 2020/04/23
- [PATCH v6 05/10] raw-format: Support BDRV_REQ_ZERO_WRITE for truncate, Kevin Wolf, 2020/04/23
- [PATCH v6 03/10] block-backend: Add flags to blk_truncate(), Kevin Wolf, 2020/04/23
- [PATCH v6 06/10] file-posix: Support BDRV_REQ_ZERO_WRITE for truncate, Kevin Wolf, 2020/04/23
- [PATCH v6 07/10] block: truncate: Don't make backing file data visible,
Kevin Wolf <=
- [PATCH v6 08/10] iotests: Filter testfiles out in filter_img_info(), Kevin Wolf, 2020/04/23
- [PATCH v6 09/10] iotests: Test committing to short backing file, Kevin Wolf, 2020/04/23
- [PATCH v6 10/10] qcow2: Forward ZERO_WRITE flag for full preallocation, Kevin Wolf, 2020/04/23