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

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

[Rocco Yue]

The kernel provides a "/proc/sys/net/ipv6/conf/<iface>/mtu"
file, which can temporarily record the mtu value of the last
received RA message when the RA mtu value is lower than the
interface mtu, but this proc has following limitations:

(1) when the interface mtu (/sys/class/net/<iface>/mtu) is
updeated, mtu6 (/proc/sys/net/ipv6/conf/<iface>/mtu) will
be updated to the value of interface mtu;
(2) mtu6 (/proc/sys/net/ipv6/conf/<iface>/mtu) only affect
ipv6 connection, and not affect ipv4.

Therefore, when the mtu option is carried in the RA message,
there will be a problem that the user sometimes cannot obtain
RA mtu value correctly by reading mtu6.

After this patch set, if a RA message carries the mtu option,
you can send a netlink msg which nlmsg_type is RTM_GETLINK,
and then by parsing the attribute of IFLA_INET6_RA_MTU to
get the mtu value carried in the RA message received on the
inet6 device. In addition, you can also get a link notification
when ra_mtu is updated so it doesn't have to poll.

In this way, if the MTU values that the device receives from
the network in the PCO IPv4 and the RA IPv6 procedures are
different, the user space process can read ra_mtu to get
the mtu value carried in the RA message without worrying
about the issue of ipv4 being stuck due to the late arrival
of RA message. After comparing the value of ra_mtu and ipv4
mtu, then the device can use the lower MTU value for both
IPv4 and IPv6.

Signed-off-by: Rocco Yue <rocco.yue@mediatek.com>
---
 include/net/if_inet6.h             |  2 +
 include/net/ndisc.h                |  1 +
 include/uapi/linux/if_link.h       |  1 +
 net/ipv6/addrconf.c                | 67 ++++++++++++++++++++++++++++++
 net/ipv6/ndisc.c                   |  6 +++
 tools/include/uapi/linux/if_link.h |  1 +
 6 files changed, 78 insertions(+)

diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h
index 42235c178b06..653e7d0f65cb 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -210,6 +210,8 @@ struct inet6_dev {
 
 	unsigned long		tstamp; /* ipv6InterfaceTable update timestamp */
 	struct rcu_head		rcu;
+
+	unsigned int		ra_mtu;
 };
 
 static inline void ipv6_eth_mc_map(const struct in6_addr *addr, char *buf)
diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index 38e4094960ce..c7b7f1b002fd 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -501,5 +501,6 @@ int ndisc_ifinfo_sysctl_strategy(struct ctl_table *ctl,
 #endif
 
 void inet6_ifinfo_notify(int event, struct inet6_dev *idev);
+void inet6_iframtu_notify(struct inet6_dev *idev);
 
 #endif
diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 5310003523ce..6dfdae1bb860 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -417,6 +417,7 @@ enum {
 	IFLA_INET6_ICMP6STATS,	/* statistics (icmpv6)		*/
 	IFLA_INET6_TOKEN,	/* device token			*/
 	IFLA_INET6_ADDR_GEN_MODE, /* implicit address generator mode */
+	IFLA_INET6_RA_MTU,	/* mtu carried in the RA message  */
 	__IFLA_INET6_MAX
 };
 
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 8381288a0d6e..3e3cdd49c295 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -5543,6 +5543,7 @@ static inline size_t inet6_ifla6_size(void)
 	     + nla_total_size(ICMP6_MIB_MAX * 8) /* IFLA_INET6_ICMP6STATS */
 	     + nla_total_size(sizeof(struct in6_addr)) /* IFLA_INET6_TOKEN */
 	     + nla_total_size(1) /* IFLA_INET6_ADDR_GEN_MODE */
+	     + nla_total_size(4) /* IFLA_INET6_RA_MTU */
 	     + 0;
 }
 
@@ -5651,6 +5652,9 @@ static int inet6_fill_ifla6_attrs(struct sk_buff *skb, struct inet6_dev *idev,
 	if (nla_put_u8(skb, IFLA_INET6_ADDR_GEN_MODE, idev->cnf.addr_gen_mode))
 		goto nla_put_failure;
 
