Am 05.08.2013 um 10:11 hat Asias He geschrieben:
> From: MORITA Kazutaka <morita.kazut...@lab.ntt.co.jp>
>
> While Asias is debugging an issue creating qcow2 images on top of
> non-file protocols. It boils down to this example using NBD:
>
> $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
>
> Notice the open -g option to set bs->growable. This means you can
> read/write beyond end of file. Reading beyond end of file is supposed
> to produce zeroes.
>
> We rely on this behavior in qcow2_create2() during qcow2 image
> creation. We create a new file and then write the qcow2 header
> structure using bdrv_pwrite(). Since QCowHeader is not a multiple of
> sector size, block.c first uses bdrv_read() on the empty file to fetch
> the first sector (should be all zeroes).
>
> Here is the output from the qemu-io NBD example above:
>
> $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
> 00000000: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
> 00000010: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
> 00000020: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
> ...
>
> We are not zeroing the buffer! As a result qcow2 image creation on top
> of protocols is not guaranteed to work even when file creation is
> supported by the protocol.
>
> Signed-off-by: MORITA Kazutaka <morita.kazut...@lab.ntt.co.jp>
> Signed-off-by: Asias He <as...@redhat.com>
> ---
> block.c | 30 +++++++++++++++++++++++++++++-
> 1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/block.c b/block.c
> index 01b66d8..deaf0a0 100644
> --- a/block.c
> +++ b/block.c
> @@ -2544,7 +2544,35 @@ static int coroutine_fn
> bdrv_co_do_readv(BlockDriverState *bs,
> }
> }
>
> - ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> + if (!bs->drv->protocol_name) {
I think !bs->growable is the right check.
Checking for the protocol name is always a hack and most times wrong.
> + ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> + } else {
> + /* NBD doesn't support reading beyond end of file. */
This is not only for NBD, make it a neutral comment like:
/* Read zeros after EOF of growable BDSes */
> + int64_t len, total_sectors, max_nb_sectors;
> +
> + len = bdrv_getlength(bs);
> + if (len < 0) {
> + ret = len;
> + goto out;
> + }
> +
> + total_sectors = len >> BDRV_SECTOR_BITS;
> + max_nb_sectors = MAX(0, total_sectors - sector_num);
> + if (max_nb_sectors > 0) {
> + ret = drv->bdrv_co_readv(bs, sector_num,
> + MIN(nb_sectors, max_nb_sectors), qiov);
> + } else {
> + ret = 0;
> + }
> +
> + /* Reading beyond end of file is supposed to produce zeroes */
> + if (ret == 0 && total_sectors < sector_num + nb_sectors) {
> + size_t offset = MAX(0, total_sectors - sector_num);
> + size_t bytes = (sector_num + nb_sectors - offset) *
> + BDRV_SECTOR_SIZE;
uint64_t for both offset and bytes, size_t can be 32 bits.
> + qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
> + }
> + }
>
> out:
> tracked_request_end(&req);
Kevin
