netdev - [PATCH net 2/2] Revert "gre: Fix IPv6 link-local address generation."

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Message-ID: <8b1ce738eb15dd841aab9ef888640cab4f6ccfea.1742418408.git.gnault@redhat.com>
Date: Wed, 19 Mar 2025 22:26:50 +0100
From: Guillaume Nault <gnault@...hat.com>
To: David Miller <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, Simon Horman <horms@...nel.org>,
	David Ahern <dsahern@...nel.org>,
	Antonio Quartulli <antonio@...delbit.com>,
	Ido Schimmel <idosch@...sch.org>, Petr Machata <petrm@...dia.com>,
	Stanislav Fomichev <stfomichev@...il.com>
Subject: [PATCH net 2/2] Revert "gre: Fix IPv6 link-local address generation."

This reverts commit 183185a18ff96751db52a46ccf93fff3a1f42815.

This patch broke net/forwarding/ip6gre_custom_multipath_hash.sh in some
circumstances (https://lore.kernel.org/netdev/Z9RIyKZDNoka53EO@mini-arch/).
Let's revert it while the problem is being investigated.

Fixes: 183185a18ff9 ("gre: Fix IPv6 link-local address generation.")
Signed-off-by: Guillaume Nault <gnault@...hat.com>
---
 net/ipv6/addrconf.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 8b6258819dad..ac8cc1076536 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3209,13 +3209,16 @@ static void add_v4_addrs(struct inet6_dev *idev)
 	struct in6_addr addr;
 	struct net_device *dev;
 	struct net *net = dev_net(idev->dev);
-	int scope, plen;
+	int scope, plen, offset = 0;
 	u32 pflags = 0;
 
 	ASSERT_RTNL();
 
 	memset(&addr, 0, sizeof(struct in6_addr));
-	memcpy(&addr.s6_addr32[3], idev->dev->dev_addr, 4);
+	/* in case of IP6GRE the dev_addr is an IPv6 and therefore we use only the last 4 bytes */
+	if (idev->dev->addr_len == sizeof(struct in6_addr))
+		offset = sizeof(struct in6_addr) - 4;
+	memcpy(&addr.s6_addr32[3], idev->dev->dev_addr + offset, 4);
 
 	if (!(idev->dev->flags & IFF_POINTOPOINT) && idev->dev->type == ARPHRD_SIT) {
 		scope = IPV6_ADDR_COMPATv4;
@@ -3526,13 +3529,7 @@ static void addrconf_gre_config(struct net_device *dev)
 		return;
 	}
 
-	/* Generate the IPv6 link-local address using addrconf_addr_gen(),
-	 * unless we have an IPv4 GRE device not bound to an IP address and
-	 * which is in EUI64 mode (as __ipv6_isatap_ifid() would fail in this
-	 * case). Such devices fall back to add_v4_addrs() instead.
-	 */
-	if (!(dev->type == ARPHRD_IPGRE && *(__be32 *)dev->dev_addr == 0 &&
-	      idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_EUI64)) {
+	if (dev->type == ARPHRD_ETHER) {
 		addrconf_addr_gen(idev, true);
 		return;
 	}
-- 
2.39.2