+	if (nla_put_u32(skb, IFLA_INET6_RA_MTU, idev->ra_mtu))
+		goto nla_put_failure;
+
 	return 0;
 
 nla_put_failure:
@@ -5767,6 +5771,9 @@ static int inet6_set_iftoken(struct inet6_dev *idev, struct in6_addr *token,
 static const struct nla_policy inet6_af_policy[IFLA_INET6_MAX + 1] = {
 	[IFLA_INET6_ADDR_GEN_MODE]	= { .type = NLA_U8 },
 	[IFLA_INET6_TOKEN]		= { .len = sizeof(struct in6_addr) },
+	[IFLA_INET6_RA_MTU]		= { .type = NLA_REJECT,
+					    .reject_message =
+					    "IFLA_INET6_RA_MTU can not be set" },
 };
 
 static int check_addr_gen_mode(int mode)
@@ -6129,6 +6136,66 @@ static void ipv6_ifa_notify(int event, struct inet6_ifaddr *ifp)
 		__ipv6_ifa_notify(event, ifp);
 }
 
+static inline size_t inet6_iframtu_msgsize(void)
+{
+	return NLMSG_ALIGN(sizeof(struct ifinfomsg))
+	     + nla_total_size(IFNAMSIZ)	/* IFLA_IFNAME */
+	     + nla_total_size(4);	/* IFLA_INET6_RA_MTU */
+}
+
+static int inet6_fill_iframtu(struct sk_buff *skb, struct inet6_dev *idev)
+{
+	struct net_device *dev = idev->dev;
+	struct ifinfomsg *hdr;
+	struct nlmsghdr *nlh;
+
+	nlh = nlmsg_put(skb, 0, 0, RTM_NEWLINK, sizeof(*hdr), 0);
+	if (!nlh)
+		return -EMSGSIZE;
+
+	hdr = nlmsg_data(nlh);
+	hdr->ifi_family = AF_INET6;
+	hdr->__ifi_pad = 0;
+	hdr->ifi_index = dev->ifindex;
+	hdr->ifi_flags = dev_get_flags(dev);
+	hdr->ifi_change = 0;
+
+	if (nla_put_string(skb, IFLA_IFNAME, dev->name) ||
+	    nla_put_u32(skb, IFLA_INET6_RA_MTU, idev->ra_mtu))
+		goto nla_put_failure;
+
+	nlmsg_end(skb, nlh);
+	return 0;
+
+nla_put_failure:
+	nlmsg_cancel(skb, nlh);
+	return -EMSGSIZE;
+}
+
+void inet6_iframtu_notify(struct inet6_dev *idev)
+{
+	struct sk_buff *skb;
+	struct net *net = dev_net(idev->dev);
+	int err = -ENOBUFS;
+
+	skb = nlmsg_new(inet6_iframtu_msgsize(), GFP_ATOMIC);
+	if (!skb)
+		goto errout;
+
+	err = inet6_fill_iframtu(skb, idev);
+	if (err < 0) {
+		/* -EMSGSIZE implies BUG in inet6_iframtu_msgsize() */
+		WARN_ON(err == -EMSGSIZE);
+		kfree_skb(skb);
+		goto errout;
+	}
+	rtnl_notify(skb, net, 0, RTNLGRP_IPV6_IFINFO, NULL, GFP_ATOMIC);
+	return;
+errout:
+	if (err < 0)
+		rtnl_set_sk_err(net, RTNLGRP_IPV6_IFINFO, err);
+}
+
 #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) {
diff --git a/tools/include/uapi/linux/if_link.h b/tools/include/uapi/linux/if_link.h
index eb15f319aa57..36a8ae4793f1 100644
--- a/tools/include/uapi/linux/if_link.h
+++ b/tools/include/uapi/linux/if_link.h
@@ -230,6 +230,7 @@ enum {
 	IFLA_INET6_ICMP6STATS,	/* statistics (icmpv6)		*/
 	IFLA_INET6_TOKEN,	/* device token			*/
 	IFLA_INET6_ADDR_GEN_MODE, /* implicit address generator mode */
+	IFLA_INET6_RA_MTU,	/* mtu carried in the RA message  */
 	__IFLA_INET6_MAX
 };
 
-- 
2.18.0

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

[Jakub Kicinski]

On Mon, 9 Aug 2021 22:01:09 +0800 Rocco Yue wrote:
> +static inline size_t inet6_iframtu_msgsize(void)
> +{
> +	return NLMSG_ALIGN(sizeof(struct ifinfomsg))
> +	     + nla_total_size(IFNAMSIZ)	/* IFLA_IFNAME */
> +	     + nla_total_size(4);	/* IFLA_INET6_RA_MTU */
> +}

Please don't use 'static inline' in C sources, static is enough

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

[David Ahern]

