Re: [PATCH v3 net] tcp: preserve timestamps across receive queue collapse

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 24 Sep 2026 23:25:29 +0800 Jason Xing wrote:
> When tcp_collapse() rebuilds skbs under memory pressure, the copy
> process doesn't include the right tstamp and hwtstamp from the
> old skb. And memcpy(nskb->cb, skb->cb, ...) copies has_rxtstamp,
> but nskb->tstamp and hwtstamps are left at zero, so
> tcp_recv_timestamp() ends up emitting no cmsg at all.
> 
> In net timestamping case, if such an skb happens to be the last
> one consumed in a recvmsg() call, the application receives no RX
> timestamp for that call.
> 
> Fix this by copying both tstamp and hwtstamp of the last skb to
> the new skb, matching tcp_try_coalesce()/tcp_add_backlog().
> 
> Note that the has_rxtstamp flag can still be inherited through
> the cb memcpy from an skb that contributes no bytes (fully covered
> skb left in the ofo tree by the tcp_ooo_try_coalesce() ->
> coalesce_done path), so set TCP_SKB_CB(nskb)->has_rxtstamp to false
> which makes the new block the only place setting it.
> 
> Fixes: 98aaa913b4ed ("tcp: Extend SOF_TIMESTAMPING_RX_SOFTWARE to TCP recvmsg")
> Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
> V3
> Link: https://lore.kernel.org/all/20260917132742.87117-1-kerneljasonxing@gmail.com/
> 1. revise the commit message with more accurate description (Eric)
> 2. fix a corner case (where a skb can contribute no bytes if OOO happens)
>    spotted by AI and Eric

Looks good now, Eric?
https://lore.kernel.org/all/20260924152529.5689-1-kerneljasonxing@gmail.com/

(Jason, please see net for the up-to-date email addr for Eric)