The 9P protocol is transport agnostic: if the guest misconfigured the
buffers, the best we can do is to set the broken flag on the device.

Since virtio_pdu_vmarshal() may be called by several active PDUs, we
check if the transport isn't broken already to avoid printing extra
error messages.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
v5: - use ssize_t variable in virtio_pdu_v[un]marshal()
    - drop remaining vdev->broken check (MST suggested to discuss calling
      virtio_error() when the device is already broken to a separate thread)
---
 hw/9pfs/9p.c               |    2 +-
 hw/9pfs/9p.h               |    2 +-
 hw/9pfs/virtio-9p-device.c |   40 ++++++++++++++++++++++++++++++++++++----
 hw/9pfs/xen-9p-backend.c   |    3 ++-
 4 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 96d268334865..da0d6da65b45 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -1664,7 +1664,7 @@ static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
     unsigned int niov;
 
     if (is_write) {
-        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov);
+        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size + skip);
     } else {
         pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size + skip);
     }
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index aac1b0b2ce3d..d1cfeaf10e4f 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -363,7 +363,7 @@ struct V9fsTransport {
     void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
                                         unsigned int *pniov, size_t size);
     void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
-                                         unsigned int *pniov);
+                                         unsigned int *pniov, size_t size);
     void        (*push_and_notify)(V9fsPDU *pdu);
 };
 
diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
index 1a68c1622d3a..62650b0a6b99 100644
--- a/hw/9pfs/virtio-9p-device.c
+++ b/hw/9pfs/virtio-9p-device.c
@@ -146,8 +146,16 @@ static ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
+    ssize_t ret;
 
-    return v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
+    ret = v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
+    if (ret < 0) {
+        VirtIODevice *vdev = VIRTIO_DEVICE(v);
+
+        virtio_error(vdev, "Failed to encode VirtFS reply type %d",
+                     pdu->id + 1);
+    }
+    return ret;
 }
 
 static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
@@ -156,28 +164,52 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
+    ssize_t ret;
+
+    ret = v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
+    if (ret < 0) {
+        VirtIODevice *vdev = VIRTIO_DEVICE(v);
 
-    return v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
+        virtio_error(vdev, "Failed to decode VirtFS request type %d", pdu->id);
+    }
+    return ret;
 }
 
-/* The size parameter is used by other transports. Do not drop it. */
 static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
                                         unsigned int *pniov, size_t size)
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
+    size_t buf_size = iov_size(elem->in_sg, elem->in_num);
+
+    if (buf_size < size) {
+        VirtIODevice *vdev = VIRTIO_DEVICE(v);
+
+        virtio_error(vdev,
+                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
+                     pdu->id + 1, size, buf_size);
+    }
 
     *piov = elem->in_sg;
     *pniov = elem->in_num;
 }
 
 static void virtio_init_out_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
-                                         unsigned int *pniov)
+                                         unsigned int *pniov, size_t size)
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
+    size_t buf_size = iov_size(elem->out_sg, elem->out_num);
+
+    if (buf_size < size) {
+        VirtIODevice *vdev = VIRTIO_DEVICE(v);
+
+        virtio_error(vdev,
+                     "VirtFS request type %d needs %zu bytes, buffer has %zu",
+                     pdu->id, size, buf_size);
+    }
 
     *piov = elem->out_sg;
     *pniov = elem->out_num;
diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
index 922cc967be63..a82cf817fe45 100644
--- a/hw/9pfs/xen-9p-backend.c
+++ b/hw/9pfs/xen-9p-backend.c
@@ -147,7 +147,8 @@ static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
 
 static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
                                            struct iovec **piov,
-                                           unsigned int *pniov)
+                                           unsigned int *pniov,
+                                           size_t size)
 {
     Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
     Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];

On Wed, 28 Jun 2017 22:44:30 +0200
Greg Kurz <groug@kaod.org> wrote:

> The 9P protocol is transport agnostic: if the guest misconfigured the
> buffers, the best we can do is to set the broken flag on the device.
> 
> Since virtio_pdu_vmarshal() may be called by several active PDUs, we
> check if the transport isn't broken already to avoid printing extra
> error messages.
> 

Oops, forgot to drop this last sentence... Will do when pushing to my tree.

> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> v5: - use ssize_t variable in virtio_pdu_v[un]marshal()
>     - drop remaining vdev->broken check (MST suggested to discuss calling
>       virtio_error() when the device is already broken to a separate thread)
> ---
>  hw/9pfs/9p.c               |    2 +-
>  hw/9pfs/9p.h               |    2 +-
>  hw/9pfs/virtio-9p-device.c |   40 ++++++++++++++++++++++++++++++++++++----
>  hw/9pfs/xen-9p-backend.c   |    3 ++-
>  4 files changed, 40 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 96d268334865..da0d6da65b45 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1664,7 +1664,7 @@ static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
>      unsigned int niov;
>  
>      if (is_write) {
> -        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov);
> +        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size + skip);
>      } else {
>          pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size + skip);
>      }
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index aac1b0b2ce3d..d1cfeaf10e4f 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -363,7 +363,7 @@ struct V9fsTransport {
>      void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
>                                          unsigned int *pniov, size_t size);
>      void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> -                                         unsigned int *pniov);
> +                                         unsigned int *pniov, size_t size);
>      void        (*push_and_notify)(V9fsPDU *pdu);
>  };
>  
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index 1a68c1622d3a..62650b0a6b99 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -146,8 +146,16 @@ static ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    ssize_t ret;
>  
> -    return v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> +    ret = v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> +    if (ret < 0) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev, "Failed to encode VirtFS reply type %d",
> +                     pdu->id + 1);
> +    }
> +    return ret;
>  }
>  
>  static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
> @@ -156,28 +164,52 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    ssize_t ret;
> +
> +    ret = v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> +    if (ret < 0) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
>  
> -    return v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> +        virtio_error(vdev, "Failed to decode VirtFS request type %d", pdu->id);
> +    }
> +    return ret;
>  }
>  
> -/* The size parameter is used by other transports. Do not drop it. */
>  static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
>                                          unsigned int *pniov, size_t size)
>  {
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    size_t buf_size = iov_size(elem->in_sg, elem->in_num);
> +
> +    if (buf_size < size) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev,
> +                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
> +                     pdu->id + 1, size, buf_size);
> +    }
>  
>      *piov = elem->in_sg;
>      *pniov = elem->in_num;
>  }
>  
>  static void virtio_init_out_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
> -                                         unsigned int *pniov)
> +                                         unsigned int *pniov, size_t size)
>  {
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    size_t buf_size = iov_size(elem->out_sg, elem->out_num);
> +
> +    if (buf_size < size) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev,
> +                     "VirtFS request type %d needs %zu bytes, buffer has %zu",
> +                     pdu->id, size, buf_size);
> +    }
>  
>      *piov = elem->out_sg;
>      *pniov = elem->out_num;
> diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> index 922cc967be63..a82cf817fe45 100644
> --- a/hw/9pfs/xen-9p-backend.c
> +++ b/hw/9pfs/xen-9p-backend.c
> @@ -147,7 +147,8 @@ static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
>  
>  static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
>                                             struct iovec **piov,
> -                                           unsigned int *pniov)
> +                                           unsigned int *pniov,
> +                                           size_t size)
>  {
>      Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
>      Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];
> 

