netdev - IPv6 unexpectedly changes MTU on wrong interface

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Message-ID: <CAB=W+o=Ar4sScW3tkn+bscMTC-XzWSqY7D5mm=XF=no-XXsb_A@mail.gmail.com>
Date:   Mon, 1 Apr 2019 15:48:37 -0600
From:   Captain Wiggum <captwiggum@...il.com>
To:     netdev@...r.kernel.org
Subject: IPv6 unexpectedly changes MTU on wrong interface

This bug exists in the LTS kernels, but has been fixed in mainline.
My problem is I have no idea what to search for in git find the commit(s).
Hopefully someone recognizes this issue and can give some tips to find it.

Summary:
Three TAHI IPv6 test cases fail on this:
Two interface test: first interface used only for IPv4, and the second
only for IPv6.
The problem is with the IPv6 PMTU (RFC 1981).
When the kernel receives an ICMPv6 "packet to big" message,
it must update the source to destination path MTU with the new value
provided in the "packet to big" message.
The kernel always uses the first interface as the local end of the path,
instead of the interface where the issue occured.
Therefore when IPv6 goes to check the Path MTU on the second interface
when transmitting a packet, it is not getting the reduced MTU value.

--John Masinter