netdev - Re: [PATCH net 1/6] net: Introduce skb ttl field to track packet loops

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Message-ID: <CAM_iQpVPnwe=YqZubeUFy78ym_TM-R3qvx3u3ydN0t6GbFnmgg@mail.gmail.com>
Date: Tue, 13 Jan 2026 11:44:57 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, andrew+netdev@...n.ch, 
	netdev@...r.kernel.org, jiri@...nulli.us, victor@...atatu.com, 
	dcaratti@...hat.com, lariel@...dia.com, daniel@...earbox.net, 
	pablo@...filter.org, kadlec@...filter.org, fw@...len.de, phil@....cc, 
	netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	zyc199902@...omail.cn, lrGerlinde@...lfence.com, jschung2@...ton.me
Subject: Re: [PATCH net 1/6] net: Introduce skb ttl field to track packet loops

On Sun, Jan 11, 2026 at 8:40 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 86737076101d..7f18b0c28728 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -840,6 +840,7 @@ enum skb_tstamp_type {
>   *     @no_fcs:  Request NIC to treat last 4 bytes as Ethernet FCS
>   *     @encapsulation: indicates the inner headers in the skbuff are valid
>   *     @encap_hdr_csum: software checksum is needed
> + *     @ttl: time to live count when a packet loops.
>   *     @csum_valid: checksum is already valid
>   *     @csum_not_inet: use CRC32c to resolve CHECKSUM_PARTIAL
>   *     @csum_complete_sw: checksum was completed by software
> @@ -1000,6 +1001,7 @@ struct sk_buff {
>         /* Indicates the inner headers are valid in the skbuff. */
>         __u8                    encapsulation:1;
>         __u8                    encap_hdr_csum:1;
> +       __u8                    ttl:2;

In the worst case it increases sk_buff size.

  ┌──────────────────────────────┬───────────────────┬───────────────────┬─────────┐
  │       Config scenario        │      Current      │   After change
  │  Delta  │
  ├──────────────────────────────┼───────────────────┼───────────────────┼─────────┤
  │ All configs enabled          │ 17 bits (3 bytes) │ 18 bits (3
bytes) │ 0       │
  ├──────────────────────────────┼───────────────────┼───────────────────┼─────────┤
  │ CONFIG_NET_REDIRECT disabled │ 16 bits           │ 18 bits
  │ +2 bits │
  ├──────────────────────────────┼───────────────────┼───────────────────┼─────────┤
  │ Minimal config               │ 7 bits (1 byte)   │ 9 bits (2
bytes)  │ +1 byte │
  └──────────────────────────────┴───────────────────┴───────────────────┴─────────┘

I think this would be a clear stopper in netdev.

Good luck to you on fighting with increasing sk_buff size!

Regards,
Cong