From b31f8f1365a386b767e17f4e0fe4f224dacfbd36 Mon Sep 17 00:00:00 2001 From: Willem de Bruijn Date: Wed, 29 Jan 2020 15:20:17 -0500 Subject: [PATCH] udp: document udp_rcv_segment special case for looped packets Commit 904b0b17ea15c ("udp: segment looped gso packets correctly") fixes an issue with rare udp gso multicast packets looped onto the receive path. The stable backport makes the narrowest change to target only these packets, when needed. As opposed to, say, expanding __udp_gso_segment, which is harder to reason to be free from unintended side-effects. But the resulting code is hardly self-describing. Document its purpose and rationale. Signed-off-by: Willem de Bruijn Signed-off-by: David S. Miller --- include/net/udp.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/include/net/udp.h b/include/net/udp.h index 4a180f2a13e32..e55d5f7658075 100644 --- a/include/net/udp.h +++ b/include/net/udp.h @@ -476,6 +476,13 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk, if (!inet_get_convert_csum(sk)) features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM; + /* UDP segmentation expects packets of type CHECKSUM_PARTIAL or + * CHECKSUM_NONE in __udp_gso_segment. UDP GRO indeed builds partial + * packets in udp_gro_complete_segment. As does UDP GSO, verified by + * udp_send_skb. But when those packets are looped in dev_loopback_xmit + * their ip_summed is set to CHECKSUM_UNNECESSARY. Reset in this + * specific case, where PARTIAL is both correct and required. + */ if (skb->pkt_type == PACKET_LOOPBACK) skb->ip_summed = CHECKSUM_PARTIAL; -- 2.39.5