Re: [Qemu-devel] [PATCH] block: Produce zeros when protocols reading beyond end of file

[Stefan Hajnoczi]

On Mon, Aug 5, 2013 at 10:11 AM, Asias He <address@hidden> wrote:
> From: MORITA Kazutaka <address@hidden>
>
> 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 <address@hidden>
> Signed-off-by: Asias He <address@hidden>
> ---
>  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) {
> +        ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> +    } else {
> +        /* NBD doesn't support reading beyond end of file. */
> +        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;
> +            qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
> +        }
> +    }

This patch breaks qemu-iotests ./check -qcow2 022.  This happens
because qcow2 temporarily sets ->growable = 1 for vmstate accesses
(which are stored beyond the end of regular image data).

static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
                              int64_t pos, int size)
{
    BDRVQcowState *s = bs->opaque;
    int growable = bs->growable;
    int ret;

    BLKDBG_EVENT(bs->file, BLKDBG_VMSTATE_LOAD);
    bs->growable = 1;
    ret = bdrv_pread(bs, qcow2_vm_state_offset(s) + pos, buf, size);
    bs->growable = growable;

    return ret;
}

Please *always* run qemu-iotests before submitting block patches:
http://qemu-project.org/Documentation/QemuIoTests

A simple but ugly way to fix this is for block.c to also have a
->zero_beyond_eof flag which enables the behavior you are adding.
qcow2_load_vmstate() would disable ->zero_beyond_eof temporarily in
addition to enabling ->growable.

It's not easy to call the internal qcow2_co_readv() from
qcow2_load_vmstate() because the vmstate functions are byte
granularity (like bdrv_pread()) while .bdrv_co_readv() is
sector-granularity.

Stefan