Re: [PATCH 1/2 net v3] ipv6: addrconf: fix temp address generation after prefix deprecation

[Ido Schimmel <idosch@nvidia.com>]

On Thu, May 07, 2026 at 03:28:27PM +0200, Fernando Fernandez Mancera wrote:
> When a router temporarily deprecates an IPv6 prefix (either by sending a
> Router Advertisement with Preferred Lifetime = 0 or by letting the
> lifetime expire) and later restores it, the kernel permanently loses its
> ability to generate temporary privacy addresses (RFC 8981) for that
> prefix.
> 
> This happens because the address worker attempts to generate a
> replacement temporary address when the current one nears expiration. As
> the base prefix is deprecated already, the generation fails after
> marking the temporary address already having spawned a replacement
> (ifp->regen_count++).
> 
> When the router eventually restores the prefix, the temporary address
> becomes active again. However, once it naturally expires, the address
> worker sees this temporary address already tried to generate one and
> skips the regeneration.
> 
> Fix this by verifying that the base prefix has sufficient preferred
> lifetime remaining before attempting to generate a new temporary
> address. In addition, make ipv6_create_tempaddr() return meaningful
> error codes. This way, we can catch if a 0-lft RA arrived just after we
> passed the verification mentioned above. If we don't have sufficient
> preferred lifetime remaining, the worker will keep the next timer as it
> is.

I didn't go through all the Sashiko comments, but at least some of them
seem valid. I wonder if we can simplify this and do the following
instead:

When updating all the temporary addresses (f.e., because we received a
RA), instead of only creating a temporary address if none exist, also
create a temporary address if all the existing ones already regenerated
a temporary address, as otherwise no temporary address will ever be
created. Something like [1]. I didn't run it through Sashiko, but I did
confirm that your test fails without it and passes with it.

[1]
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 5476b6536eb7..3eaa583bb08d 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2597,6 +2597,7 @@ static void manage_tempaddrs(struct inet6_dev *idev,
 {
 	u32 flags;
 	struct inet6_ifaddr *ift;
+	bool all_regen = true;
 
 	read_lock_bh(&idev->lock);
 	/* update all temporary addresses in the list */
@@ -2637,6 +2638,8 @@ static void manage_tempaddrs(struct inet6_dev *idev,
 		ift->tstamp = now;
 		if (prefered_lft > 0)
 			ift->flags &= ~IFA_F_DEPRECATED;
+		if (!ift->regen_count)
+			all_regen = false;
 
 		spin_unlock(&ift->lock);
 		if (!(flags&IFA_F_TENTATIVE))
@@ -2644,12 +2647,14 @@ static void manage_tempaddrs(struct inet6_dev *idev,
 	}
 
 	/* Also create a temporary address if it's enabled but no temporary
-	 * address currently exists.
+	 * address currently exists or if all the temporary addresses already
+	 * regenerated an address.
 	 * However, we get called with valid_lft == 0, prefered_lft == 0, create == false
 	 * as part of cleanup (ie. deleting the mngtmpaddr).
 	 * We don't want that to result in creating a new temporary ip address.
 	 */
-	if (list_empty(&idev->tempaddr_list) && (valid_lft || prefered_lft))
+	if ((list_empty(&idev->tempaddr_list) || all_regen)
+	    && (valid_lft || prefered_lft))
 		create = true;
 
 	if (create && READ_ONCE(idev->cnf.use_tempaddr) > 0) {