From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [Patch iproute2] ipaddress: do not add ifa_flags when not
 necessary
Date: Fri, 04 Apr 2014 09:59:05 -0400 (EDT)
Message-ID: <20140404.095905.2169727474578538143.davem@davemloft.net>
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Cc: netdev@vger.kernel.org, jiri@resnulli.us,
	stephen@networkplumber.org
To: xiyou.wangcong@gmail.com
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From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Thu,  3 Apr 2014 23:05:03 -0700

> commit 37c9b94ed21d5779acc23d89a4 (add support for extended ifa_flags)
> introduced a regression:
> 
> 	# ./ip/ip addr add 192.168.0.1/24 dev eth0
> 	RTNETLINK answers: Invalid argument
> 
> This is due to old kernels don't support IFA_FLAGS flag, it should be skipped
> if we don't use this feature at all.
> 
> Cc: Jiri Pirko <jiri@resnulli.us>
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

Your patch doesn't really fully fix the problem.

If the kernel rejects because of presence of the IFA_FLAGS attribute,
it will do so for all such cases.

Therefore the guard you need is not whether ifa_flags is NULL, but
whether a large enough flag bit is set such that IFA_FLAGS is required.

IFA_FLAGS was added because ifa.ifa_flags is too small for the new
values we needed to add.