Re: [PATCH 1/1] virtio-9pfs: don't truncate response

[Stefano Stabellini]

On Thu, 14 May 2020, Christian Schoenebeck wrote:
> Commit SHA-1 16724a173049ac29c7b5ade741da93a0f46edff7 introduced
> truncating the response to the currently available transport buffer size,
> which was supposed to fix an 9pfs error on Xen boot where transport buffer
> might still be smaller than required for response.
> 
> Unfortunately this change broke small reads (with less than 12 bytes).
> 
> To fix this introduced bug for virtio at least, let's revert this change
> for the virtio transport. Unlike with Xen, we should never come into
> this situation with virtio that the available transport buffer would be
> too small for delivering any response to client. So truncating the buffer
> is not necessary with virtio in the first place.
> 
> This bug still needs to be addressed for Xen appropriately though.
> 
> Fixes: 16724a173049ac29c7b5ade741da93a0f46edff7 (for virtio only)
> Fixes: https://bugs.launchpad.net/bugs/1877688 (for virtio only)
> Signed-off-by: Christian Schoenebeck <address@hidden>

Reviewed-by: Stefano Stabellini <address@hidden>


> ---
>  hw/9pfs/virtio-9p-device.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index 536447a355..bb6154945a 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -154,16 +154,13 @@ static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, 
> struct iovec **piov,
>      VirtQueueElement *elem = v->elems[pdu->idx];
>      size_t buf_size = iov_size(elem->in_sg, elem->in_num);
>  
> -    if (buf_size < P9_IOHDRSZ) {
> +    if (buf_size < *size) {
>          VirtIODevice *vdev = VIRTIO_DEVICE(v);
>  
>          virtio_error(vdev,
> -                     "VirtFS reply type %d needs %zu bytes, buffer has %zu, 
> less than minimum",
> +                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
>                       pdu->id + 1, *size, buf_size);
>      }
> -    if (buf_size < *size) {
> -        *size = buf_size;
> -    }
>  
>      *piov = elem->in_sg;
>      *pniov = elem->in_num;
> -- 
> 2.20.1
>