netdev - Re: [PATCH net-next v6 04/19] tcp: add datapath logic for PSP with inline key exchange

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Message-ID: <0c1dd072-8dae-429e-846f-08de9a69366f@gmail.com>
Date: Thu, 14 Aug 2025 10:43:34 -0400
From: Daniel Zahka <daniel.zahka@...il.com>
To: Paolo Abeni <pabeni@...hat.com>, Donald Hunter <donald.hunter@...il.com>,
 Jakub Kicinski <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>
Cc: Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 Tariq Toukan <tariqt@...dia.com>, Boris Pismenny <borisp@...dia.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn
 <willemb@...gle.com>, David Ahern <dsahern@...nel.org>,
 Neal Cardwell <ncardwell@...gle.com>, Patrisious Haddad
 <phaddad@...dia.com>, Raed Salem <raeds@...dia.com>,
 Jianbo Liu <jianbol@...dia.com>, Dragos Tatulea <dtatulea@...dia.com>,
 Rahul Rameshbabu <rrameshbabu@...dia.com>,
 Stanislav Fomichev <sdf@...ichev.me>,
 Toke Høiland-Jørgensen <toke@...hat.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>,
 Kiran Kella <kiran.kella@...adcom.com>,
 Jacob Keller <jacob.e.keller@...el.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v6 04/19] tcp: add datapath logic for PSP with
 inline key exchange

On 8/14/25 9:18 AM, Paolo Abeni wrote:
> On 8/12/25 2:29 AM, Daniel Zahka wrote:
>> @@ -2070,7 +2076,9 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
>>   	     (TCPHDR_ECE | TCPHDR_CWR | TCPHDR_AE)) ||
>>   	    !tcp_skb_can_collapse_rx(tail, skb) ||
>>   	    thtail->doff != th->doff ||
>> -	    memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)))
>> +	    memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)) ||
>> +	    /* prior to PSP Rx policy check, retain exact PSP metadata */
>> +	    psp_skb_coalesce_diff(tail, skb))
>>   		goto no_coalesce;
> The TCP stack will try to coalesce skbs in other places, too (i.e.
> tcp_try_coalesce(), tcp_collapse()...) Why a similar check is not needed
> there?

We handle coalescing of skb's in various places. On the tx path, we 
place a call to tcp_write_collapse_fence() in psp_sock_assoc_set_tx(), 
to prevent data written into the socket as cleartext from being merged 
with data written after the tx-assoc netlink operation has been 
performed. On the rx path, for dealing with coalescing before the skb is 
in the socket receive queue we call psp_skb_coalesce_diff() from 
tcp_add_backlog() and gro_list_prepare(). For skb's that are already on 
the socket receive queue, we rely on the fact that psp skb's will have 
skb->decrypted set, and all tcp functions that try to collapse skb's on 
the receive queue should call skb_cmp_decrypted() at some point. If we 
have missed a case, than that would be a bug.