[RFC net-next v2 03/18] ipv6: allow output fragmentation to reuse a Fragment Header
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From: Ralf Lici <ralf@mandelbit.com>
Date: 2026-10-24 09:26:13
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Subsystem:
networking [general], networking [ipv4/ipv6], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, David Ahern, Ido Schimmel, Linus Torvalds
IPv4-to-IPv6 translation constructs an IPv6 Fragment Header before the target route is selected. If that route has a smaller MTU, IPv6 output must split the translated fragment again. Adding another Fragment Header would create a new fragment domain instead of subfragmenting the original datagram representation. Allow ip6_fragment to reuse a locally constructed Fragment Header, preserving its identification, datagram-relative offset and More Fragments state. This mirrors IPv4 refragmentation, whose fragment state carries the original offset and whether more fragments follow the input fragment. Signed-off-by: Ralf Lici <ralf@mandelbit.com> --- New patch in RFC v2 replacing ipxlat's v1 IPv4 pre-fragmentation machinery. include/linux/ipv6.h | 2 ++ include/net/ipv6.h | 1 + net/ipv6/ip6_output.c | 53 +++++++++++++++++++++++++++++++++++++++---- 3 files changed, 51 insertions(+), 5 deletions(-)
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index a7421382a916..558260074842 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h@@ -177,6 +177,8 @@ struct inet6_skb_parm { #define IP6SKB_L3SLAVE 64 #define IP6SKB_JUMBOGRAM 128 #define IP6SKB_SEG6 256 +/* the local stack owns fragmentation of this IPv6 representation */ +#define IP6SKB_FRAG_SOURCE 512 #define IP6SKB_MULTIPATH 1024 #define IP6SKB_MCROUTE 2048 };
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index e3f245d640ac..a1cca04fd17a 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h@@ -189,6 +189,7 @@ struct ip6_frag_state { int hroom; int troom; __be32 frag_id; + bool not_last_frag; u8 nexthdr; };
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 25fe0ba98b1a..73633483f1ab 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c@@ -204,7 +204,8 @@ static int __ip6_finish_output(struct net *net, struct sock *sk, struct sk_buff return ip6_finish_output_gso(net, sk, skb, mtu); if (unlikely(skb->len > mtu || - (IP6CB(skb)->frag_max_size && skb->len > IP6CB(skb)->frag_max_size))) + (IP6CB(skb)->frag_max_size && + skb->len > IP6CB(skb)->frag_max_size))) return ip6_fragment(net, sk, skb, ip6_finish_output2); return ip6_finish_output2(net, sk, skb);
@@ -805,6 +806,7 @@ void ip6_frag_init(struct sk_buff *skb, unsigned int hlen, unsigned int mtu, state->troom = needed_tailroom; state->offset = 0; + state->not_last_frag = false; } EXPORT_SYMBOL(ip6_frag_init);
@@ -873,7 +875,7 @@ struct sk_buff *ip6_frag_next(struct sk_buff *skb, struct ip6_frag_state *state) state->left -= len; fh->frag_off = htons(state->offset); - if (state->left > 0) + if (state->left > 0 || state->not_last_frag) fh->frag_off |= htons(IP6_MF); ipv6_hdr(frag)->payload_len = htons(frag->len - sizeof(struct ipv6hdr));
@@ -891,10 +893,13 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, struct rt6_info *rt = dst_rt6_info(skb_dst(skb)); struct ipv6_pinfo *np = skb->sk && !dev_recursion_level() ? inet6_sk(skb->sk) : NULL; + struct frag_hdr old_fh; u8 tstamp_type = skb->tstamp_type; struct ip6_frag_state state; unsigned int mtu, hlen, nexthdr_offset; + unsigned int old_data_len, old_offset, unfrag_len; ktime_t tstamp = skb->tstamp; + bool frag_source, refrag; int hroom, err = 0; __be32 frag_id; u8 *prevhdr, nexthdr = 0;
@@ -910,6 +915,31 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, hlen = err; nexthdr = *prevhdr; nexthdr_offset = prevhdr - skb_network_header(skb); + frag_source = IP6CB(skb)->flags & IP6SKB_FRAG_SOURCE; + refrag = frag_source && nexthdr == NEXTHDR_FRAGMENT; + if (unlikely(refrag)) { + /* The current fragment header describes this locally built + * IPv6 representation. We need to replace it with smaller + * fragments rather than nesting another frag hdr around it. + */ + if (unlikely(skb->len < hlen + sizeof(old_fh) || + skb_copy_bits(skb, skb_network_offset(skb) + hlen, + &old_fh, sizeof(old_fh)))) { + err = -EINVAL; + goto fail; + } + + old_data_len = skb->len - hlen - sizeof(old_fh); + old_offset = ntohs(old_fh.frag_off) & IP6_OFFSET; + unfrag_len = hlen - sizeof(struct ipv6hdr); + if (unlikely(old_offset + unfrag_len > IPV6_MAXPLEN || + old_data_len > + IPV6_MAXPLEN - old_offset - unfrag_len)) { + err = -EINVAL; + goto fail; + } + nexthdr = old_fh.nexthdr; + } mtu = ip6_skb_dst_mtu(skb);
@@ -939,8 +969,11 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, goto fail_toobig; mtu -= hlen + sizeof(struct frag_hdr); - frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr, - &ipv6_hdr(skb)->saddr); + if (refrag) + frag_id = old_fh.identification; + else + frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr, + &ipv6_hdr(skb)->saddr); if (skb->ip_summed == CHECKSUM_PARTIAL && (err = skb_checksum_help(skb)))
@@ -953,7 +986,8 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, struct ip6_fraglist_iter iter; struct sk_buff *frag2; - if (first_len - hlen > mtu || + if (refrag || + first_len - hlen > mtu || ((first_len - hlen) & 7) || skb_cloned(skb) || skb_headroom(skb) < (hroom + sizeof(struct frag_hdr)))
@@ -1038,6 +1072,15 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb, ip6_frag_init(skb, hlen, mtu, rt->dst.dev->needed_tailroom, LL_RESERVED_SPACE(rt->dst.dev), prevhdr, nexthdr, frag_id, &state); + if (refrag) { + /* Mirror IPv4 refragmentation: start at the old datagram + * offset and retain its MF value on the final new fragment. + */ + state.left -= sizeof(old_fh); + state.ptr += sizeof(old_fh); + state.offset = old_offset; + state.not_last_frag = old_fh.frag_off & htons(IP6_MF); + } /* * Keep copying data until we run out.
--
2.55.0