Re: [PATCH] block/io: accept NULL qiov in bdrv_pad_request

[Stefan Hajnoczi]

On Tue, Mar 19, 2024 at 10:13:41AM +0100, Fiona Ebner wrote:
> From: Stefan Reiter <s.reiter@proxmox.com>
> 
> Some operations, e.g. block-stream, perform reads while discarding the
> results (only copy-on-read matters). In this case, they will pass NULL
> as the target QEMUIOVector, which will however trip bdrv_pad_request,
> since it wants to extend its passed vector. In particular, this is the
> case for the blk_co_preadv() call in stream_populate().
> 
> If there is no qiov, no operation can be done with it, but the bytes
> and offset still need to be updated, so the subsequent aligned read
> will actually be aligned and not run into an assertion failure.
> 
> In particular, this can happen when the request alignment of the top
> node is larger than the allocated part of the bottom node, in which
> case padding becomes necessary. For example:
> 
> > ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M -o cluster_size=32768
> > ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2
> > ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F qcow2
> > ./qemu-system-x86_64 --qmp stdio \
> > --blockdev 
> > qcow2,node-name=node0,file.driver=file,file.filename=/tmp/top.qcow2 \
> > <<EOF
> > {"execute": "qmp_capabilities"}
> > {"execute": "blockdev-add", "arguments": { "driver": "compress", "file": 
> > "node0", "node-name": "node1" } }
> > {"execute": "block-stream", "arguments": { "job-id": "stream0", "device": 
> > "node1" } }
> > EOF

Hi Fiona,
Can you add a qemu-iotests test case for this issue?

Thanks,
Stefan

> 
> Originally-by: Stefan Reiter <s.reiter@proxmox.com>
> Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
> [FE: do update bytes and offset in any case
>      add reproducer to commit message]
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>  block/io.c | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index 33150c0359..395bea3bac 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1726,22 +1726,29 @@ static int bdrv_pad_request(BlockDriverState *bs,
>          return 0;
>      }
>  
> -    sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
> -                                  &sliced_head, &sliced_tail,
> -                                  &sliced_niov);
> +    /*
> +     * For prefetching in stream_populate(), no qiov is passed along, because
> +     * only copy-on-read matters.
> +     */
> +    if (qiov && *qiov) {
> +        sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
> +                                      &sliced_head, &sliced_tail,
> +                                      &sliced_niov);
>  
> -    /* Guaranteed by bdrv_check_request32() */
> -    assert(*bytes <= SIZE_MAX);
> -    ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
> -                                  sliced_head, *bytes);
> -    if (ret < 0) {
> -        bdrv_padding_finalize(pad);
> -        return ret;
> +        /* Guaranteed by bdrv_check_request32() */
> +        assert(*bytes <= SIZE_MAX);
> +        ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
> +                                      sliced_head, *bytes);
> +        if (ret < 0) {
> +            bdrv_padding_finalize(pad);
> +            return ret;
> +        }
> +        *qiov = &pad->local_qiov;
> +        *qiov_offset = 0;
>      }
> +
>      *bytes += pad->head + pad->tail;
>      *offset -= pad->head;
> -    *qiov = &pad->local_qiov;
> -    *qiov_offset = 0;
>      if (padded) {
>          *padded = true;
>      }
> -- 
> 2.39.2
> 
>

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