vmxnet3 驱动丢包问题，具体原因

看代码udp报文, L4头大小竟然也按tcp算, 使在 (dataSize)<(tcpHead-udpHead) 时 (headSize)>(skb->len), 

导致后面pskb_may_pull()中判断异常---丢包。(skb->len是实际报文[头+数据]大小，应该大于headSize)

补丁修正了udp时l4_hdr_size赋值，并在后面做了防御性修改 '= min(.,.);' 

ctx->l4_hdr_size =
compat_skb_tcp_header(skb)->doff * 4;
else if (iph->protocol == IPPROTO_UDP)
- /*
-  * Use TCP header size so that bytes to
-  * copied are more than the minimum
-  * required by the backend.
-  */
ctx->l4_hdr_size =
- sizeof(struct tcphdr);
+ sizeof(struct udphdr);
else
ctx->l4_hdr_size = 0;
} else {
/* for simplicity, don't copy L4 headers */
ctx->l4_hdr_size = 0;
}

- ctx->copy_size = ctx->eth_ip_hdr_size +
-                  ctx->l4_hdr_size;
+ /* make sure that copy size is not exceeding pkt len */
+ ctx->copy_size = min((ctx->eth_ip_hdr_size +
+                       ctx->l4_hdr_size), skb->len);

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