ipv6: Remove rcu_read_lock() in fib6_get_table().

Once allocated, the IPv6 routing table is not freed until
netns is dismantled.

fib6_get_table() uses rcu_read_lock() while iterating
net->ipv6.fib_table_hash[], but it's not needed and
rather confusing.

Because some callers have this pattern,

table = fib6_get_table();

rcu_read_lock();
/* ... use table here ... */
rcu_read_unlock();

[ See: addrconf_get_prefix_route(), ip6_route_del(),
rt6_get_route_info(), rt6_get_dflt_router() ]

and this looks illegal but is actually safe.

Let's remove rcu_read_lock() in fib6_get_table() and pass true
to the last argument of hlist_for_each_entry_rcu() to bypass
the RCU check.

Note that protection is not needed but RCU helper is used to
avoid data-race.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/ip6_fib.c

···
 
 struct fib6_table *fib6_get_table(struct net *net, u32 id)
 {
-	struct fib6_table *tb;
 	struct hlist_head *head;
-	unsigned int h;
+	struct fib6_table *tb;
 
-	if (id == 0)
+	if (!id)
 		id = RT6_TABLE_MAIN;
-	h = id & (FIB6_TABLE_HASHSZ - 1);
-	rcu_read_lock();
-	head = &net->ipv6.fib_table_hash[h];
-	hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
-		if (tb->tb6_id == id) {
-			rcu_read_unlock();
+
+	head = &net->ipv6.fib_table_hash[id & (FIB6_TABLE_HASHSZ - 1)];
+
+	/* See comment in fib6_link_table().  RCU is not required,
+	 * but rcu_dereference_raw() is used to avoid data-race.
+	 */
+	hlist_for_each_entry_rcu(tb, head, tb6_hlist, true)
+		if (tb->tb6_id == id)
 			return tb;
-		}
-	}
-	rcu_read_unlock();
 
 	return NULL;
 }