linux-kernel - devinet.c inet_abc_len() breaking ifconfig?

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Message-ID: <c2cbddb5-da35-c522-b181-30cd4f327e96@landley.net>
Date:   Mon, 15 Oct 2018 09:42:41 -0500
From:   Rob Landley <rob@...dley.net>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: devinet.c inet_abc_len() breaking ifconfig?

Dave Taht pointed out to me that this doesn't work in toybox:

  $ ifconfig eth0 242.2.0.1 netmask 255.255.255.0 broadcast 242.2.0.255
  ifconfig: ioctl 8916: Invalid argument

Because of this in the kernel:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/devinet.c#n940

What is the inet_abc_len() function _for_? When's the last time "class A, Class
B, and Class C" IPv4 addresses actually _existed_ as a thing? (Somewhere around
1995?)

He suggested I switch to the netlink API, which doesn't enforce this, but... why
is the kernel enforcing it for one api but not the other?

Rob