Hi Eric, On 01/11/17 - 19:21:31, Eric Dumazet wrote: > On Wed, 2017-11-01 at 18:00 -0700, Eric Dumazet wrote: > > On Wed, 2017-11-01 at 17:10 -0700, Christoph Paasch wrote: > > > > > Yes, that looks good to me. Thanks! > > > > > > But we still need to clean up the skb in tcp_v4_send_reset and > > > tcp_v4_send_ack, as the fields (e.g., tcp_skb_cb->seq) have been set when > > > coming from tcp_v4_rcv. > > > > You might be confused : ip_send_unicast_reply() does not send back the > > incoming skb. > > > > A fresh skb is allocated, then appended/sent. > > > > And commit 24a2d43d8886f5a29c did the changes to provide to > > __ip_options_echo() the proper IPCB header location. > > > > More details : > > Fields written by tcp_init_nondata_skb() on the synack packet : > > ->seq (32bits) at offset 0 of skb->cb[] > ->end_seq (32bits) at offset 4 of skb->cb[] > ->tcp_gso_segs (16bits) at offset 8 > ->tcp_flags (8bits) at offset 12 value (TCPHDR_SYN | TCPHDR_ACK -> > 0x12)
In my build I see tcp_flags at offset 16, because ktime_t is 64 bits. > ->sacked (8bits) at offset 13 > > IPCB fields sharing these 14 bytes : > > iif /* 32bits, offset 0 */ > opt.faddr (32bits) offset 4 > opt.nexthop (32bits) offset 8 /* value 1 */ > opt.optlen (8bits) offset 12 /* value 0x12 */ > opt.srr (8bits) offset 13 > > IP6CB fields sharing these 14 bytes : > > iif /* 32bits, offset 0 */ > ra /* 16 bits, offset 4 */ > dst0 /* 16 bits offset 6 */ > srcrt /* 16 bits offset 8 */ -> 0x0001 > dst1 /* 16 bits offset 10 */ (not mangled -> 0) > lastopt /* 16 bits offset 12 */ -> 0x12 Thus, because what I mention above, we are writing here into flags which sits at offset 16. So, IP6SKB_FORWARDED and IP6SKB_FRAGMENTED will be set on the outgoing skb. Especially for IP6SKB_FORWARDED, which is checked at ip6_finish_output2() I am concerned. Even in general, having these fields set looks brittle to me. What do you think? > At xmit : > > IPV4 uses ip_build_and_send_pkt() to transmit the SYNACK, so skb->cb[] > is not used. > > IPv6 uses other fields. > > So I really wonder what exact issue you observed, please share your > drugs ;) No drugs, only belgian beer :) Christoph
