Re: [PATCH net-next v3] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

[David Ahern <dsahern@gmail.com>]

On 8/10/21 6:33 AM, Rocco Yue wrote:
> On Mon, 2021-08-09 at 16:43 -0600, David Ahern wrote:
>> On 8/9/21 8:01 AM, Rocco Yue wrote:
> 
>> +
>>>  #ifdef CONFIG_SYSCTL
>>>  
>>>  static int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
>>> diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
>>> index c467c6419893..a04164cbd77f 100644
>>> --- a/net/ipv6/ndisc.c
>>> +++ b/net/ipv6/ndisc.c
>>> @@ -1496,6 +1496,12 @@ static void ndisc_router_discovery(struct sk_buff *skb)
>>>  		memcpy(&n, ((u8 *)(ndopts.nd_opts_mtu+1))+2, sizeof(mtu));
>>>  		mtu = ntohl(n);
>>>  
>>> +		if (in6_dev->ra_mtu != mtu) {
>>> +			in6_dev->ra_mtu = mtu;
>>> +			inet6_iframtu_notify(in6_dev);
>>> +			ND_PRINTK(2, info, "update ra_mtu to %d\n", in6_dev->ra_mtu);
>>> +		}
>>> +
>>>  		if (mtu < IPV6_MIN_MTU || mtu > skb->dev->mtu) {
>>>  			ND_PRINTK(2, warn, "RA: invalid mtu: %d\n", mtu);
>>>  		} else if (in6_dev->cnf.mtu6 != mtu) {
>>
>> Since this MTU is getting reported via af_info infrastructure,
>> rtmsg_ifinfo should be sufficient.
>>
>> From there use 'ip monitor' to make sure you are not generating multiple
>> notifications; you may only need this on the error path.
> 
> Hi David,
> 
> To avoid generating multiple notifications, I added a separate ramtu notify
> function in this patch, and I added RTNLGRP_IPV6_IFINFO nl_mgrp to the ipmonitor.c
> to verify this patch was as expected.
> 
> I look at the rtmsg_ifinfo code, it should be appropriate and I will use it and
> verify it.
> 
> But there's one thing, I'm sorry I didn't fully understand the meaning of this
> sentence "you may only need this on the error path". Honestly, I'm not sure what
> the error patch refers to, do you mean "if (mtu < IPV6_MIN_MTU || mtu > skb->dev->mtu)" ?
> 

looks like nothing under:
    if (ndopts.nd_opts_mtu && in6_dev->cnf.accept_ra_mtu) {

    }

is going to send a link notification so you can just replace
inet6_iframtu_notify with rtmsg_ifinfo in your proposed change.