[PATCH net v4 0/4] ipmr: forward GSO multicast packets

With fraglist GRO or UDP GRO forwarding on, a routed IPv4 multicast
stream arrives as GSO packets. ipmr_prepare_xmit() drops every DF
aggregate larger than the MTU, and ip_encap() sets no tunnel GSO state,
so a GSO packet sent to a VIFF_TUNNEL vif is dropped during
segmentation. With that state set, a fraglist aggregate reaches
skb_segment_list() with tunnel headers, which can write before the head
of a frag_list member; this also happens today when such a packet is
routed into a tunnel device. Checking the segment length instead of
skb->len relies on skb_gso_transport_seglen(), which counts the inner
UDP header of an encapsulated UDP GSO packet 8 bytes short.

Patch 1 makes room for the tunnel headers in skb_segment_list().
Patch 2 counts the inner UDP header of encapsulated UDP GSO packets.
Patch 3 sets up IPXIP4 GSO state, outer IP IDs and length in ip_encap().
Patch 4 lets DF GSO packets pass when their segments fit the MTU.

Tested in QEMU (KASAN) on veth, one run per arm: an MFC entry to a plain
vif and a tunnel vif, 40 x 1316-byte datagrams per GSO packet, 4000
datagrams per arm, UDP_SEGMENT, fraglist GRO and UDP GRO forwarding, DF
set and clear, software and device segmentation on egress:
- net (7076f684d513): every DF aggregate dropped on both vifs (FragFails
  2 per aggregate), and with software segmentation nothing arrives over
  the tunnel vif; fraglist GRO routed into VXLAN reports a KASAN
  slab-out-of-bounds write in skb_segment_list(), then panics.
- this series, all 12 arms: 4000 of 4000 datagrams on the plain vif,
  FragFails 0; on the tunnel vif 4000 of 4000 IPIP packets with unique
  outer IDs with software segmentation, and the 100 GSO packets of 52688
  bytes whole with device segmentation.
- VXLAN with DF, 1404-byte outer segments, route MTU 1400: net and this
  series without patch 2 forward all 1000 segments at 1404 bytes through
  ip_forward(), and the series without patch 2 also through the MFC
  entry; with patch 2 none leave and FragFails counts them.
- a GRE GSO packet (skb->encapsulation set) multicast-routed to both
  vifs: all 1000 segments on the plain vif, none on the tunnel vif.
- fraglist GRO routed into VXLAN and IPIP with software segmentation:
  every segment reaches the peer, no KASAN report.

Changes in v4:
- Add patch 2 (Sashiko).
- Patch 3: drop GSO packets that are already encapsulated, and set the
  outer length with iph_set_totlen() (Sashiko).
- Drop v3 patch 2 (TCP fixed-ID flag on encapsulation): multicast
  routing does not carry TCP, and the flag also needs the decap and
  GRE/UDP segmentation paths, which go to a separate series (Sashiko).
- Rebase onto net.
- v3: https://lore.kernel.org/netdev/cover.1792293419.git.julius@bairaktaris.de/ (local)

Changes in v3:
- Patch 1: name the VXLAN out-of-bounds write (Sashiko).
- Add patch 2 (Sashiko).
- Patch 3: reserve one outer IP ID per segment (Sashiko).
- v2: https://lore.kernel.org/netdev/20261011162957.1328474-1-julius@bairaktaris.de/ (local)

Changes in v2:
- Add patches 1 and 3; v1 let DF GSO packets reach ip_encap() on tunnel
  vifs, where they were dropped during segmentation (Sashiko).
- Keep the additive MTU comparison and check mtu > encap before
  subtracting, so a route MTU below 20 cannot wrap (Sashiko).
- v1: https://lore.kernel.org/netdev/20261010131842.2157365-1-julius@bairaktaris.de/ (local)

I used Claude (Anthropic) to help write these patches.

Julius Bairaktaris (4):
  net: make room for the tunnel header in skb_segment_list()
  net: gso: count the inner UDP header of encapsulated UDP GSO packets
  ipmr: set up GSO state for tunnel vifs
  ipmr: do not drop GSO packets that fit the MTU after segmentation

 net/core/gso.c    |  2 ++
 net/core/skbuff.c |  4 ++++
 net/ipv4/ipmr.c   | 29 ++++++++++++++++++++++-------
 3 files changed, 28 insertions(+), 7 deletions(-)


base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
-- 
2.53.0