[PATCH v2 2/6] block: truncate: Don't make backing file data visible

[Kevin Wolf]

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>
---
 block/io.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/block/io.c b/block/io.c
index 003f4ea38c..6a5144f8d2 100644
--- a/block/io.c
+++ b/block/io.c
@@ -3385,12 +3385,44 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, 
int64_t offset, bool exact,
     ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not refresh total sector count");
+        goto fail_refresh_total_sectors;
     } else {
         offset = bs->total_sectors * BDRV_SECTOR_SIZE;
     }
+
+    /*
+     * 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 area where backing file and new area overlap.
+     *
+     * 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 && prealloc == PREALLOC_MODE_OFF) {
+        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) {
+            ret = bdrv_co_do_pwrite_zeroes(
+                    bs, old_size, MIN(new_bytes, backing_len - old_size),
+                    BDRV_REQ_ZERO_WRITE | BDRV_REQ_MAY_UNMAP);
+            if (ret < 0) {
+                goto out;
+            }
+        }
+    }
+
     /* It's possible that truncation succeeded but refresh_total_sectors
      * failed, but the latter doesn't affect how we should finish the request.
      * Pass 0 as the last parameter so that dirty bitmaps etc. are handled. */
+fail_refresh_total_sectors:
     bdrv_co_write_req_finish(child, offset - new_bytes, new_bytes, &req, 0);
 
 out:
-- 
2.20.1