Re: [Qemu-devel] [PATCH V2] virtio-net: unbreak any layout

[Jason Wang <jasowang@redhat.com>]

On 07/16/2015 03:29 PM, Michael S. Tsirkin wrote:
> On Thu, Jul 16, 2015 at 02:49:48PM +0800, Jason Wang wrote:
>> > 
>> > 
>> > On 07/16/2015 02:42 PM, Michael S. Tsirkin wrote:
>>> > > On Wed, Jul 15, 2015 at 03:56:07PM +0800, Jason Wang wrote:
>>>> > >> Commit 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
>>>> > >> ("virtio-net: byteswap virtio-net header") breaks any layout by
>>>> > >> requiring out_sg[0].iov_len >= n->guest_hdr_len. Fixing this by
>>>> > >> copying header to temporary buffer if swap is needed, and then use
>>>> > >> this buffer as part of out_sg.
>>>> > >>
>>>> > >> Fixes 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
>>>> > >> ("virtio-net: byteswap virtio-net header")
>>>> > >> Cc: qemu-stable@nongnu.org
>>>> > >> Cc: clg@fr.ibm.com
>>>> > >> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>> > >> ---
>>>> > >> Changes from V1:
>>>> > >> - avoid header copying if there's no need to do header swap
>>>> > >> - don't write the header back
>>>> > >> ---
>>>> > >>  hw/net/virtio-net.c               | 17 ++++++++++++++---
>>>> > >>  include/hw/virtio/virtio-access.h |  9 +++++++++
>>>> > >>  2 files changed, 23 insertions(+), 3 deletions(-)
>>>> > >>
>>>> > >> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>>>> > >> index e3c2db3..12322bd 100644
>>>> > >> --- a/hw/net/virtio-net.c
>>>> > >> +++ b/hw/net/virtio-net.c
>>>> > >> @@ -1142,7 +1142,8 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
>>>> > >>          ssize_t ret, len;
>>>> > >>          unsigned int out_num = elem.out_num;
>>>> > >>          struct iovec *out_sg = &elem.out_sg[0];
>>>> > >> -        struct iovec sg[VIRTQUEUE_MAX_SIZE];
>>>> > >> +        struct iovec sg[VIRTQUEUE_MAX_SIZE], sg2[VIRTQUEUE_MAX_SIZE];
>>>> > >> +        struct virtio_net_hdr hdr;
>>>> > >>  
>>>> > >>          if (out_num < 1) {
>>>> > >>              error_report("virtio-net header not in first element");
>>>> > >> @@ -1150,11 +1151,21 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
>>>> > >>          }
>>>> > >>  
>>>> > >>          if (n->has_vnet_hdr) {
>>>> > >> -            if (out_sg[0].iov_len < n->guest_hdr_len) {
>>>> > >> +            if (iov_size(out_sg, out_num) < n->guest_hdr_len) {
>>>> > >>                  error_report("virtio-net header incorrect");
>>>> > >>                  exit(1);
>>>> > >>              }
>>> > > this scans the iov unnecessarily. How about checking return
>>> > > code from iov_to_buf instead?
>> > 
>> > Looks ok since hdr is very short anyway.
> but iov_size scans the whole iov, not just the header.
>

Right.