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

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 14 Aug 2025 10:43:34 -0400 Daniel Zahka wrote:
> 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.

Stating the obvious, but please incorporate the answers to the questions
asked in the review in the commit message. I think this came up before..