TCP connection repair
Subsystems (3)
cover TCP connection repair
Hi.
Here's another approach to transparent TCP connection hijacking (previous one was here [1]). This one is _much_ more straightforward and patches TCP code a little bit more. I'd like to have comments on the idea itself early, so here is the very-very basic functionality, just to demonstrate the concept.
The idea briefly is -- introduce the "repair" mode of a TCP socket. In this mode any API call to the socket (bind/connect/sendmsg/etc.) does not result in packets sent over the network, but instead modifies the socket locally in an expected way.
I.e., the connect() in the repair mode assigns peer's credentials to the sock and just turns one into the connected state without issuing SYN-s or whatever. The bind() call on the socket under repair forcibly binds one to the desired IP and port ignoring any (potential) local conflicts (just like if everybody else has the SO_REUSEADDR set). The sendmsg() just queues data for transmission (however, this is not implemented in this set, need more thinking on how to make it symmetrical to the receive queue), etc.
The protocol sequences are also to be get/set, so a couple of another sockopts are introduced for this.
I think, that it makes sense to have this ability in a form of non-obscure API, since the connection migration can be used not only by checkpoint/restore project, but also by various load balancing solutions. E.g., a server can accept the connection, read the app-level header out of the stream, take the balancing decision based on _it_ (rather than just TCP and/or IP header) and then pass the existing connection to another host.
Of course, switching socket to the repair mode is only allowed for CAP_SYS_ADMIN.
What do you think?
With the set provided you can do only simple tricks, e.g. transparently passing a connection between echo server and telnet to another echo server task and proceed with echoing messages.
Thanks,
Pavel
PATCH 1/2 tcp: Move code around
This is just the preparation patch, which makes the needed for TCP repair code ready for use.
--- include/net/tcp.h | 4 +++ net/ipv4/tcp_input.c | 68 ++++++++++++++++++++++++++++--------------------- net/ipv4/tcp_output.c | 10 ++++++- 3 files changed, 51 insertions(+), 31 deletions(-)
include/net/tcp.h +4 -0
@@ -434,6 +434,8 @@ extern struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst, struct request_values *rvp);extern int tcp_disconnect (struct sock *sk,int flags); +void tcp_connect_init (struct sock *sk); +void tcp_finish_connect (struct sock *sk,struct sk_buff *skb);/* From syncookies.c */ extern __u32 syncookie_secret[2 ][16 -4 +SHA_DIGEST_WORDS ];
@@ -608,6 +610,8 @@ static inline u32 tcp_receive_window(const struct tcp_sock *tp)*/ extern u32 __tcp_select_window (struct sock *sk); +void tcp_send_window_probe (struct sock *sk); +/* TCP timestamps are only 32-bits, this causes a slight * complication on 64-bit systems since we store a snapshot * of jiffies in the buffer control blocks below. We decided
net/ipv4/tcp_input.c +39 -29
81 lines not on this page · raw message
net/ipv4/tcp_output.c +7 -2
@@ -2561,7 +2561,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,EXPORT_SYMBOL (tcp_make_synack);/* Do all connect socket setups that can be done AF independent. */ -static void tcp_connect_init (struct sock *sk) +void tcp_connect_init (struct sock *sk) {const struct dst_entry *dst =__sk_dst_get (sk);struct tcp_sock *tp =tcp_sk (sk);
@@ -2619,6 +2619,7 @@ static void tcp_connect_init(struct sock *sk) tp->rcv_nxt =0 ; tp->rcv_wup =0 ; tp->copied_seq =0 ; + tp->snd_nxt = tp->write_seq;inet_csk (sk)->icsk_rto =TCP_TIMEOUT_INIT ;inet_csk (sk)->icsk_retransmits =0 ;
@@ -2641,7 +2642,6 @@ int tcp_connect(struct sock *sk)/* Reserve space for headers. */ skb_reserve (buff,MAX_TCP_HEADER ); - tp->snd_nxt = tp->write_seq;tcp_init_nondata_skb (buff, tp->write_seq++,TCPHDR_SYN );TCP_ECN_send_syn (sk, buff);
