2.6.27-stable review patch. If anyone has any objections, please let us know. ------------------ From: Tom Quetchenbach commit f5fff5dc8a7a3f395b0525c02ba92c95d42b7390 upstream. I'm trying to use the TCP_MAXSEG option to setsockopt() to set the MSS for both sides of a bidirectional connection. man tcp says: "If this option is set before connection establishment, it also changes the MSS value announced to the other end in the initial packet." However, the kernel only uses the MTU/route cache to set the advertised MSS. That means if I set the MSS to, say, 500 before calling connect(), I will send at most 500-byte packets, but I will still receive 1500-byte packets in reply. This is a bug, either in the kernel or the documentation. This patch (applies to latest net-2.6) reduces the advertised value to that requested by the user as long as setsockopt() is called before connect() or accept(). This seems like the behavior that one would expect as well as that which is documented. I've tried to make sure that things that depend on the advertised MSS are set correctly. Signed-off-by: Tom Quetchenbach Signed-off-by: David S. Miller Cc: Willy Tarreau Signed-off-by: Greg Kroah-Hartman --- net/ipv4/tcp_ipv4.c | 4 ++++ net/ipv4/tcp_output.c | 13 ++++++++++--- 2 files changed, 14 insertions(+), 3 deletions(-) --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -1364,6 +1364,10 @@ struct sock *tcp_v4_syn_recv_sock(struct tcp_mtup_init(newsk); tcp_sync_mss(newsk, dst_mtu(dst)); newtp->advmss = dst_metric(dst, RTAX_ADVMSS); + if (tcp_sk(sk)->rx_opt.user_mss && + tcp_sk(sk)->rx_opt.user_mss < newtp->advmss) + newtp->advmss = tcp_sk(sk)->rx_opt.user_mss; + tcp_initialize_rcv_mss(newsk); #ifdef CONFIG_TCP_MD5SIG --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2252,6 +2252,7 @@ struct sk_buff *tcp_make_synack(struct s struct sk_buff *skb; struct tcp_md5sig_key *md5; __u8 *md5_hash_location; + int mss; skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC); if (skb == NULL) @@ -2262,13 +2263,17 @@ struct sk_buff *tcp_make_synack(struct s skb->dst = dst_clone(dst); + mss = dst_metric(dst, RTAX_ADVMSS); + if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss) + mss = tp->rx_opt.user_mss; + if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */ __u8 rcv_wscale; /* Set this up on the first call only */ req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW); /* tcp_full_space because it is guaranteed to be the first packet */ tcp_select_initial_window(tcp_full_space(sk), - dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), + mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), &req->rcv_wnd, &req->window_clamp, ireq->wscale_ok, @@ -2283,8 +2288,7 @@ struct sk_buff *tcp_make_synack(struct s else #endif TCP_SKB_CB(skb)->when = tcp_time_stamp; - tcp_header_size = tcp_synack_options(sk, req, - dst_metric(dst, RTAX_ADVMSS), + tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, &md5) + sizeof(struct tcphdr); @@ -2353,6 +2357,9 @@ static void tcp_connect_init(struct sock if (!tp->window_clamp) tp->window_clamp = dst_metric(dst, RTAX_WINDOW); tp->advmss = dst_metric(dst, RTAX_ADVMSS); + if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss) + tp->advmss = tp->rx_opt.user_mss; + tcp_initialize_rcv_mss(sk); tcp_select_initial_window(tcp_full_space(sk), -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/