On Wed, Jun 28, 2017 at 10:44:30PM +0200, Greg Kurz wrote:
> The 9P protocol is transport agnostic: if the guest misconfigured the
> buffers, the best we can do is to set the broken flag on the device.
> 
> Since virtio_pdu_vmarshal() may be called by several active PDUs, we
> check if the transport isn't broken already to avoid printing extra
> error messages.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> v5: - use ssize_t variable in virtio_pdu_v[un]marshal()
>     - drop remaining vdev->broken check (MST suggested to discuss calling
>       virtio_error() when the device is already broken to a separate thread)
> ---
>  hw/9pfs/9p.c               |    2 +-
>  hw/9pfs/9p.h               |    2 +-
>  hw/9pfs/virtio-9p-device.c |   40 ++++++++++++++++++++++++++++++++++++----
>  hw/9pfs/xen-9p-backend.c   |    3 ++-
>  4 files changed, 40 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 96d268334865..da0d6da65b45 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1664,7 +1664,7 @@ static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
>      unsigned int niov;
>  
>      if (is_write) {
> -        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov);
> +        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size + skip);
>      } else {
>          pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size + skip);
>      }
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index aac1b0b2ce3d..d1cfeaf10e4f 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -363,7 +363,7 @@ struct V9fsTransport {
>      void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
>                                          unsigned int *pniov, size_t size);
>      void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> -                                         unsigned int *pniov);
> +                                         unsigned int *pniov, size_t size);
>      void        (*push_and_notify)(V9fsPDU *pdu);
>  };
>  
> diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> index 1a68c1622d3a..62650b0a6b99 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -146,8 +146,16 @@ static ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    ssize_t ret;
>  
> -    return v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> +    ret = v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> +    if (ret < 0) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev, "Failed to encode VirtFS reply type %d",
> +                     pdu->id + 1);
> +    }
> +    return ret;
>  }
>  
>  static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
> @@ -156,28 +164,52 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    ssize_t ret;
> +
> +    ret = v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> +    if (ret < 0) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
>  
> -    return v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> +        virtio_error(vdev, "Failed to decode VirtFS request type %d", pdu->id);
> +    }
> +    return ret;
>  }
>  
> -/* The size parameter is used by other transports. Do not drop it. */
>  static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
>                                          unsigned int *pniov, size_t size)
>  {
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    size_t buf_size = iov_size(elem->in_sg, elem->in_num);
> +
> +    if (buf_size < size) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev,
> +                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
> +                     pdu->id + 1, size, buf_size);
> +    }
>  
>      *piov = elem->in_sg;
>      *pniov = elem->in_num;
>  }
>  
>  static void virtio_init_out_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
> -                                         unsigned int *pniov)
> +                                         unsigned int *pniov, size_t size)
>  {
>      V9fsState *s = pdu->s;
>      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
>      VirtQueueElement *elem = v->elems[pdu->idx];
> +    size_t buf_size = iov_size(elem->out_sg, elem->out_num);
> +
> +    if (buf_size < size) {
> +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> +
> +        virtio_error(vdev,
> +                     "VirtFS request type %d needs %zu bytes, buffer has %zu",
> +                     pdu->id, size, buf_size);
> +    }
>  
>      *piov = elem->out_sg;
>      *pniov = elem->out_num;
> diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> index 922cc967be63..a82cf817fe45 100644
> --- a/hw/9pfs/xen-9p-backend.c
> +++ b/hw/9pfs/xen-9p-backend.c
> @@ -147,7 +147,8 @@ static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
>  
>  static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
>                                             struct iovec **piov,
> -                                           unsigned int *pniov)
> +                                           unsigned int *pniov,
> +                                           size_t size)
>  {
>      Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
>      Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];

On Fri, 30 Jun 2017 02:33:22 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jun 28, 2017 at 10:44:30PM +0200, Greg Kurz wrote:
> > The 9P protocol is transport agnostic: if the guest misconfigured the
> > buffers, the best we can do is to set the broken flag on the device.
> > 
> > Since virtio_pdu_vmarshal() may be called by several active PDUs, we
> > check if the transport isn't broken already to avoid printing extra
> > error messages.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>  
> 
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> 

Oops, I've already sent a pull request and it got merged.

Thanks for the Reviewed-by's anyway.

