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Subject: Re: [PATCH] net: ipv6: don't generate link-local address in any
 addr_gen_mode
To:     Rocco Yue <rocco.yue@mediatek.com>
Cc:     "David S . Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, wsd_upstream@mediatek.com,
        rocco.yue@gmail.com, chao.song@mediatek.com,
        kuohong.wang@mediatek.com, zhuoliang.zhang@mediatek.com
References: <62c9f5b7-84bd-d809-4e33-39fed7a9d780@gmail.com>
 <20210706123702.29375-1-rocco.yue@mediatek.com>
From:   David Ahern <dsahern@gmail.com>
Message-ID: <40951584-8f53-4e95-4a6b-14ae1cf7f011@gmail.com>
Date:   Wed, 7 Jul 2021 08:39:36 -0600
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On 7/6/21 6:37 AM, Rocco Yue wrote:
> On Mon, 2021-07-05 at 10:35 -0600, David Ahern wrote:
>> On 7/1/21 2:51 AM, Rocco Yue wrote:
>>> On Wed, 2021-06-30 at 22:41 -0600, David Ahern wrote:
>>>
>>> For mobile operators that don't need to support RFC7217, setting
>>> addr_gen_mode == 1 is sufficient;
>>>
>>> But for some other mobile operators that need to support RFC7217, such as AT&T,
>>> the mobile device's addr_gen_mode will be switched to the
>>> IN6_ADDR_GEN_MODE_STABLE_PRIVACY, instead of using IN6_ADDR_GEN_MODE_NONE.
>>> The purpose is: in the IN6_ADDR_GEN_MODE_STABLE_PRIVACY mode, kernel can
>>> gererate a stable privacy global ipv6 address after receiveing RA, and
>>> network processes can use this global address to communicate with the
>>> outside network.
>>>
>>> Of course, mobile operators that need to support RFC7217 should also meet
>>> the requirement of 3GPP TS 29.061, that is, MT should use IID assigned by
>>> the GGSN to build its ipv6 link-local address and use this address to send RS.
>>> We don't want the kernel to automatically generate an ipv6 link-local address
>>> when addr_gen_mode == 2. Otherwise, using the stable privacy ipv6 link-local
>>> address automatically generated by the kernel to send RS message, GGSN will
>>> not be able to respond to the RS and reply a RA message.
>>>
>>> Therefore, after this patch, kernel will not generate ipv6 link-local address
>>> for the corresponding device when addr_gen_mode == 1 or addr_gen_mode == 2.
>>>
>>
>> I think another addr_gen_mode is better than a separate sysctl. It looks
>> like IN6_ADDR_GEN_MODE_STABLE_PRIVACY and IN6_ADDR_GEN_MODE_RANDOM are
>> the ones used for RAs, so add something like:
>>
>> IN6_ADDR_GEN_MODE_STABLE_PRIVACY_NO_LLA,
>> IN6_ADDR_GEN_MODE_RANDOM_NO_LLA,
>>
>> to in6_addr_gen_mode.
>>
> 
> Hi David,
> 
> Thanks for your reply.
> 
> According to your suggestion, I checked the ipv6 code again. In my
> opinion, adding another addr_gen_mode may not be suitable.
> 
> (1)
> In the user space, the process enable the ipv6 stable privacy mode by
> setting the "/proc/sys/net/ipv6/conf/<iface>/stable_secret".
> 
> In the kernel, the addr_gen_mode of a networking device is switched to
> IN6_ADDR_GEN_MODE_STABLE_PRIVACY by judging the bool value of
> "cnf.stable_secret.initialized".

and that can be updated. If the default (inherited) setting is
IN6_ADDR_GEN_MODE_STABLE_PRIVACY_NO_LLA, then do not change to
IN6_ADDR_GEN_MODE_STABLE_PRIVACY.

> 
> So, although adding an additional IN6_ADDR_GEN_MODE_STABLE_PRIVACY_NO_LLA,
> user space process has some trouble to let kernel switch the iface's
> addr_gen_mode to the IN6_ADDR_GEN_MODE_STABLE_PRIVACY_NO_LLA.
> 
> This is not as flexible as adding a separate sysctl.
> 
> (2)
> After adding "proc/sys/net/ipv6/<iface>/disable_gen_linklocal_addr",
> so that kernel can keep the original code logic of the stable_secret
> proc file, and expand when the subsequent kernel adds a new add_gen_mode
> more flexibility and applicability.
> 
> And we only need to care about the networking device that do not
> generate an ipv6 link-local address, and not the addr_gen_mode that
> this device is using.
> 
> Maybe adding a separate sysctl is a better choice.
> Looking forward to your professional reply again.

per device sysctl's are not free. I do not see a valid reason for a
separate disable knob.