Re: [PATCH v6 07/10] block: truncate: Don't make backing file data visible
23.04.2020 18:01, Kevin Wolf wrote:
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:
overlay.qcow2:
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-
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
Reviewed-by: Alberto Garcia
---
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));
this fit in one line with backing_len definition
+if (backing_len < 0) {
+ret = backing_len;
with errp set, as noted by Max:
Reviewed-by: Vladimir Sementsov-Ogievskiy
+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");
--
Best regards,
Vladimir
Re: [PATCH v6 07/10] block: truncate: Don't make backing file data visible
On 23.04.20 17:01, Kevin Wolf wrote:
> 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:
> overlay.qcow2:
>
> 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-
> 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
> Reviewed-by: Alberto Garcia
Seems like Berto gave you a rather broad R-b in v4. :)
> ---
> 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;
Shouldn’t we set errp?
Max
> +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");
>
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Re: [PATCH v6 07/10] block: truncate: Don't make backing file data visible
On 4/23/20 10:01 AM, Kevin Wolf wrote: 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: overlay.qcow2: 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- 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 Reviewed-by: Alberto Garcia --- block/io.c | 24 1 file changed, 24 insertions(+) Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
