Re: [PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[Alice Mikityanska <alice.kernel@fastmail.im>]

On Tue, 06 Jan 2026 at 10:52:40 +0100, Mariusz Klimek wrote:
> This series fixes the MTU validation of BIG TCP jumbograms and removes the
> existing IP6SKB_FAKEJUMBO work-around that only fixes the issue in one
> location.

My series removes IPv6 HBH in BIG TCP entirely:

https://lore.kernel.org/netdev/20260113212655.116122-1-alice.kernel@fastmail.im/T/

I believe, it makes this fix no longer necessary.

> For GSO packets, the length that matters for MTU validation is the segment
> length, not the total length of the packet. skb_gso_network_seglen is used
> by skb_gso_validate_network_len to calculate the segment length including
> the network and transport headers and to then verify that the segment
> length is below the MTU.
> 
> skb_gso_network_seglen assumes that the headers of the segments are
> identical to those of the unsegmented packet, but that assumption is
> incorrect for BIG TCP jumbograms which have an added HBH header that is
> removed upon segmentation. The calculated segment length ends up being 8
> bytes more than the actual segment length.
> 
> The actual segment length is set according to the MSS, so the segment
> length calculated by skb_gso_network_seglen is greater than the MTU,
> causing the skb_gso_validate_network_len check to fail despite the fact
> that the actual segment length is lower than the MTU.
> 
> There is currently a work-around that fixes this bug in some cases:
> ip6_xmit sets the IP6SKB_FAKEJUMBO flag for BIG TCP jumbograms, which
> causes the MTU validation in ip6_finish_output_gso to be skipped
> (intentionally). However, this work-around doesn't apply to MTU validations
> performed in other places such as in ip6_forward. BIG TCP jumbograms don't
> pass the MTU validation when forwarded locally and are therefore dropped,
> unless the MTU of the originating interface is lower than the MTUs of the
> rest of the interfaces the packets are forwarded through.
> 
> v2:
>   fix jumbogram check in skb_gso_network_seglen
>   add jumbogram check to skb_gso_mac_seglen as well
> 
> v1:
>   Link: https://lore.kernel.org/netdev/20251127091325.7248-1-maklimek97@gmail.com/
> 
> Mariusz Klimek (3):
>   net: gso: do not include jumbogram HBH header in seglen calculation
>   ipv6: remove IP6SKB_FAKEJUMBO flag
>   selftests/net: remove unnecessary MTU config in big_tcp.sh
> 
>  include/linux/ipv6.h                   |  1 -
>  net/core/gso.c                         | 14 +++++++++-----
>  net/ipv6/ip6_output.c                  |  4 +---
>  tools/testing/selftests/net/big_tcp.sh |  1 -
>  4 files changed, 10 insertions(+), 10 deletions(-)
> 
> -- 
> 2.47.3
> 
>