linux-kernel - Re: [RFC PATCH v2 1/3] ipv4: Support setting src port in sendmsg().

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Message-ID: <CAHTyZGwm4pHZg1eSdF6bbk7_WcJ4k25YzWtRKbfWNC_e6gUNVQ@mail.gmail.com>
Date: Sun, 22 Sep 2024 18:23:19 +0200
From: ericnetdev dumazet <erdnetdev@...il.com>
To: Tiago Lam <tiagolam@...udflare.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Willem de Bruijn <willemdebruijn.kernel@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Jakub Sitnicki <jakub@...udflare.com>, 
	kernel-team@...udflare.com
Subject: Re: [RFC PATCH v2 1/3] ipv4: Support setting src port in sendmsg().

Le ven. 20 sept. 2024 à 19:03, Tiago Lam <tiagolam@...udflare.com> a écrit :
>
> sendmsg() doesn't currently allow users to set the src port from which
> egress traffic should be sent from. This is possible if a user wants to
> configure the src address from which egress traffic should be sent from
> - with the IP_PKTINFO ancillary message, a user is currently able to
>   specify a source address to egress from when calling sendmsg().
> However, this still requires the user to set the IP_TRANSPARENT flag
> using setsockopt(), which happens to require special privileges in the
> case of IPv4.
>
> To support users setting the src port for egress traffic when using
> sendmsg(), this patch extends the ancillary messages supported by
> sendmsg() to support the IP_ORIGDSTADDR ancillary message, reusing the
> same cmsg and struct used in recvmsg() - which already supports
> specifying a port.
>
> Additionally, to avoid having to have special configurations, such as
> IP_TRANSPARENT, this patch allows egress traffic that's been configured
> using (the newly added) IP_ORIGDSTADDR to proceed if there's an ingress
> socket lookup (sk_lookup) that matches that traffic - by performing a
> reserve sk_lookup. Thus, if the sk_lookup reverse call returns a socket
> that matches the egress socket, we also let the egress traffic through -
> following the principle of, allowing return traffic to proceed if
> ingress traffic is allowed in. In case no match is found in the reverse
> sk_lookup, traffic falls back to the regular egress path.
>
> This reverse lookup is only performed in case an sk_lookup ebpf program
> is attached and the source address and/or port for the return traffic
> have been modified using the (newly added) IP_ORIGDSTADDR in sendmsg.

Is it compatible with SO_REUSEPORT ?

Most heavy duty UDP servers use SO_REUSEPORT to spread incoming
packets to multiple sockets.

How is the reverse lookup going to choose the 'right' socket ?