Merge branch 'ipv6-avoid-atomic-fragment-on-gso-output'

Yan Zhai says:

====================
ipv6: avoid atomic fragment on GSO output

When the ipv6 stack output a GSO packet, if its gso_size is larger than
dst MTU, then all segments would be fragmented. However, it is possible
for a GSO packet to have a trailing segment with smaller actual size
than both gso_size as well as the MTU, which leads to an "atomic
fragment". Atomic fragments are considered harmful in RFC-8021. An
Existing report from APNIC also shows that atomic fragments are more
likely to be dropped even it is equivalent to a no-op [1].

The series contains following changes:
* drop feature RTAX_FEATURE_ALLFRAG, which has been broken. This helps
  simplifying other changes in this set.
* refactor __ip6_finish_output code to separate GSO and non-GSO packet
  processing, mirroring IPv4 side logic.
* avoid generating atomic fragment on GSO packets.

Link: https://www.potaroo.net/presentations/2022-03-01-ipv6-frag.pdf [1]

V4: https://lore.kernel.org/netdev/cover.1698114636.git.yan@cloudflare.com/
V3: https://lore.kernel.org/netdev/cover.1697779681.git.yan@cloudflare.com/
V2: https://lore.kernel.org/netdev/ZS1%2Fqtr0dZJ35VII@debian.debian/
====================

Link: https://lore.kernel.org/r/cover.1698156966.git.yan@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 1 insertions, 19 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 2866ccbccde07..ca4d7594efd48 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1698,14 +1698,6 @@ static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
 	 */
 	mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
 
-	/* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
-	if (icsk->icsk_af_ops->net_frag_header_len) {
-		const struct dst_entry *dst = __sk_dst_get(sk);
-
-		if (dst && dst_allfrag(dst))
-			mss_now -= icsk->icsk_af_ops->net_frag_header_len;
-	}
-
 	/* Clamp it (mss_clamp does not include tcp options) */
 	if (mss_now > tp->rx_opt.mss_clamp)
 		mss_now = tp->rx_opt.mss_clamp;
@@ -1733,21 +1725,11 @@ int tcp_mss_to_mtu(struct sock *sk, int mss)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	int mtu;
 
-	mtu = mss +
+	return mss +
 	      tp->tcp_header_len +
 	      icsk->icsk_ext_hdr_len +
 	      icsk->icsk_af_ops->net_header_len;
-
-	/* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
-	if (icsk->icsk_af_ops->net_frag_header_len) {
-		const struct dst_entry *dst = __sk_dst_get(sk);
-
-		if (dst && dst_allfrag(dst))
-			mtu += icsk->icsk_af_ops->net_frag_header_len;
-	}
-	return mtu;
 }
 EXPORT_SYMBOL(tcp_mss_to_mtu);