| Message ID | 20191122160511.8377-5-kwolf@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block: Fix resize (extending) of short overlays | expand |
On 11/22/19 10:05 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: 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 <kwolf@redhat.com> > --- > block/io.c | 33 +++++++++++++++++++++++++++++++++ > 1 file changed, 33 insertions(+) Reviewed-by: Eric Blake <eblake@redhat.com>
On 22.11.19 17:05, 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: 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 <kwolf@redhat.com> > --- > block/io.c | 33 +++++++++++++++++++++++++++++++++ > 1 file changed, 33 insertions(+) Reviewed-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/io.c b/block/io.c index 42e7558954..61a63d9dc2 100644 --- a/block/io.c +++ b/block/io.c @@ -3392,12 +3392,45 @@ 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 fail_refresh_total_sectors; + } + + 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 | + (no_fallback ? BDRV_REQ_NO_FALLBACK : 0)); + if (ret < 0) { + goto fail_refresh_total_sectors; + } + } + } + /* 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:
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 <kwolf@redhat.com> --- block/io.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+)