rtnetlink: do not acquire RTNL in rtnl_getlink() with RTEXT_FILTER_NAME_ONLY

When RTEXT_FILTER_NAME_ONLY is requested, rtnl_fill_ifinfo()
is dumping device attributes which do not need RTNL protection.

Many shell scripts invoke iproute2 commands specifying a device by
its name. After this patch, they will no longer add RTNL pressure.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260525083542.1565964-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index cd1004410dd7f5c45ebfdc329b461dde7b1d9411..6041e008b22dbfd164ede6d50a77d2db5d7e2e23 100644 (file)
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -2068,7 +2068,6 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
        struct nlmsghdr *nlh;
        struct Qdisc *qdisc;
 
-       ASSERT_RTNL();
        nlh = nlmsg_put(skb, pid, seq, type, sizeof(*ifm), flags);
        if (nlh == NULL)
                return -EMSGSIZE;
@@ -2091,6 +2090,7 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
        if (ext_filter_mask & RTEXT_FILTER_NAME_ONLY)
                goto end;
 
+       ASSERT_RTNL();
        if (tgt_netnsid >= 0 &&
            nla_put_s32(skb, IFLA_TARGET_NETNSID, tgt_netnsid))
                goto nla_put_failure;
@@ -3468,6 +3468,21 @@ static struct net_device *rtnl_dev_get(struct net *net,
        return __dev_get_by_name(net, ifname);
 }
 
+static struct net_device *rtnl_dev_get_rcu(struct net *net,
+                                          struct nlattr *tb[])
+{
+       char ifname[ALTIFNAMSIZ];
+
+       if (tb[IFLA_IFNAME])
+               nla_strscpy(ifname, tb[IFLA_IFNAME], IFNAMSIZ);
+       else if (tb[IFLA_ALT_IFNAME])
+               nla_strscpy(ifname, tb[IFLA_ALT_IFNAME], ALTIFNAMSIZ);
+       else
+               return NULL;
+
+       return dev_get_by_name_rcu(net, ifname);
+}
+
 static int rtnl_setlink(struct sk_buff *skb, struct nlmsghdr *nlh,
                        struct netlink_ext_ack *extack)
 {
@@ -4187,14 +4202,16 @@ static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
                        struct netlink_ext_ack *extack)
 {
        struct net *net = sock_net(skb->sk);
+       struct nlattr *tb[IFLA_MAX + 1];
+       netdevice_tracker dev_tracker;
+       struct net_device *dev = NULL;
        struct net *tgt_net = net;
+       u32 ext_filter_mask = 0;
        struct ifinfomsg *ifm;
-       struct nlattr *tb[IFLA_MAX+1];
-       struct net_device *dev = NULL;
        struct sk_buff *nskb;
        int netnsid = -1;
+       bool need_rtnl;
        int err;
-       u32 ext_filter_mask = 0;
 
        err = rtnl_valid_getlink_req(skb, nlh, tb, extack);
        if (err < 0)
@@ -4214,43 +4231,65 @@ static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
        if (tb[IFLA_EXT_MASK])
                ext_filter_mask = nla_get_u32(tb[IFLA_EXT_MASK]);
 
-       err = -EINVAL;
        ifm = nlmsg_data(nlh);
-       if (ifm->ifi_index > 0)
-               dev = __dev_get_by_index(tgt_net, ifm->ifi_index);
-       else if (tb[IFLA_IFNAME] || tb[IFLA_ALT_IFNAME])
-               dev = rtnl_dev_get(tgt_net, tb);
-       else
+       rcu_read_lock();
+       if (ifm->ifi_index > 0) {
+               dev = dev_get_by_index_rcu(tgt_net, ifm->ifi_index);
+       } else if (tb[IFLA_IFNAME] || tb[IFLA_ALT_IFNAME]) {
+               dev = rtnl_dev_get_rcu(tgt_net, tb);
+       } else {
+               rcu_read_unlock();
+               err = -EINVAL;
                goto out;
+       }
+       netdev_hold(dev, &dev_tracker, GFP_ATOMIC);
+       rcu_read_unlock();
 
        err = -ENODEV;
        if (dev == NULL)
                goto out;
 
+       need_rtnl = !(ext_filter_mask & RTEXT_FILTER_NAME_ONLY);
+
+retry:
+       if (need_rtnl) {
+               rtnl_lock();
+               /* Synchronize the carrier state so we don't report a state
+                * that we're not actually going to honour immediately; if
+                * the driver just did a carrier off->on transition, we can
+                * only TX if link watch work has run, but without this we'd
+                * already report carrier on, even if it doesn't work yet.
+                */
+               linkwatch_sync_dev(dev);
+       }
+
        err = -ENOBUFS;
        nskb = nlmsg_new_large(if_nlmsg_size(dev, ext_filter_mask));
-       if (nskb == NULL)
-               goto out;
+       if (nskb)
+               err = rtnl_fill_ifinfo(nskb, dev, net,
+                                      RTM_NEWLINK, NETLINK_CB(skb).portid,
+                                      nlh->nlmsg_seq, 0, 0, ext_filter_mask,
+                                      0, NULL, 0, netnsid, GFP_KERNEL);
 
-       /* Synchronize the carrier state so we don't report a state
-        * that we're not actually going to honour immediately; if
-        * the driver just did a carrier off->on transition, we can
-        * only TX if link watch work has run, but without this we'd
-        * already report carrier on, even if it doesn't work yet.
-        */
-       linkwatch_sync_dev(dev);
+       if (need_rtnl)
+               rtnl_unlock();
 
-       err = rtnl_fill_ifinfo(nskb, dev, net,
-                              RTM_NEWLINK, NETLINK_CB(skb).portid,
-                              nlh->nlmsg_seq, 0, 0, ext_filter_mask,
-                              0, NULL, 0, netnsid, GFP_KERNEL);
        if (err < 0) {
-               /* -EMSGSIZE implies BUG in if_nlmsg_size */
-               WARN_ON(err == -EMSGSIZE);
                kfree_skb(nskb);
-       } else
+               if (err == -EMSGSIZE) {
+                       if (!need_rtnl) {
+                               /* Some altnames were added, retry with RTNL. */
+                               need_rtnl = true;
+                               goto retry;
+                       }
+                       /* -EMSGSIZE implies BUG in if_nlmsg_size */
+                       WARN_ON_ONCE(1);
+               }
+       } else {
                err = rtnl_unicast(nskb, net, NETLINK_CB(skb).portid);
+       }
 out:
+       netdev_put(dev, &dev_tracker);
        if (netnsid >= 0)
                put_net(tgt_net);
 
@@ -7117,7 +7156,8 @@ static const struct rtnl_msg_handler rtnetlink_rtnl_msg_handlers[] __initconst =
        {.msgtype = RTM_DELLINK, .doit = rtnl_dellink,
         .flags = RTNL_FLAG_DOIT_PERNET_WIP},
        {.msgtype = RTM_GETLINK, .doit = rtnl_getlink,
-        .dumpit = rtnl_dump_ifinfo, .flags = RTNL_FLAG_DUMP_SPLIT_NLM_DONE},
+        .dumpit = rtnl_dump_ifinfo,
+        .flags = RTNL_FLAG_DUMP_SPLIT_NLM_DONE | RTNL_FLAG_DOIT_UNLOCKED},
        {.msgtype = RTM_SETLINK, .doit = rtnl_setlink,
         .flags = RTNL_FLAG_DOIT_PERNET_WIP},
        {.msgtype = RTM_GETADDR, .dumpit = rtnl_dump_all},