On 8/9/21 8:01 AM, Rocco Yue wrote:
> @@ -6129,6 +6136,66 @@ static void ipv6_ifa_notify(int event, struct inet6_ifaddr *ifp)
>  		__ipv6_ifa_notify(event, ifp);
>  }
>  
> +static inline size_t inet6_iframtu_msgsize(void)
> +{
> +	return NLMSG_ALIGN(sizeof(struct ifinfomsg))
> +	     + nla_total_size(IFNAMSIZ)	/* IFLA_IFNAME */
> +	     + nla_total_size(4);	/* IFLA_INET6_RA_MTU */
> +}
> +
> +static int inet6_fill_iframtu(struct sk_buff *skb, struct inet6_dev *idev)
> +{
> +	struct net_device *dev = idev->dev;
> +	struct ifinfomsg *hdr;
> +	struct nlmsghdr *nlh;
> +
> +	nlh = nlmsg_put(skb, 0, 0, RTM_NEWLINK, sizeof(*hdr), 0);
> +	if (!nlh)
> +		return -EMSGSIZE;
> +
> +	hdr = nlmsg_data(nlh);
> +	hdr->ifi_family = AF_INET6;
> +	hdr->__ifi_pad = 0;
> +	hdr->ifi_index = dev->ifindex;
> +	hdr->ifi_flags = dev_get_flags(dev);
> +	hdr->ifi_change = 0;
> +
> +	if (nla_put_string(skb, IFLA_IFNAME, dev->name) ||
> +	    nla_put_u32(skb, IFLA_INET6_RA_MTU, idev->ra_mtu))
> +		goto nla_put_failure;
> +
> +	nlmsg_end(skb, nlh);
> +	return 0;
> +
> +nla_put_failure:
> +	nlmsg_cancel(skb, nlh);
> +	return -EMSGSIZE;
> +}
> +
> +void inet6_iframtu_notify(struct inet6_dev *idev)
> +{
> +	struct sk_buff *skb;
> +	struct net *net = dev_net(idev->dev);
> +	int err = -ENOBUFS;
> +
> +	skb = nlmsg_new(inet6_iframtu_msgsize(), GFP_ATOMIC);
> +	if (!skb)
> +		goto errout;
> +
> +	err = inet6_fill_iframtu(skb, idev);
> +	if (err < 0) {
> +		/* -EMSGSIZE implies BUG in inet6_iframtu_msgsize() */
> +		WARN_ON(err == -EMSGSIZE);
> +		kfree_skb(skb);
> +		goto errout;
> +	}
> +	rtnl_notify(skb, net, 0, RTNLGRP_IPV6_IFINFO, NULL, GFP_ATOMIC);
> +	return;
> +errout:
> +	if (err < 0)
> +		rtnl_set_sk_err(net, RTNLGRP_IPV6_IFINFO, err);
> +}

pretty sure you don't need to build a new notify function.

> +
>  #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.

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

[Rocco Yue]

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)" ?

Thanks
Rocco

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

[David Ahern]

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.

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

[David Ahern]

On 8/11/21 7:56 AM, David Ahern wrote:
> 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.
> 

Taking a deeper dive on the code, you do not need to call rtmsg_ifinfo.
Instead, the existing:

        /*
         *      Send a notify if RA changed managed/otherconf flags or
timer settings
         */
        if (send_ifinfo_notify)
                inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);

is called too early. For one the RA can change the MTU and that is done
after this notify.

I think if you moved this down to the out:

out:
        /*
         *      Send a notify if RA changed managed/otherconf flags or
timer settings
         */
        if (send_ifinfo_notify)
                inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);

and then set send_ifinfo_notify when the mtu is *changed* by the RA you
should be good.

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

[Rocco Yue]

On Wed, 2021-08-11 at 12:05 -0600, David Ahern wrote:
> On 8/11/21 7:56 AM, David Ahern wrote:
>> On 8/10/21 6:33 AM, Rocco Yue wrote:
>>> On Mon, 2021-08-09 at 16:43 -0600, David Ahern wrote:
>>>>
>>>> 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.
>> 
> 
> Taking a deeper dive on the code, you do not need to call rtmsg_ifinfo.
> Instead, the existing:
> 
>         /*
>          *      Send a notify if RA changed managed/otherconf flags or
> timer settings
>          */
>         if (send_ifinfo_notify)
>                 inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
> 
> is called too early. For one the RA can change the MTU and that is done
> after this notify.
> 
> I think if you moved this down to the out:
> 
> out:
>         /*
>          *      Send a notify if RA changed managed/otherconf flags or
> timer settings
>          */
>         if (send_ifinfo_notify)
>                 inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
> 
> and then set send_ifinfo_notify when the mtu is *changed* by the RA you
> should be good.

Hi David,

Thanks for your suggestion,
this looks like a better choice without adding a separate notification function,
I will modify it and push the next iteration .

Best Regards,
Rocco