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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
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