[PATCH net-next v4] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

[PATCH net-next v4] 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 can obtain the correct ipv6 ra_mtu value
and compare 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/uapi/linux/if_link.h       |  1 +
 net/ipv6/addrconf.c                |  8 ++++++++
 net/ipv6/ndisc.c                   | 17 +++++++++++------
 tools/include/uapi/linux/if_link.h |  1 +
 5 files changed, 23 insertions(+), 6 deletions(-)

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/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 5310003523ce..957ec9873e70 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..3eb957777b31 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -394,6 +394,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 		ndev->cnf.addr_gen_mode = IN6_ADDR_GEN_MODE_STABLE_PRIVACY;
 
 	ndev->cnf.mtu6 = dev->mtu;
+	ndev->ra_mtu = U32_MIN;
 	ndev->nd_parms = neigh_parms_alloc(dev, &nd_tbl);
 	if (!ndev->nd_parms) {
 		kfree(ndev);
@@ -5543,6 +5544,7 @@ 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 +5653,9 @@ 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 +5772,9 @@ 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't be set" },
 };
 
 static int check_addr_gen_mode(int mode)
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index c467c6419893..23e690769857 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1391,12 +1391,6 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 		}
 	}
 
-	/*
-	 *	Send a notify if RA changed managed/otherconf flags or timer settings
-	 */
-	if (send_ifinfo_notify)
-		inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
-
 skip_linkparms:
 
 	/*
@@ -1496,6 +1490,11 @@ 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;
+			send_ifinfo_notify = true;
+		}
+
 		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) {
@@ -1519,6 +1518,12 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 		ND_PRINTK(2, warn, "RA: invalid RA options\n");
 	}
 out:
+	/* Send a notify if RA changed managed/otherconf flags or timer
+	 * settings or ra_mtu value
+	 */
+	if (send_ifinfo_notify)
+		inet6_ifinfo_notify(RTM_NEWLINK, in6_dev);
+
 	fib6_info_release(rt);
 	if (neigh)
 		neigh_release(neigh);
diff --git a/tools/include/uapi/linux/if_link.h b/tools/include/uapi/linux/if_link.h
index eb15f319aa57..b3610fdd1fee 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 v4] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

[David Ahern]

On 8/17/21 1:26 AM, Rocco Yue wrote:
> @@ -1496,6 +1490,11 @@ 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;
> +			send_ifinfo_notify = true;
> +		}
> +
>  		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) {


If an RA no longer carries an MTU or if accept_ra_mtu is reset, then
in6_dev->ra_mtu should be reset to 0 right?

rest of the change looks good to me.

Re: [PATCH net-next v4] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

[Rocco Yue]

On Tue, 2021-08-17 at 09:05 -0600, David Ahern wrote:
On 8/17/21 1:26 AM, Rocco Yue wrote:
>> @@ -1496,6 +1490,11 @@ 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;
>> +			send_ifinfo_notify = true;
>> +		}
>> +
>>  		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) {
> 
> 
> If an RA no longer carries an MTU or if accept_ra_mtu is reset, then
> in6_dev->ra_mtu should be reset to 0 right?
> 
> rest of the change looks good to me.

Hi David,

Thanks for your review.

In this patch, if an RA no longer carries an MTU or if accept_ra_mtu is reset,
in6_dev->ra_mtu will not be reset to 0, its value will remain the previous
accept_ra_mtu=1 and the value of the mtu carried in the RA msg. This behavior
is same with mtu6. This should be reasonable, it would show that the device
had indeed received the ra_mtu before set accept_ra_mtu to 0 or an RA no longer
carries an mtu value. I am willing to listen to your suggestions and make
changes if needed, maybe it needs to add a new separate proc handler for
accept_ra_mtu.

In addition, at your prompt, I find that this patch maybe have a defect for
some types of virtual devices, that is, when the state of the virtual device
updates the value of ra_mtu during the UP period, when its state is set to
DOWN, ra_mtu is not reset to 0, so that its ra_mtu value remains the previous
value after the interface is re-UP. I think I need to fix it.

Thanks
Rocco

Re: [PATCH net-next v4] ipv6: add IFLA_INET6_RA_MTU to expose mtu value in the RA message

[David Ahern]

On 8/21/21 12:10 AM, Rocco Yue wrote:
> In this patch, if an RA no longer carries an MTU or if accept_ra_mtu is reset,
> in6_dev->ra_mtu will not be reset to 0, its value will remain the previous
> accept_ra_mtu=1 and the value of the mtu carried in the RA msg. This behavior
> is same with mtu6. This should be reasonable, it would show that the device
> had indeed received the ra_mtu before set accept_ra_mtu to 0 or an RA no longer
> carries an mtu value. I am willing to listen to your suggestions and make
> changes if needed, maybe it needs to add a new separate proc handler for
> accept_ra_mtu.

fair point. Consistency is important.


> 
> In addition, at your prompt, I find that this patch maybe have a defect for
> some types of virtual devices, that is, when the state of the virtual device
> updates the value of ra_mtu during the UP period, when its state is set to
> DOWN, ra_mtu is not reset to 0, so that its ra_mtu value remains the previous
> value after the interface is re-UP. I think I need to fix it.
> 

Please do. Also, that problem should apply to all netdev's not just
virtual devices if you are referring to admin down (e.g., ip link set
$DEV down)