netdev - Re: [PATCH net-next] tcp: enable data-less, empty-cookie SYN with TFO_SERVER_COOKIE_NOT

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Date:   Tue, 17 Aug 2021 06:35:17 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Luke Hsiao <luke.w.hsiao@...il.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Luke Hsiao <lukehsiao@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] tcp: enable data-less, empty-cookie SYN with
 TFO_SERVER_COOKIE_NOT_REQD

On Mon, 16 Aug 2021 20:51:06 +0000 Luke Hsiao wrote:
> From: Luke Hsiao <lukehsiao@...gle.com>
> 
> Since the original TFO server code was implemented in commit
> 168a8f58059a22feb9e9a2dcc1b8053dbbbc12ef ("tcp: TCP Fast Open Server -
> main code path") the TFO server code has supported the sysctl bit flag
> TFO_SERVER_COOKIE_NOT_REQD. Currently, when the TFO_SERVER_ENABLE and
> TFO_SERVER_COOKIE_NOT_REQD sysctl bit flags are set, a server connection
> will accept a SYN with N bytes of data (N > 0) that has no TFO cookie,
> create a new fast open connection, process the incoming data in the SYN,
> and make the connection ready for accepting. After accepting, the
> connection is ready for read()/recvmsg() to read the N bytes of data in
> the SYN, ready for write()/sendmsg() calls and data transmissions to
> transmit data.
> 
> This commit changes an edge case in this feature by changing this
> behavior to apply to (N >= 0) bytes of data in the SYN rather than only
> (N > 0) bytes of data in the SYN. Now, a server will accept a data-less
> SYN without a TFO cookie if TFO_SERVER_COOKIE_NOT_REQD is set.
> 
> Caveat! While this enables a new kind of TFO (data-less empty-cookie
> SYN), some firewall rules setup may not work if they assume such packets
> are not legit TFOs and will filter them.
> 
> Signed-off-by: Luke Hsiao <lukehsiao@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks!