ipv6: flowlabel: take ip6_fl_lock across mem_check and fl_intern

mem_check() in net/ipv6/ip6_flowlabel.c reads fl_size without
holding ip6_fl_lock. fl_intern() takes the lock immediately
afterwards. The two checks therefore race against concurrent
fl_intern, ip6_fl_gc and ip6_fl_purge writers, which makes the
mem_check budget check approximate.

Move spin_lock_bh(&ip6_fl_lock) and the matching unlock from
fl_intern() into its only caller ipv6_flowlabel_get(). The
mem_check() call now runs under the same critical section as the
fl_intern() insert, so the budget check is exact.

With all writers and the read of fl_size under ip6_fl_lock,
convert fl_size from atomic_t to plain int. The four sites that
update or read fl_size are fl_intern (insert path), ip6_fl_gc
(garbage collector, the !sched check and the per-entry decrement),
ip6_fl_purge (per-netns purge), and mem_check (budget check), and
all four now run under ip6_fl_lock.

This is a prerequisite for adding a per-netns budget alongside
fl_size. The follow-up patch adds netns_ipv6::flowlabel_count and
folds it into mem_check().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Suggested-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Maoyi Xie <maoyi.xie@ntu.edu.sg>
Link: https://patch.msgid.link/20260506082416.2259567-2-maoyixie.tju@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 21 insertions, 13 deletions

diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c
index c92f98c6f6ec..a8974643195a 100644
--- a/net/ipv6/ip6_flowlabel.c
+++ b/net/ipv6/ip6_flowlabel.c
@@ -40,7 +40,7 @@
 #define FL_HASH_MASK	255
 #define FL_HASH(l)	(ntohl(l)&FL_HASH_MASK)
 
-static atomic_t fl_size = ATOMIC_INIT(0);
+static int fl_size;
 static struct ip6_flowlabel __rcu *fl_ht[FL_HASH_MASK+1];
 
 static void ip6_fl_gc(struct timer_list *unused);
@@ -163,7 +163,7 @@ static void ip6_fl_gc(struct timer_list *unused)
 				if (time_after_eq(now, ttd)) {
 					*flp = fl->next;
 					fl_free(fl);
-					atomic_dec(&fl_size);
+					fl_size--;
 					continue;
 				}
 				if (!sched || time_before(ttd, sched))
@@ -172,7 +172,7 @@ static void ip6_fl_gc(struct timer_list *unused)
 			flp = &fl->next;
 		}
 	}
-	if (!sched && atomic_read(&fl_size))
+	if (!sched && fl_size)
 		sched = now + FL_MAX_LINGER;
 	if (sched) {
 		mod_timer(&ip6_fl_gc_timer, sched);
@@ -196,7 +196,7 @@ static void __net_exit ip6_fl_purge(struct net *net)
 			    atomic_read(&fl->users) == 0) {
 				*flp = fl->next;
 				fl_free(fl);
-				atomic_dec(&fl_size);
+				fl_size--;
 				continue;
 			}
 			flp = &fl->next;
@@ -210,10 +210,10 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 {
 	struct ip6_flowlabel *lfl;
 
+	lockdep_assert_held(&ip6_fl_lock);
+
 	fl->label = label & IPV6_FLOWLABEL_MASK;
 
-	rcu_read_lock();
-	spin_lock_bh(&ip6_fl_lock);
 	if (label == 0) {
 		for (;;) {
 			fl->label = htonl(get_random_u32())&IPV6_FLOWLABEL_MASK;
@@ -235,8 +235,6 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 		lfl = __fl_lookup(net, fl->label);
 		if (lfl) {
 			atomic_inc(&lfl->users);
-			spin_unlock_bh(&ip6_fl_lock);
-			rcu_read_unlock();
 			return lfl;
 		}
 	}
@@ -244,9 +242,7 @@ static struct ip6_flowlabel *fl_intern(struct net *net,
 	fl->lastuse = jiffies;
 	fl->next = fl_ht[FL_HASH(fl->label)];
 	rcu_assign_pointer(fl_ht[FL_HASH(fl->label)], fl);
-	atomic_inc(&fl_size);
-	spin_unlock_bh(&ip6_fl_lock);
-	rcu_read_unlock();
+	fl_size++;
 	return NULL;
 }
 
@@ -464,10 +460,14 @@ done:
 
 static int mem_check(struct sock *sk)
 {
-	int room = FL_MAX_SIZE - atomic_read(&fl_size);
+	int room;
 	struct ipv6_fl_socklist *sfl;
 	int count = 0;
 
+	lockdep_assert_held(&ip6_fl_lock);
+
+	room = FL_MAX_SIZE - fl_size;
+
 	if (room > FL_MAX_SIZE - FL_MAX_PER_SOCK)
 		return 0;
 
@@ -692,11 +692,19 @@ release:
 	if (!sfl1)
 		goto done;
 
+	rcu_read_lock();
+	spin_lock_bh(&ip6_fl_lock);
 	err = mem_check(sk);
+	if (err == 0)
+		fl1 = fl_intern(net, fl, freq->flr_label);
+	else
+		fl1 = NULL;
+	spin_unlock_bh(&ip6_fl_lock);
+	rcu_read_unlock();
+
 	if (err != 0)
 		goto done;
 
-	fl1 = fl_intern(net, fl, freq->flr_label);
 	if (fl1)
 		goto recheck;