Re: [PATCH] 9p: init_in_iov_from_pdu can truncate the size

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Donnerstag, 19. Dezember 2019 01:42:51 CET Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> init_in_iov_from_pdu might not be able to allocate the full buffer size
> requested, which comes from the client and could be larger than the
> transport has available at the time of the request. Specifically, this
> can happen with read operations, with the client requesting a read up to
> the max allowed, which might be more than the transport has available at
> the time.

I haven't looked thoroughly at this yet, but that's about addressing a 
temporary, not a permanent transport buffer size limitation, right? Because if 
it was a permanent one, then probably an adjusted (lowered) msize should be 
returned on R_version response to client as well.

I wonder why I never triggered this issue, because I was experimenting with 
huge msize values for 9pfs performance checks. Was there anything specific to 
trigger this issue?

> Today the implementation of init_in_iov_from_pdu throws an error, both
> Xen and Virtio.
> 
> Instead, change the V9fsTransport interface so that the size becomes a
> pointer and can be limited by the implementation of
> init_in_iov_from_pdu.
> 
> Change both the Xen and Virtio implementations to set the size to the
> size of the buffer they managed to allocate, instead of throwing an
> error.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> CC: groug@kaod.org
> CC: anthony.perard@citrix.com
> ---
>  hw/9pfs/9p.c               | 22 +++++++++++++++-------
>  hw/9pfs/9p.h               |  2 +-
>  hw/9pfs/virtio-9p-device.c | 10 +++-------
>  hw/9pfs/xen-9p-backend.c   | 12 ++++--------
>  4 files changed, 23 insertions(+), 23 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index bdf7919abf..d6c89ce608 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1682,22 +1682,30 @@ out_nofid:
>   * with qemu_iovec_destroy().
>   */
>  static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
> -                                    size_t skip, size_t size,
> +                                    size_t skip, size_t *size,
>                                      bool is_write)
>  {
>      QEMUIOVector elem;
>      struct iovec *iov;
>      unsigned int niov;
> +    size_t alloc_size = *size + skip;
> 
>      if (is_write) {
> -        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size +
> skip); +        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov,
> alloc_size); } else {
> -        pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size +
> skip); +        pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov,
> &alloc_size); +    }
> +
> +    if (alloc_size < skip)
> +    {
> +        *size = 0;
> +    } else {
> +        *size = alloc_size - skip;
>      }
>

Code style nitpicking:  

ERROR: that open brace { should be on the previous line
#56: FILE: hw/9pfs/9p.c:1699:
+    if (alloc_size < skip)
+    {

> 
>  static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
> @@ -1722,7 +1730,7 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu,
> V9fsFidState *fidp, }
>      offset += err;
> 
> -    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, read_count, false);
> +    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, &read_count, false);
>      err = v9fs_pack(qiov_full.iov, qiov_full.niov, 0,
>                      ((char *)fidp->fs.xattr.value) + off,
>                      read_count);
> @@ -1852,7 +1860,7 @@ static void coroutine_fn v9fs_read(void *opaque)
>          QEMUIOVector qiov;
>          int32_t len;
> 
> -        v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset + 4, max_count,
> false); +        v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset + 4,
> &max_count, false); qemu_iovec_init(&qiov, qiov_full.niov);
>          do {
>              qemu_iovec_reset(&qiov);
> @@ -2085,7 +2093,7 @@ static void coroutine_fn v9fs_write(void *opaque)
>          return;
>      }
>      offset += err;
> -    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, count, true);
> +    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, &count, true);
>      trace_v9fs_write(pdu->tag, pdu->id, fid, off, count, qiov_full.niov);
> 
>      fidp = get_fid(pdu, fid);
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index 8883761b2c..50f7e21da6 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -365,7 +365,7 @@ struct V9fsTransport {
>      ssize_t     (*pdu_vunmarshal)(V9fsPDU *pdu, size_t offset, const char
> *fmt, va_list ap);
>      void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> -                                        unsigned int *pniov, size_t size);
> +                                        unsigned int *pniov, size_t *size);
> void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
> 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 775e8ff766..68873c3f5f 100644
> --- a/hw/9pfs/virtio-9p-device.c
> +++ b/hw/9pfs/virtio-9p-device.c
> @@ -145,19 +145,15 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu,
> size_t offset, }
> 
>  static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
> -                                        unsigned int *pniov, size_t size)
> +                                        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);
> +    if (buf_size < *size) {
> +        *size = buf_size;
>      }
> 
>      *piov = elem->in_sg;
> diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> index 3f54a21c76..3994a356d4 100644
> --- a/hw/9pfs/xen-9p-backend.c
> +++ b/hw/9pfs/xen-9p-backend.c
> @@ -187,7 +187,7 @@ static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
> static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU *pdu,
>                                            struct iovec **piov,
>                                            unsigned int *pniov,
> -                                          size_t size)
> +                                          size_t *size)
>  {
>      Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
>      Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];
> @@ -197,15 +197,11 @@ static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU
> *pdu, g_free(ring->sg);
> 
>      ring->sg = g_malloc0(sizeof(*ring->sg) * 2);
> -    xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, size);
> +    xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, *size);
> 
>      buf_size = iov_size(ring->sg, num);
> -    if (buf_size  < size) {
> -        xen_pv_printf(&xen_9pfs->xendev, 0, "Xen 9pfs request type %d"
> -                "needs %zu bytes, buffer has %zu\n", pdu->id, size,
> -                buf_size);
> -        xen_be_set_state(&xen_9pfs->xendev, XenbusStateClosing);
> -        xen_9pfs_disconnect(&xen_9pfs->xendev);
> +    if (buf_size  < *size) {
> +        *size = buf_size;
>      }
> 
>      *piov = ring->sg;

Best regards,
Christian Schoenebeck