Re: [PATCH net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 9/29/26 9:12 PM, Eric Dumazet wrote:
> __ip_make_skb() reserves a single IP ID for UDP GSO packets, but GSO
> assigns one IP ID per segment. Following packets then reuse these IDs,
> either from inet->inet_id or from the shared generator.
>
> Unless IP_PMTUDISC_DO/PROBE is used, DF is not set on UDP GSO packets,
> so segments can be fragmented on the path and IP ID reuse can lead to
> incorrect reassembly.
>
> Reserve one IP ID per segment, using the same test as udp_send_skb().
>
> Fixes: bec1f6f69736 ("udp: generate gso with UDP_SEGMENT")
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


> ---
>   net/ipv4/ip_output.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index a24cc8ee11d3ea3069bcc0d4d12e6867c2c475f7..b1cf0c6bfc79c8d234cc81ff32332116c1ee3401 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1410,6 +1410,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>   	struct iphdr *iph;
>   	u8 pmtudisc, ttl;
>   	__be16 df = 0;
> +	int segs;
>   
>   	skb = __skb_dequeue(queue);
>   	if (!skb)
> @@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
>   	iph->ttl = ttl;
>   	iph->protocol = sk->sk_protocol;
>   	ip_copy_addrs(iph, fl4);
> -	ip_select_ident(net, skb, sk);
> +
> +	/* UDP GSO packets are segmented later (see udp_send_skb()):
> +	 * reserve one IP ID per segment.
> +	 */
> +	segs = 1;
> +	if (cork->gso_size) {
> +		int datalen = skb->len - skb_transport_offset(skb) -
> +			      sizeof(struct udphdr);
> +
> +		if (datalen > cork->gso_size)
> +			segs = DIV_ROUND_UP(datalen, cork->gso_size);
> +	}
> +	ip_select_ident_segs(net, skb, sk, segs);
>   
>   	if (opt) {
>   		iph->ihl += opt->optlen >> 2;