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SubjectRe: [TEST FIX] Re: SSH Hangs in 2.5.59 and 2.5.55 but not 2.4.x,
Date
Hello!

> BTW, Alexey, please please explain to me how that trick made
> by tcp_trim_head() works. :-) I am talking about how it is
> setting ip_summed to CHECKSUM_HARDWARE blindly and not even
> bothering to set skb->csum correctly.

skb->csum is not used inside TCP when skb->ip_summed==CHECKSUM_HW:

void tcp_v4_send_check(struct sock *sk, struct tcphdr *th, int len,
struct sk_buff *skb)
{
struct inet_opt *inet = inet_sk(sk);

if (skb->ip_summed == CHECKSUM_HW) {
th->check = ~tcp_v4_check(th, len, inet->saddr, inet->daddr, 0);
skb->csum = offsetof(struct tcphdr, check);

And when pushing segment down to IP, it is initialized to offset of th->check.

So, it is safe to make skb->ip_summed := CHECKSUM_HW any moment when
we are lazy to recalculate checksum. Frankly speaking, it is not very good,
I was confused _a_ _lot_ when seeing wrong checksums on those bogus
zero-length packets in tcpdumps made by Christopher. But saves some
source lines.

Alexey
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