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

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Message-ID: <137ab4f7-80af-4e00-a5bb-b1d4f4c75a67@arista.com>
Date:   Wed, 29 Nov 2023 19:57:53 +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 18:34, Eric Dumazet wrote:
> On Wed, Nov 29, 2023 at 7:14 PM Dmitry Safonov <dima@...sta.com> wrote:
>>
>> 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)
>>
> 
> Oh well, this seems quite strange to have such a limitation.

I guess so. On the other side, it seems that there aren't many
non-hobbyist 32bit platforms: ia32 compatible layer will even be limited
with a boot parameter/compile option. Maybe I'm not aware of, but it
seems that arm64/ppc64/risc-v/x86_64 are the ones everyone interested in
these days.

> 
>> 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)
>>
> 
> You have not commented on where these are read without the socket lock held ?

Sorry for missing this, the SNEs are used with this helper
tcp_ao_compute_sne(), so these places are (in square brackets AFAICS,
there is a chance that I miss something obvious from your message):

- tcp_v4_send_reset() => tcp_ao_prepare_reset() [rcu_read_lock()]
- __tcp_transmit_skb() => tcp_ao_transmit_skb() [TX softirq]
- tcp_v4_rcv() => tcp_inbound_ao_hash() [RX softirq]


> tcp_ao_get_repair() can lock the socket.

It can, sure.

> In TW state, I guess these values can not be changed ?

Currently, they are considered constant on TW. The incoming segments are
not verified on twsk (so no need for SNEs). And from ACK side not
expecting SEQ roll-over (tcp_ao_compute_sne() is not called) - this may
change, but not quite critical it seems.

If we go with this patch in question, I'll have to update this:
:		key.sne = READ_ONCE(ao_info->snd_sne);
(didn't adjust it for higher-bytes shift)

> I think you can remove all these READ_ONCE()/WRITE_ONCE() which are not needed,
> or please add a comment if they really are.

Not sure if I answered above..

> Then, you might be able to remove the 64BIT dependency ...

At this moment I fail to imagine anyone running BGP + TCP-AO on 32bit
kernel. I may be wrong, for sure.

Thanks,
             Dmitry