456 lines
13 KiB
C
456 lines
13 KiB
C
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// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* Syncookies implementation for the Linux kernel
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*
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* Copyright (C) 1997 Andi Kleen
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* Based on ideas by D.J.Bernstein and Eric Schenk.
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*/
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#include <linux/tcp.h>
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#include <linux/siphash.h>
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#include <linux/kernel.h>
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#include <linux/export.h>
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#include <net/secure_seq.h>
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#include <net/tcp.h>
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#include <net/route.h>
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static siphash_aligned_key_t syncookie_secret[2];
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#define COOKIEBITS 24 /* Upper bits store count */
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#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
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/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
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* stores TCP options:
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*
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* MSB LSB
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* | 31 ... 6 | 5 | 4 | 3 2 1 0 |
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* | Timestamp | ECN | SACK | WScale |
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*
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* When we receive a valid cookie-ACK, we look at the echoed tsval (if
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* any) to figure out which TCP options we should use for the rebuilt
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* connection.
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*
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* A WScale setting of '0xf' (which is an invalid scaling value)
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* means that original syn did not include the TCP window scaling option.
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*/
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#define TS_OPT_WSCALE_MASK 0xf
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#define TS_OPT_SACK BIT(4)
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#define TS_OPT_ECN BIT(5)
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/* There is no TS_OPT_TIMESTAMP:
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* if ACK contains timestamp option, we already know it was
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* requested/supported by the syn/synack exchange.
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*/
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#define TSBITS 6
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#define TSMASK (((__u32)1 << TSBITS) - 1)
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static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
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u32 count, int c)
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{
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net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
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return siphash_4u32((__force u32)saddr, (__force u32)daddr,
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(__force u32)sport << 16 | (__force u32)dport,
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count, &syncookie_secret[c]);
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}
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/*
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* when syncookies are in effect and tcp timestamps are enabled we encode
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* tcp options in the lower bits of the timestamp value that will be
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* sent in the syn-ack.
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* Since subsequent timestamps use the normal tcp_time_stamp value, we
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* must make sure that the resulting initial timestamp is <= tcp_time_stamp.
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*/
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u64 cookie_init_timestamp(struct request_sock *req, u64 now)
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{
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struct inet_request_sock *ireq;
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u32 ts, ts_now = tcp_ns_to_ts(now);
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u32 options = 0;
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ireq = inet_rsk(req);
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options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
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if (ireq->sack_ok)
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options |= TS_OPT_SACK;
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if (ireq->ecn_ok)
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options |= TS_OPT_ECN;
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ts = ts_now & ~TSMASK;
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ts |= options;
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if (ts > ts_now) {
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ts >>= TSBITS;
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ts--;
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ts <<= TSBITS;
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ts |= options;
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}
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return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ);
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}
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static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
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__be16 dport, __u32 sseq, __u32 data)
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{
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/*
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* Compute the secure sequence number.
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* The output should be:
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* HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
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* + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
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* Where sseq is their sequence number and count increases every
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* minute by 1.
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* As an extra hack, we add a small "data" value that encodes the
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* MSS into the second hash value.
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*/
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u32 count = tcp_cookie_time();
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return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
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sseq + (count << COOKIEBITS) +
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((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
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& COOKIEMASK));
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}
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/*
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* This retrieves the small "data" value from the syncookie.
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* If the syncookie is bad, the data returned will be out of
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* range. This must be checked by the caller.
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*
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* The count value used to generate the cookie must be less than
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* MAX_SYNCOOKIE_AGE minutes in the past.
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* The return value (__u32)-1 if this test fails.
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*/
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static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
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__be16 sport, __be16 dport, __u32 sseq)
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{
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u32 diff, count = tcp_cookie_time();
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/* Strip away the layers from the cookie */
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cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
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/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
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diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
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if (diff >= MAX_SYNCOOKIE_AGE)
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return (__u32)-1;
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return (cookie -
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cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
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& COOKIEMASK; /* Leaving the data behind */
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}
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/*
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* MSS Values are chosen based on the 2011 paper
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* 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
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* Values ..
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* .. lower than 536 are rare (< 0.2%)
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* .. between 537 and 1299 account for less than < 1.5% of observed values
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* .. in the 1300-1349 range account for about 15 to 20% of observed mss values
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* .. exceeding 1460 are very rare (< 0.04%)
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*
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* 1460 is the single most frequently announced mss value (30 to 46% depending
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* on monitor location). Table must be sorted.
