On Mon, Jul 27, 2026 at 06:23:25PM +0900, Akihiko Odaki wrote:
> The blob size is encoded as a 64-bit integer in the virtio protocol, but
> it is encoded as a 32-bit integer in the migration stream, and
> a large blob whose size cannot be expressed in a 32-bit integer will
> be corrupted after migration. Deny creating such large blobs.
Is there a valid use case for such large blobs ? If so, rather than
rejecting the blob, we could dynamically add a migration blocker.
That would allow the valid use cases, while preventing the migration
problem.
>
> Fixes: f66767f75c9c ("virtio-gpu: add virtio-gpu/blob vmstate subsection")
> Fixes: 10b9ddbc83b9 ("Revert "virtio-gpu: block migration of VMs with
> blob=true"")
> Signed-off-by: Akihiko Odaki <[email protected]>
> ---
> Changes in v2:
> - Add the Fixes: tag for f66767f75c9c.
> - Add a TODO to encode blob_size as 64 bits in virtio-gpu/blob
> VMState version 2.
> - Link to v1:
> https://lore.kernel.org/qemu-devel/[email protected]
> ---
> hw/display/virtio-gpu.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
> index 718ba3039290..987b1b3ea860 100644
> --- a/hw/display/virtio-gpu.c
> +++ b/hw/display/virtio-gpu.c
> @@ -352,6 +352,14 @@ static void virtio_gpu_resource_create_blob(VirtIOGPU *g,
> return;
> }
>
> + if (cblob.size > UINT32_MAX) {
> + qemu_log_mask(LOG_GUEST_ERROR,
> + "%s: blob too large (%" PRIu64 "> %" PRIu32 ")\n",
> + __func__, cblob.size, UINT32_MAX);
> + cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
> + return;
> + }
> +
> if (virtio_gpu_find_resource(g, cblob.resource_id)) {
> qemu_log_mask(LOG_GUEST_ERROR, "%s: resource already exists %d\n",
> __func__, cblob.resource_id);
> @@ -1435,7 +1443,14 @@ static int virtio_gpu_blob_save(QEMUFile *f, void
> *opaque, size_t size,
> }
> assert(!res->image);
> qemu_put_be32(f, res->resource_id);
> +
> + /*
> + * The migration stream encodes blob_size as a 32-bit integer for
> + * compatibility though virtio encodes it as a 64-bit integer. A
> newer
> + * version of the migration stream should encode it as a 64-bit
> integer.
> + */
> qemu_put_be32(f, res->blob_size);
> +
> qemu_put_be32(f, res->iov_cnt);
> for (i = 0; i < res->iov_cnt; i++) {
> qemu_put_be64(f, res->addrs[i]);
>
> ---
> base-commit: 006a22cb26998998385b104db1ff9466ef2f3153
> change-id: 20260725-blob-8a6a1a85601a
>
> Best regards,
> --
> Akihiko Odaki <[email protected]>
>
>
With regards,
Daniel
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