while (left > 0) { len = left; /* IF: it doesn't fit, use 'mtu' - the data space left */ if (len > mtu) len = mtu; /* IF: we are not sending up to and including the packet end then align the next start on an eight byte boundary */ if (len < left) { len &= ~7; }
/* * Copy a block of the IP datagram. */ if (skb_copy_bits(skb, ptr, skb_transport_header(skb2), len)) BUG(); left -= len;
设置 IP 头的偏移和分片标志。
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/* * Fill in the new header fields. */ iph = ip_hdr(skb2); iph->frag_off = htons((offset >> 3));
if (IPCB(skb)->flags & IPSKB_FRAG_PMTU) iph->frag_off |= htons(IP_DF);
如果是第一个分片就尝试更新 IP options。
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/* ANK: dirty, but effective trick. Upgrade options only if * the segment to be fragmented was THE FIRST (otherwise, * options are already fixed) and make it ONCE * on the initial skb, so that all the following fragments * will inherit fixed options. */ if (offset == 0) ip_options_fragment(skb);
/* * Added AC : If we are fragmenting a fragment that's not the * last fragment then keep MF on each bit */ if (left > 0 || not_last_frag) iph->frag_off |= htons(IP_MF); ptr += len; offset += len;
/* * Put this fragment into the sending queue. */ iph->tot_len = htons(len + hlen);
/* Determine the position of this fragment. */ end = offset + skb->len - skb_network_offset(skb) - ihl; err = -EINVAL;
如果是最后一个 fragment，那么不应该超过 q.len，或者已经有了最后一个了，但是 end 和 q.len 不一致，所以有一些 corruption。如果检查没问题，就更新q.flasg 标记为最后一个和把 end 赋值给q.len。
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/* Is this the final fragment? */ if ((flags & IP_MF) == 0) { /* If we already have some bits beyond end * or have different end, the segment is corrupted. */ if (end < qp->q.len || ((qp->q.flags & INET_FRAG_LAST_IN) && end != qp->q.len)) goto err; qp->q.flags |= INET_FRAG_LAST_IN; qp->q.len = end;
如果不是最后一个，长度要与 8 对齐，然后更新 q.len。
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} else { if (end&7) { end &= ~7; if (skb->ip_summed != CHECKSUM_UNNECESSARY) skb->ip_summed = CHECKSUM_NONE; } if (end > qp->q.len) { /* Some bits beyond end -> corruption. */ if (qp->q.flags & INET_FRAG_LAST_IN) goto err; qp->q.len = end; }
err = -ENOMEM; if (!pskb_pull(skb, skb_network_offset(skb) + ihl)) goto err;
err = pskb_trim_rcsum(skb, end - offset); if (err) goto err;
/* Find out which fragments are in front and at the back of us * in the chain of fragments so far. We must know where to put * this fragment, right? */ prev = qp->q.fragments_tail; if (!prev || FRAG_CB(prev)->offset < offset) { next = NULL; goto found; } prev = NULL; for (next = qp->q.fragments; next != NULL; next = next->next) { if (FRAG_CB(next)->offset >= offset) break; /* bingo! */ prev = next; }
如果和前面的分组有重叠，就把重叠的部分去掉，CHECKSUM_NONE 可以使当前的校验和失效。
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if (prev) { int i = (FRAG_CB(prev)->offset + prev->len) - offset;
if (i > 0) { offset += i; err = -EINVAL; if (end <= offset) goto err; err = -ENOMEM; if (!pskb_pull(skb, i)) goto err; if (skb->ip_summed != CHECKSUM_UNNECESSARY) skb->ip_summed = CHECKSUM_NONE; } }
然后向后检查有没有重叠，并且把重叠的部分去掉，如果重叠的部分比 next 本身还要大，直接把 next 删掉。
while (next && FRAG_CB(next)->offset < end) { int i = end - FRAG_CB(next)->offset; /* overlap is 'i' bytes */
if (i < next->len) { /* Eat head of the next overlapped fragment * and leave the loop. The next ones cannot overlap. */ if (!pskb_pull(next, i)) goto err; FRAG_CB(next)->offset += i; qp->q.meat -= i; if (next->ip_summed != CHECKSUM_UNNECESSARY) next->ip_summed = CHECKSUM_NONE; break; } else { struct sk_buff *free_it = next;
/* Old fragment is completely overridden with * new one drop it. */ next = next->next;
if (prev) prev->next = next; else qp->q.fragments = next;