netdev - Re: [PATCH net V2] net: always reevalulate autoflowlabel setting for reset packet

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Message-Id: <20171220.130333.2284740049571493817.davem@davemloft.net>
Date:   Wed, 20 Dec 2017 13:03:33 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     shli@...nel.org
Cc:     netdev@...r.kernel.org, Kernel-team@...com, shli@...com,
        kafai@...com
Subject: Re: [PATCH net V2] net: always reevalulate autoflowlabel setting
 for reset packet

From: Shaohua Li <shli@...nel.org>
Date: Tue, 19 Dec 2017 12:58:21 -0800

> From: Shaohua Li <shli@...com>
> 
> ipv6_pinfo.autoflowlabel is set in sock creation. Later if we change
> sysctl.ip6.auto_flowlabels, the ipv6_pinfo.autoflowlabel isn't changed,
> so the sock will keep the old behavior in terms of auto flowlabel. Reset
> packet is suffering from this problem, because reset packset is sent
> from a special control socket, which is created at boot time. Since
> sysctl.ipv6.auto_flowlabels is 2 by default, the control socket will
> always have its ipv6_pinfo.autoflowlabel set, even after user set
> sysctl.ipv6.auto_flowlabels to 1, so reset packset will always have
> flowlabel.
> 
> To fix this, we always reevaluate autoflowlabel setting for reset
> packet. Normal sock has the same issue too, but since the
> sysctl.ipv6.auto_flowlabels is usually set at host startup, this isn't a
> big issue for normal sock.
> 
> Cc: Martin KaFai Lau <kafai@...com>
> Signed-off-by: Shaohua Li <shli@...com>

This definitely goes into the category of hack.

Special casing resets is not the answer.

What about normal user sockets openned before the sysctl setting is
modified?

No user is going to be happy about this nice little surprise.

What you're saying is that we should specially treat the kernel
sockets that emit resets, in response to this sysctl setting being
modified.

But that is not what you are doing.  You are treating all sockets
in the system in this way, and only for reset packets.  That's
really terrible.