Re: [PATCH 6/7] virtio-net: Copy received header to buffer

[Akihiko Odaki]

On 2024/10/21 17:22, Jason Wang wrote:
On Mon, Oct 21, 2024 at 4:21 PM Jason Wang <jasow...@redhat.com> wrote:

On Sun, Sep 15, 2024 at 9:07 AM Akihiko Odaki <akihiko.od...@daynix.com> wrote:

receive_header() used to cast the const qualifier of the pointer to the
received packet away to modify the header. Avoid this by copying the
received header to buffer.

Signed-off-by: Akihiko Odaki <akihiko.od...@daynix.com>
---
  hw/net/virtio-net.c | 85 +++++++++++++++++++++++++++++------------------------
  1 file changed, 46 insertions(+), 39 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 3fc1d10cb9e0..ca4e22344f78 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1685,41 +1685,44 @@ static void virtio_net_hdr_swap(VirtIODevice *vdev, 
struct virtio_net_hdr *hdr)
   * cache.
   */
  static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
-                                        uint8_t *buf, size_t size)
+                                        size_t *hdr_len, const uint8_t *buf,
+                                        size_t buf_size, size_t *buf_offset)
  {
      size_t csum_size = ETH_HLEN + sizeof(struct ip_header) +
                         sizeof(struct udp_header);

+    buf += *buf_offset;
+    buf_size -= *buf_offset;
+
      if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
-        (size >= csum_size && size < 1500) && /* normal sized MTU */
+        (buf_size >= csum_size && buf_size < 1500) && /* normal sized MTU */
          (buf[12] == 0x08 && buf[13] == 0x00) && /* ethertype == IPv4 */
          (buf[23] == 17) && /* ip.protocol == UDP */
          (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */
-        net_checksum_calculate(buf, size, CSUM_UDP);
+        memcpy((uint8_t *)hdr + *hdr_len, buf, csum_size);
+        net_checksum_calculate((uint8_t *)hdr + *hdr_len, csum_size, CSUM_UDP);
          hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
+        *hdr_len += csum_size;
+        *buf_offset += csum_size;
      }
  }

-static void receive_header(VirtIONet *n, const struct iovec *iov, int iov_cnt,
-                           const void *buf, size_t size)
+static size_t receive_header(VirtIONet *n, struct virtio_net_hdr *hdr,
+                             const void *buf, size_t buf_size,
+                             size_t *buf_offset)
  {
-    if (n->has_vnet_hdr) {
-        /* FIXME this cast is evil */
-        void *wbuf = (void *)buf;
-        work_around_broken_dhclient(wbuf, wbuf + n->host_hdr_len,
-                                    size - n->host_hdr_len);
+    size_t hdr_len = n->guest_hdr_len;

-        if (n->needs_vnet_hdr_swap) {
-            virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
-        }
-        iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
-    } else {
-        struct virtio_net_hdr hdr = {
-            .flags = 0,
-            .gso_type = VIRTIO_NET_HDR_GSO_NONE
-        };
-        iov_from_buf(iov, iov_cnt, 0, &hdr, sizeof hdr);
+    memcpy(hdr, buf, sizeof(struct virtio_net_hdr));
+
+    *buf_offset = n->host_hdr_len;
+    work_around_broken_dhclient(hdr, &hdr_len, buf, buf_size, buf_offset);
+
+    if (n->needs_vnet_hdr_swap) {
+        virtio_net_hdr_swap(VIRTIO_DEVICE(n), hdr);
      }
+
+    return hdr_len;
  }

  static int receive_filter(VirtIONet *n, const uint8_t *buf, int size)
@@ -1887,6 +1890,13 @@ static int virtio_net_process_rss(NetClientState *nc, 
const uint8_t *buf,
      return (index == new_index) ? -1 : new_index;
  }

+typedef struct Header {
+    struct virtio_net_hdr_v1_hash virtio_net;
+    struct eth_header eth;
+    struct ip_header ip;
+    struct udp_header udp;
+} Header;

I don't see too much value in having this structure especially
considering eth/ip/udp is not even used.

Any reason we can simply use an array as a buffer in virtio_net_receive_rcu()?

We still need to access virtio_net and it needs to be properly aligned. 
We can still make the latter part an array, but it does not look nicer:

typedef struct Header {
    struct virtio_net_hdr_v1_hash virtio_net;
    uint8_t eth[sizeof(struct eth_header) + sizeof(struct ip_header) + 
sizeof(struct udp_header)];
} Header;


Btw, since the code was for ancient guests and userspace, I wonder how
it is tested.

I didn't test with dhclient, but I tested with UDP packets synthesized 
with iperf.

Regards,
Akihiko Odaki