> > ---
> > v5: - use ssize_t variable in virtio_pdu_v[un]marshal()
> >     - drop remaining vdev->broken check (MST suggested to discuss calling
> >       virtio_error() when the device is already broken to a separate thread)
> > ---
> >  hw/9pfs/9p.c               |    2 +-
> >  hw/9pfs/9p.h               |    2 +-
> >  hw/9pfs/virtio-9p-device.c |   40 ++++++++++++++++++++++++++++++++++++----
> >  hw/9pfs/xen-9p-backend.c   |    3 ++-
> >  4 files changed, 40 insertions(+), 7 deletions(-)
> > 
> > diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> > index 96d268334865..da0d6da65b45 100644
> > --- a/hw/9pfs/9p.c
> > +++ b/hw/9pfs/9p.c
> > @@ -1664,7 +1664,7 @@ static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
> >      unsigned int niov;
> >  
> >      if (is_write) {
> > -        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov);
> > +        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size + skip);
> >      } else {
> >          pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size + skip);
> >      }
> > diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> > index aac1b0b2ce3d..d1cfeaf10e4f 100644
> > --- a/hw/9pfs/9p.h
> > +++ b/hw/9pfs/9p.h
> > @@ -363,7 +363,7 @@ struct V9fsTransport {
> >      void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> >                                          unsigned int *pniov, size_t size);
> >      void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> > -                                         unsigned int *pniov);
> > +                                         unsigned int *pniov, size_t size);
> >      void        (*push_and_notify)(V9fsPDU *pdu);
> >  };
> >  
> > diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
> > index 1a68c1622d3a..62650b0a6b99 100644
> > --- a/hw/9pfs/virtio-9p-device.c
> > +++ b/hw/9pfs/virtio-9p-device.c
> > @@ -146,8 +146,16 @@ static ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
> >      V9fsState *s = pdu->s;
> >      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
> >      VirtQueueElement *elem = v->elems[pdu->idx];
> > +    ssize_t ret;
> >  
> > -    return v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> > +    ret = v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
> > +    if (ret < 0) {
> > +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> > +
> > +        virtio_error(vdev, "Failed to encode VirtFS reply type %d",
> > +                     pdu->id + 1);
> > +    }
> > +    return ret;
> >  }
> >  
> >  static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
> > @@ -156,28 +164,52 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
> >      V9fsState *s = pdu->s;
> >      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
> >      VirtQueueElement *elem = v->elems[pdu->idx];
> > +    ssize_t ret;
> > +
> > +    ret = v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> > +    if (ret < 0) {
> > +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> >  
> > -    return v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
> > +        virtio_error(vdev, "Failed to decode VirtFS request type %d", pdu->id);
> > +    }
> > +    return ret;
> >  }
> >  
> > -/* The size parameter is used by other transports. Do not drop it. */
> >  static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
> >                                          unsigned int *pniov, size_t size)
> >  {
> >      V9fsState *s = pdu->s;
> >      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
> >      VirtQueueElement *elem = v->elems[pdu->idx];
> > +    size_t buf_size = iov_size(elem->in_sg, elem->in_num);
> > +
> > +    if (buf_size < size) {
> > +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> > +
> > +        virtio_error(vdev,
> > +                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
> > +                     pdu->id + 1, size, buf_size);
> > +    }
> >  
> >      *piov = elem->in_sg;
> >      *pniov = elem->in_num;
> >  }
> >  
> >  static void virtio_init_out_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
> > -                                         unsigned int *pniov)
> > +                                         unsigned int *pniov, size_t size)
> >  {
> >      V9fsState *s = pdu->s;
> >      V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
> >      VirtQueueElement *elem = v->elems[pdu->idx];
> > +    size_t buf_size = iov_size(elem->out_sg, elem->out_num);
> > +
> > +    if (buf_size < size) {
> > +        VirtIODevice *vdev = VIRTIO_DEVICE(v);
> > +
> > +        virtio_error(vdev,
> > +                     "VirtFS request type %d needs %zu bytes, buffer has %zu",
> > +                     pdu->id, size, buf_size);
> > +    }
> >  
> >      *piov = elem->out_sg;
> >      *pniov = elem->out_num;
> > diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> > index 922cc967be63..a82cf817fe45 100644
> > --- a/hw/9pfs/xen-9p-backend.c
> > +++ b/hw/9pfs/xen-9p-backend.c
> > @@ -147,7 +147,8 @@ static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
> >  
> >  static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
> >                                             struct iovec **piov,
> > -                                           unsigned int *pniov)
> > +                                           unsigned int *pniov,
> > +                                           size_t size)
> >  {
> >      Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
> >      Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];