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*/
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static __u16 const msstab[] = {
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536,
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1300,
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1440, /* 1440, 1452: PPPoE */
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1460,
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};
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/*
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* Generate a syncookie. mssp points to the mss, which is returned
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* rounded down to the value encoded in the cookie.
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*/
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u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
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u16 *mssp)
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{
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int mssind;
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const __u16 mss = *mssp;
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for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
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if (mss >= msstab[mssind])
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break;
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*mssp = msstab[mssind];
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return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
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th->source, th->dest, ntohl(th->seq),
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mssind);
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}
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EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
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__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
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{
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const struct iphdr *iph = ip_hdr(skb);
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const struct tcphdr *th = tcp_hdr(skb);
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return __cookie_v4_init_sequence(iph, th, mssp);
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}
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/*
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* Check if a ack sequence number is a valid syncookie.
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* Return the decoded mss if it is, or 0 if not.
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*/
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int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
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u32 cookie)
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{
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__u32 seq = ntohl(th->seq) - 1;
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__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
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th->source, th->dest, seq);
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return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
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}
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EXPORT_SYMBOL_GPL(__cookie_v4_check);
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struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
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struct request_sock *req,
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struct dst_entry *dst, u32 tsoff)
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{
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struct inet_connection_sock *icsk = inet_csk(sk);
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struct sock *child;
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bool own_req;
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child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
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NULL, &own_req);
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if (child) {
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refcount_set(&req->rsk_refcnt, 1);
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tcp_sk(child)->tsoffset = tsoff;
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sock_rps_save_rxhash(child, skb);
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if (rsk_drop_req(req)) {
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reqsk_put(req);
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return child;
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}
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if (inet_csk_reqsk_queue_add(sk, req, child))
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return child;
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bh_unlock_sock(child);
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sock_put(child);
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}
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__reqsk_free(req);
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return NULL;
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}
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EXPORT_SYMBOL(tcp_get_cookie_sock);
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/*
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* when syncookies are in effect and tcp timestamps are enabled we stored
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* additional tcp options in the timestamp.
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* This extracts these options from the timestamp echo.
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*
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* return false if we decode a tcp option that is disabled
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* on the host.
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*/
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bool cookie_timestamp_decode(const struct net *net,
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struct tcp_options_received *tcp_opt)
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{
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/* echoed timestamp, lowest bits contain options */
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u32 options = tcp_opt->rcv_tsecr;
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if (!tcp_opt->saw_tstamp) {
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tcp_clear_options(tcp_opt);
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return true;
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}
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if (!READ_ONCE(net->ipv4.sysctl_tcp_timestamps))
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return false;
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tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
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if (tcp_opt->sack_ok && !READ_ONCE(net->ipv4.sysctl_tcp_sack))
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return false;
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if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
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return true; /* no window scaling */
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tcp_opt->wscale_ok = 1;
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tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
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return READ_ONCE(net->ipv4.sysctl_tcp_window_scaling) != 0;
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}
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EXPORT_SYMBOL(cookie_timestamp_decode);
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bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
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const struct net *net, const struct dst_entry *dst)
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{
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bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
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if (!ecn_ok)
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return false;
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if (READ_ONCE(net->ipv4.sysctl_tcp_ecn))
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return true;
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return dst_feature(dst, RTAX_FEATURE_ECN);
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}
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EXPORT_SYMBOL(cookie_ecn_ok);
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struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
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const struct tcp_request_sock_ops *af_ops,
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struct sock *sk,
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struct sk_buff *skb)
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{
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struct tcp_request_sock *treq;
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struct request_sock *req;
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if (sk_is_mptcp(sk))
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req = mptcp_subflow_reqsk_alloc(ops, sk, false);
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else
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req = inet_reqsk_alloc(ops, sk, false);
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if (!req)
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return NULL;
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treq = tcp_rsk(req);
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/* treq->af_specific might be used to perform TCP_MD5 lookup */
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treq->af_specific = af_ops;
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treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
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#if IS_ENABLED(CONFIG_MPTCP)
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treq->is_mptcp = sk_is_mptcp(sk);
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if (treq->is_mptcp) {
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int err = mptcp_subflow_init_cookie_req(req, sk, skb);
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if (err) {
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reqsk_free(req);
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return NULL;
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}
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}
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#endif
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return req;
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}
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EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
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/* On input, sk is a listener.
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* Output is listener if incoming packet would not create a child
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* NULL if memory could not be allocated.
