netdev - SCTP ABI breakage?

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Date:   Tue, 21 Dec 2021 14:50:13 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     in Long <lucien.xin@...il.com>, vyasevich@...il.com,
        nhorman@...driver.com, marcelo.leitner@...il.com,
        davem@...emloft.net, kuba@...nel.org
Cc:     linux-sctp@...r.kernel.org, netdev@...r.kernel.org
Subject: SCTP ABI breakage?

Looks like changes to SCTP events create kernel ABI breakage in applications.

If application is compiled with new header but attempts to run on older kernel, it doesn't work:

Reported here: https://osmocom.org/issues/5366

Looks like bad design assumptions about how setsockopt here:
static int sctp_setsockopt_events(struct sock *sk, __u8 *sn_type,
				  unsigned int optlen)
{
	struct sctp_sock *sp = sctp_sk(sk);
	struct sctp_association *asoc;
	int i;

	if (optlen > sizeof(struct sctp_event_subscribe))
		return -EINVAL;

Because of that the commits that add new events cause code built with the new
header to not run on older kernels.