|
| From: | Ivan Ren |
| Subject: | Re: [Qemu-block] [Qemu-devel] [PATCH] qcow2: fix preallocation with metadata on bare block device |
| Date: | Fri, 11 May 2018 18:46:47 +0800 |
On 05/08/2018 07:27 AM, Ivan Ren wrote:
> Create a qcow2 directly on bare block device with
> "-o preallocation=metadata" option. When read this qcow2, it will
> return dirty data of block device.
Yes, reading garbage is expected.
> This patch add QCOW_OFLAG_ZERO
> for all preallocated l2 entry if the underlying device is a bare
> block device.
This says what the patch does, but not why. What is the actual use case
scenario where changing semantics to have the qcow2 overwrite the
garbage to read 0 instead of any pre-existing garbage, when dealing with
portions of the disk that have not yet been written by the guest? Are
you trying to prevent a security leak of previous information that may
be resident on the block device?
>
> Signed-off-by: Ivan Ren <address@hidden>
> ---
> block/qcow2-cluster.c | 5 +++--
> block/qcow2.c | 19 ++++++++++++++++---
> block/qcow2.h | 3 ++-
> 3 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 1aee726..b9e0ceb 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -919,7 +919,8 @@ fail:
> return ret;
> }
>
> -int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
> +int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m,
> + uint64_t flags)
> {
> BDRVQcow2State *s = bs->opaque;
> int i, j = 0, l2_index, ret;
> @@ -969,7 +970,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
> }
>
> l2_slice[l2_index + i] = cpu_to_be64((cluster_offset +
> - (i << s->cluster_bits)) | QCOW_OFLAG_COPIED);
> + (i << s->cluster_bits)) | QCOW_OFLAG_COPIED | flags);
> }
>
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 2f36e63..093735c 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2044,7 +2044,7 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
> while (l2meta != NULL) {
> QCowL2Meta *next;
>
> - ret = qcow2_alloc_cluster_link_l2(bs, l2meta);
> + ret = qcow2_alloc_cluster_link_l2(bs, l2meta, 0);
> if (ret < 0) {
> goto fail;
> }
> @@ -2534,6 +2534,7 @@ static void coroutine_fn preallocate_co(void *opaque)
> uint64_t host_offset = 0;
> unsigned int cur_bytes;
> int ret;
> + struct stat st;
> QCowL2Meta *meta;
>
> qemu_co_mutex_lock(&s->lock);
> @@ -2552,7 +2553,19 @@ static void coroutine_fn preallocate_co(void *opaque)
> while (meta) {
> QCowL2Meta *next = meta->next;
>
> - ret = qcow2_alloc_cluster_link_l2(bs, meta);
> + /* Check the underlying device type.
> + * If the underlying device is a block device, we add
> + * QCOW_OFLAG_ZERO for all preallocated l2 entry to ignore dirty
> + * data on block device.
> + * If the underlying device can't be used with stat(return < 0),
> + * treat it as a regular file.
> + */
> + if (stat(bs->filename, &st) < 0 || !S_ISBLK(st.st_mode)) {
Won't work. You cannot guarantee that bs->filename is a local file; it
could be a remote protocol, such as NBD or gluster. If you need to know
whether bs->filename has a property that it reads as all zeroes when
first initialized, we already have
BlockDriverInfo::unallocated_blocks_are_zero, which is set to true for
regular files and false for block devices.
> + ret = qcow2_alloc_cluster_link_l2(bs, meta, 0);
> + } else {
> + ret = qcow2_alloc_cluster_link_l2(bs, meta, QCOW_OFLAG_ZERO);
Why would we not want to pass QCOW_OFLAG_ZERO always, rather than
special-casing it based on what the underlying protocol might already
have in place?
> + }
> +
> if (ret < 0) {
> qcow2_free_any_clusters(bs, meta->alloc_offset,
> meta->nb_clusters, QCOW2_DISCARD_NEVER);
> @@ -3458,7 +3471,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
> };
> qemu_co_queue_init(&allocation.dependent_requests);
>
> - ret = qcow2_alloc_cluster_link_l2(bs, &allocation);
> + ret = qcow2_alloc_cluster_link_l2(bs, &allocation, 0);
> if (ret < 0) {
> error_setg_errno(errp, -ret, "Failed to update L2 tables");
> qcow2_free_clusters(bs, host_offset,
> diff --git a/block/qcow2.h b/block/qcow2.h
> index adf5c39..9a59602 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -617,7 +617,8 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
> uint64_t offset,
> int compressed_size);
>
> -int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m);
> +int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m,
> + uint64_t flags);
> int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset,
> uint64_t bytes, enum qcow2_discard_type type,
> bool full_discard);
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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