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*/
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struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
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{
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struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
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struct tcp_options_received tcp_opt;
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struct inet_request_sock *ireq;
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struct tcp_request_sock *treq;
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struct tcp_sock *tp = tcp_sk(sk);
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const struct tcphdr *th = tcp_hdr(skb);
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__u32 cookie = ntohl(th->ack_seq) - 1;
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struct sock *ret = sk;
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struct request_sock *req;
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int full_space, mss;
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struct rtable *rt;
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__u8 rcv_wscale;
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struct flowi4 fl4;
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u32 tsoff = 0;
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if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_syncookies) ||
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!th->ack || th->rst)
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goto out;
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if (tcp_synq_no_recent_overflow(sk))
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goto out;
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mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
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if (mss == 0) {
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__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
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goto out;
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}
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__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
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/* check for timestamp cookie support */
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memset(&tcp_opt, 0, sizeof(tcp_opt));
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tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
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if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
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tsoff = secure_tcp_ts_off(sock_net(sk),
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ip_hdr(skb)->daddr,
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ip_hdr(skb)->saddr);
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tcp_opt.rcv_tsecr -= tsoff;
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}
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if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
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goto out;
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ret = NULL;
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req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops,
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&tcp_request_sock_ipv4_ops, sk, skb);
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if (!req)
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goto out;
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ireq = inet_rsk(req);
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treq = tcp_rsk(req);
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treq->rcv_isn = ntohl(th->seq) - 1;
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treq->snt_isn = cookie;
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treq->ts_off = 0;
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treq->txhash = net_tx_rndhash();
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req->mss = mss;
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ireq->ir_num = ntohs(th->dest);
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ireq->ir_rmt_port = th->source;
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sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
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sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
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ireq->ir_mark = inet_request_mark(sk, skb);
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ireq->snd_wscale = tcp_opt.snd_wscale;
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ireq->sack_ok = tcp_opt.sack_ok;
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ireq->wscale_ok = tcp_opt.wscale_ok;
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ireq->tstamp_ok = tcp_opt.saw_tstamp;
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req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
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treq->snt_synack = 0;
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treq->tfo_listener = false;
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if (IS_ENABLED(CONFIG_SMC))
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ireq->smc_ok = 0;
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ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
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/* We throwed the options of the initial SYN away, so we hope
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* the ACK carries the same options again (see RFC1122 4.2.3.8)
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*/
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RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));
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||
|
|
||
|
if (security_inet_conn_request(sk, skb, req)) {
|
||
|
reqsk_free(req);
|
||
|
goto out;
|
||
|
}
|
||
|
|
||
|
req->num_retrans = 0;
|
||
|
|
||
|
/*
|
||
|
* We need to lookup the route here to get at the correct
|
||
|
* window size. We should better make sure that the window size
|
||
|
* hasn't changed since we received the original syn, but I see
|
||
|
* no easy way to do this.
|
||
|
*/
|
||
|
flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
|
||
|
RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
|
||
|
inet_sk_flowi_flags(sk),
|
||
|
opt->srr ? opt->faddr : ireq->ir_rmt_addr,
|
||
|
ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
|
||
|
security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
|
||
|
rt = ip_route_output_key(sock_net(sk), &fl4);
|
||
|
if (IS_ERR(rt)) {
|
||
|
reqsk_free(req);
|
||
|
goto out;
|
||
|
}
|
||
|
|
||
|
/* Try to redo what tcp_v4_send_synack did. */
|
||
|
req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
|
||
|
/* limit the window selection if the user enforce a smaller rx buffer */
|
||
|
full_space = tcp_full_space(sk);
|
||
|
if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
|
||
|
(req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
|
||
|
req->rsk_window_clamp = full_space;
|
||
|
|
||
|
tcp_select_initial_window(sk, full_space, req->mss,
|
||
|
&req->rsk_rcv_wnd, &req->rsk_window_clamp,
|
||
|
ireq->wscale_ok, &rcv_wscale,
|
||
|
dst_metric(&rt->dst, RTAX_INITRWND));
|
||
|
|
||
|
ireq->rcv_wscale = rcv_wscale;
|
||
|
ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
|
||
|
|
||
|
ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
|
||
|
/* ip_queue_xmit() depends on our flow being setup
|
||
|
* Normal sockets get it right from inet_csk_route_child_sock()
|
||
|
*/
|
||
|
if (ret)
|
||
|
inet_sk(ret)->cork.fl.u.ip4 = fl4;
|
||
|
out: return ret;
|
||
|
}
|