linux-kernel - Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao

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Message-ID: <df55eb1d-b63a-4652-8103-d2bd7b5d7eda@arista.com>
Date:   Wed, 29 Nov 2023 18:14:16 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Ahern <dsahern@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Francesco Ruggeri <fruggeri05@...il.com>,
        Salam Noureddine <noureddine@...sta.com>,
        Simon Horman <horms@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v4 6/7] net/tcp: Store SNEs + SEQs on ao_info

On 11/29/23 18:09, Eric Dumazet wrote:
> On Wed, Nov 29, 2023 at 5:57 PM Dmitry Safonov <dima@...sta.com> wrote:
>>
>> RFC 5925 (6.2):
>>> TCP-AO emulates a 64-bit sequence number space by inferring when to
>>> increment the high-order 32-bit portion (the SNE) based on
>>> transitions in the low-order portion (the TCP sequence number).
>>
>> snd_sne and rcv_sne are the upper 4 bytes of extended SEQ number.
>> Unfortunately, reading two 4-bytes pointers can't be performed
>> atomically (without synchronization).
>>
>> In order to avoid locks on TCP fastpath, let's just double-account for
>> SEQ changes: snd_una/rcv_nxt will be lower 4 bytes of snd_sne/rcv_sne.
>>
> 
> This will not work on 32bit kernels ?

Yeah, unsure if there's someone who wants to run BGP on 32bit box, so at
this moment it's already limited:

config TCP_AO
	bool "TCP: Authentication Option (RFC5925)"
	select CRYPTO
	select TCP_SIGPOOL
	depends on 64BIT && IPV6 != m # seq-number extension needs WRITE_ONCE(u64)

Probably, if there will be a person who is interested in this, it can
get a spinlock for !CONFIG_64BIT.

> Unless ao->snd_sne and ao->rcv_sneare only read/written under the
> socket lock (and in this case no READ_ONCE()/WRITE_ONCE() should be
> necessary)

Thanks,
            Dmitry