有关skb_copy_expand函数

lawrence2013

我需要在IP协议头和传输层协议头之间加一个新的协议头，我用skb_copy_expand来扩展skb。但似乎用了memcpy以后，tcp的payload发送就有问题了，对端能收到数据包，但payload接收不正确的。但其他协议，例如UDP和ICMP，通讯都正常。只要不执行memcpy操作，对端接收就没问题。请问我改如何解决这个问题？我明白skb_copy_expand本身就执行了相关的copy操作，但我希望把内容调整到所希望的位置，似乎只能再执行memcpy？我调用skb_copy_expand的相关代码如下。谢谢！

        tskb = skb_copy_expand(nskb, skb_headroom(nskb), skb_tailroom(nskb) + new_len, GFP_ATOMIC);
        if(tskb == NULL){
                printk("Expension error\n");
        }

        memcpy(tskb->data, nskb->data, iph_len);
        memcpy(tskb->data + iph_len + new_len, nskb->data + iph_len, nskb->len - iph_len);

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回复 1# lawrence2013

我需要在IP协议头和传输层协议头之间加一个新的协议头，我用skb_copy_expand来扩展skb。但似乎用了memcpy以后，tcp的payload发送就有问题了，对端能收到数据包，但payload接收不正确的。但其他协议，例如UDP和ICMP，通讯都正常。只要不执行memcpy操作，对端接收就没问题。请问我改如何解决这个问题？我明白skb_copy_expand本身就执行了相关的copy操作，但我希望把内容调整到所希望的位置，似乎只能再执行memcpy？我调用skb_copy_expand的相关代码如下。谢谢！

        tskb = skb_copy_expand(nskb, skb_headroom(nskb), skb_tailroom(nskb) + new_len, GFP_ATOMIC);
        if(tskb == NULL){
                printk("Expension error\n");
        }

        memcpy(tskb->data, nskb->data, iph_len);
        memcpy(tskb->data + iph_len + new_len, nskb->data + iph_len, nskb->len - iph_len);

Maybe need to check skb_put() function.





   

lawrence2013

Thanks for the suggestion. I tried to use skb_put, but it does not work. The problem is caused by the following sentence: memcpy(tskb->data + iph_len + new_len, nskb->data + iph_len, nskb->len - iph_len), which does not correctly copy the application payload. It is a bit strange since there is no problem for the UDP and ICMP packets, even the TCP packets without PSH flag. May I know if you have any suggestion to fix it? Thank you very much!

firkraag

Have you modified the 'Total Length','Protocol' and the 'Header Checksum' fields in the ip header?

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回复 3# lawrence2013
Please have a look http://bbs.chinaunix.net/thread-1960367-1-1.html by Godbach!

   

lawrence2013

回复 4# firkraag

我修改了相关信息了，似乎问题是memcpy无法把完整内容拷贝出来。

lawrence2013

回复 5# 瀚海书香

Thanks for sharing. Yes, skb_copy_expand() can fix the problem when the packer headers are not extended. But I did not have a way to efficiently copy the payloads when the new packets have the extended headers. Thanks again!