netdev - Re: [PATCH net] net: reenable NETIF_F_IPV6

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Message-ID: <CANn89iJP4unWmk2T36t1LiFrchy+DSGkbZWz_i42mb1eCDXyeg@mail.gmail.com>
Date: Thu, 2 Jan 2025 12:51:00 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, benoit.monin@....fr, Willem de Bruijn <willemb@...gle.com>, 
	syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets

On Wed, Jan 1, 2025 at 5:49 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> The blamed commit disabled hardware offoad of IPv6 packets with
> extension headers on devices that advertise NETIF_F_IPV6_CSUM,
> based on the definition of that feature in skbuff.h:
>
>  *   * - %NETIF_F_IPV6_CSUM
>  *     - Driver (device) is only able to checksum plain
>  *       TCP or UDP packets over IPv6. These are specifically
>  *       unencapsulated packets of the form IPv6|TCP or
>  *       IPv6|UDP where the Next Header field in the IPv6
>  *       header is either TCP or UDP. IPv6 extension headers
>  *       are not supported with this feature. This feature
>  *       cannot be set in features for a device with
>  *       NETIF_F_HW_CSUM also set. This feature is being
>  *       DEPRECATED (see below).
>
> The change causes skb_warn_bad_offload to fire for BIG TCP
> packets.
>
> [  496.310233] WARNING: CPU: 13 PID: 23472 at net/core/dev.c:3129 skb_warn_bad_offload+0xc4/0xe0
>
> [  496.310297]  ? skb_warn_bad_offload+0xc4/0xe0
> [  496.310300]  skb_checksum_help+0x129/0x1f0
> [  496.310303]  skb_csum_hwoffload_help+0x150/0x1b0
> [  496.310306]  validate_xmit_skb+0x159/0x270
> [  496.310309]  validate_xmit_skb_list+0x41/0x70
> [  496.310312]  sch_direct_xmit+0x5c/0x250
> [  496.310317]  __qdisc_run+0x388/0x620
>
> BIG TCP introduced an IPV6_TLV_JUMBO IPv6 extension header to
> communicate packet length, as this is an IPv6 jumbogram. But, the
> feature is only enabled on devices that support BIG TCP TSO. The
> header is only present for PF_PACKET taps like tcpdump, and not
> transmitted by physical devices.
>
> For this specific case of extension headers that are not
> transmitted, return to the situation before the blamed commit
> and support hardware offload.
>
> ipv6_has_hopopt_jumbo() tests not only whether this header is present,
> but also that it is the only extension header before a terminal (L4)
> header.
>
> Fixes: 04c20a9356f2 ("net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Reported-by: Eric Dumazet <edumazet@...gle.com>
> Closes: https://lore.kernel.org/netdev/CANn89iK1hdC3Nt8KPhOtTF8vCPc1AHDCtse_BTNki1pWxAByTQ@mail.gmail.com/
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks Willem !