e1000e: Always copy ethernet header

e1000e_receive_internal() used to check the iov length to determine
copy the iovs to a contiguous buffer, but the check is flawed in two
ways:
- It does not ensure that iovcnt > 0.
- It does not take virtio-net header into consideration.

The size of this copy is just 18 octets, which can be even less than
the code size required for checks. This (wrong) optimization is probably
not worth so just remove it.

Fixes: 6f3fbe4 ("net: Introduce e1000e device emulation")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/e1000e_core.c

@@ -1686,12 +1686,9 @@ static ssize_t
 e1000e_receive_internal(E1000ECore *core, const struct iovec *iov, int iovcnt,
                         bool has_vnet)
 {
-    static const int maximum_ethernet_hdr_len = (ETH_HLEN + 4);
-
     uint32_t n = 0;
-    uint8_t min_buf[ETH_ZLEN];
+    uint8_t buf[ETH_ZLEN];
     struct iovec min_iov;
-    uint8_t *filter_buf;
     size_t size, orig_size;
     size_t iov_ofs = 0;
     E1000E_RxRing rxr;
@@ -1714,24 +1711,21 @@ e1000e_receive_internal(E1000ECore *core, const struct iovec *iov, int iovcnt,
         net_rx_pkt_unset_vhdr(core->rx_pkt);
     }
 
-    filter_buf = iov->iov_base + iov_ofs;
     orig_size = iov_size(iov, iovcnt);
     size = orig_size - iov_ofs;
 
     /* Pad to minimum Ethernet frame length */
-    if (size < sizeof(min_buf)) {
-        iov_to_buf(iov, iovcnt, iov_ofs, min_buf, size);
-        memset(&min_buf[size], 0, sizeof(min_buf) - size);
+    if (size < sizeof(buf)) {
+        iov_to_buf(iov, iovcnt, iov_ofs, buf, size);
+        memset(&buf[size], 0, sizeof(buf) - size);
         e1000x_inc_reg_if_not_full(core->mac, RUC);
-        min_iov.iov_base = filter_buf = min_buf;
-        min_iov.iov_len = size = sizeof(min_buf);
+        min_iov.iov_base = buf;
+        min_iov.iov_len = size = sizeof(buf);
         iovcnt = 1;
         iov = &min_iov;
         iov_ofs = 0;
-    } else if (iov->iov_len < maximum_ethernet_hdr_len) {
-        /* This is very unlikely, but may happen. */
-        iov_to_buf(iov, iovcnt, iov_ofs, min_buf, maximum_ethernet_hdr_len);
-        filter_buf = min_buf;
+    } else {
+        iov_to_buf(iov, iovcnt, iov_ofs, buf, ETH_HLEN + 4);
     }
 
     /* Discard oversized packets if !LPE and !SBP. */
@@ -1740,9 +1734,9 @@ e1000e_receive_internal(E1000ECore *core, const struct iovec *iov, int iovcnt,
     }
 
     net_rx_pkt_set_packet_type(core->rx_pkt,
-        get_eth_packet_type(PKT_GET_ETH_HDR(filter_buf)));
+        get_eth_packet_type(PKT_GET_ETH_HDR(buf)));
 
-    if (!e1000e_receive_filter(core, filter_buf, size)) {
+    if (!e1000e_receive_filter(core, buf, size)) {
         trace_e1000e_rx_flt_dropped();
         return orig_size;
     }