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

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Date: Wed, 29 Nov 2023 19:34:07 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Dmitry Safonov <dima@...sta.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 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.

> 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 ?

tcp_ao_get_repair() can lock the socket.

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

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

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