@@ -2790,6 +2790,11 @@ static int tcp_xmit_probe_skb(struct sock *sk, int urgent)return tcp_transmit_skb (sk, skb,0 ,GFP_ATOMIC ); } +void tcp_send_window_probe (struct sock *sk) +{ +tcp_xmit_probe_skb (sk,0 ); +} +/* Initiate keepalive or window probe from timer. */ int tcp_write_wakeup (struct sock *sk) {
Signature · 1 line
1.5.5.6
PATCH 2/2 tcp: Initial repair mode
In reply to Pavel EmelyanovThis includes (according the the previous description):
* TCP_REPAIR sockoption
* Sequences sockoptions
* Ability to forcibly bind a socket to a port
* Immediate connect modification
* Silent close modification
--- include/linux/tcp.h | 6 ++++- net/ipv4/inet_connection_sock.c | 3 ++ net/ipv4/tcp.c | 43 ++++++++++++++++++++++++++++++++++++++- net/ipv4/tcp_ipv4.c | 19 ++++++++++++++-- net/ipv4/tcp_output.c | 1 - 5 files changed, 66 insertions(+), 6 deletions(-)
include/linux/tcp.h +5 -1
@@ -106,6 +106,9 @@ enum {#define TCP_THIN_LINEAR_TIMEOUTS 16/* Use linear timeouts for thin streams*/ #define TCP_THIN_DUPACK 17/* Fast retrans. after 1 dupack */ #define TCP_USER_TIMEOUT 18/* How long for loss retry before timeout */ +#define TCP_REPAIR 19/* TCP sock is under repair right now */ +#define TCP_WRITE_SEQ 20 +#define TCP_RCV_NXT 21/* for TCP_INFO socket option */ #define TCPI_OPT_TIMESTAMPS 1
@@ -353,7 +356,8 @@ struct tcp_sock { u8 nonagle : 4,/* Disable Nagle algorithm? */ thin_lto : 1,/* Use linear timeouts for thin streams */ thin_dupack : 1,/* Fast retransmit on first dupack */ - unused : 2; + repair : 1, + unused : 1;/* RTT measurement */ u32 srtt;/* smoothed round trip time << 3 */
net/ipv4/inet_connection_sock.c +3 -0
@@ -172,6 +172,9 @@ have_snum: goto tb_not_found; tb_found:if (!hlist_empty (&tb->owners)) { +if (sk->sk_reuse ==2 ) + goto success; +if (tb->fastreuse >0 && sk->sk_reuse && sk->sk_state !=TCP_LISTEN && smallest_size ==-1 ) {
net/ipv4/tcp.c +44 -1
69 lines not on this page · raw message
net/ipv4/tcp_ipv4.c +16 -3
@@ -137,6 +137,14 @@ int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp) }EXPORT_SYMBOL_GPL (tcp_twsk_unique); +static int tcp_repair_connect (struct sock *sk) +{ +tcp_connect_init (sk); +tcp_finish_connect (sk,NULL ); + +return 0 ; +} +/* This will initiate an outgoing connection. */ int tcp_v4_connect (struct sock *sk,struct sockaddr *uaddr,int addr_len) {
@@ -195,7 +203,8 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)/* Reset inherited state */ tp->rx_opt.ts_recent =0 ; tp->rx_opt.ts_recent_stamp =0 ; - tp->write_seq =0 ; +if (!tp->repair) + tp->write_seq =0 ; } if (tcp_death_row.sysctl_tw_recycle &&
@@ -246,7 +255,7 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len) sk->sk_gso_type =SKB_GSO_TCPV4 ;sk_setup_caps (sk, &rt->dst); -if (!tp->write_seq) +if (!tp->write_seq && !tp->repair) tp->write_seq = secure_tcp_sequence_number(inet->inet_saddr, inet->inet_daddr, inet->inet_sport,
@@ -254,7 +263,11 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len) inet->inet_id = tp->write_seq ^ jiffies; - err =tcp_connect (sk); +if (likely (!tp->repair)) + err =tcp_connect (sk); +else + err =tcp_repair_connect (sk); + rt =NULL ;if (err) goto failure;
Signature · 1 line
1.5.5.6
David Miller
maintainerRe: [PATCH 2/2] tcp: Initial repair mode
From: Pavel Emelyanov <[email protected]>
Date: Wed, 29 Feb 2012 19:14:22 +0400
Quoted 9 lines
This includes (according the the previous description):
*
TCP_REPAIRsockoption
* Sequences sockoptions
* Ability to forcibly bind a socket to a port
* Immediate connect modification
* Silent close modificationSigned-off-by Pavel Emelyanov <[email protected]>
There are some assumptions about the relationship between the receive queue, copied_seq, and these other sequence numbers which keep track of what's happening.
There are warning and assertions that trigger any time these values appear to be out-of-whack, for example the WARN() checks in tcp_cleanup_rbuf() and tcp_recvmsg().
I don't see anything in this code which makes sure the other aspects of the TCP socket are in a state such that these assertions will not potentially trigger.