ipv6: remove nexthop_fib6_nh_bh()

After blamed commit, nexthop_fib6_nh_bh() and nexthop_fib6_nh()
are the same.

Delete nexthop_fib6_nh_bh(), and convert /proc/net/ipv6_route
to standard rcu to avoid this splat:

[ 5723.180080] WARNING: suspicious RCU usage
[ 5723.180083] -----------------------------
[ 5723.180084] include/net/nexthop.h:516 suspicious rcu_dereference_check() usage!
[ 5723.180086]
other info that might help us debug this:

[ 5723.180087]
rcu_scheduler_active = 2, debug_locks = 1
[ 5723.180089] 2 locks held by cat/55856:
[ 5723.180091] #0: ffff9440a582afa8 (&p->lock){+.+.}-{3:3}, at: seq_read_iter (fs/seq_file.c:188)
[ 5723.180100] #1: ffffffffaac07040 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire (include/linux/rcupdate.h:326)
[ 5723.180109]
stack backtrace:
[ 5723.180111] CPU: 14 PID: 55856 Comm: cat Tainted: G S        I        6.3.0-dbx-DEV #528
[ 5723.180115] Call Trace:
[ 5723.180117]  <TASK>
[ 5723.180119] dump_stack_lvl (lib/dump_stack.c:107)
[ 5723.180124] dump_stack (lib/dump_stack.c:114)
[ 5723.180126] lockdep_rcu_suspicious (include/linux/context_tracking.h:122)
[ 5723.180132] ipv6_route_seq_show (include/net/nexthop.h:?)
[ 5723.180135] ? ipv6_route_seq_next (net/ipv6/ip6_fib.c:2605)
[ 5723.180140] seq_read_iter (fs/seq_file.c:272)
[ 5723.180145] seq_read (fs/seq_file.c:163)
[ 5723.180151] proc_reg_read (fs/proc/inode.c:316 fs/proc/inode.c:328)
[ 5723.180155] vfs_read (fs/read_write.c:468)
[ 5723.180160] ? up_read (kernel/locking/rwsem.c:1617)
[ 5723.180164] ksys_read (fs/read_write.c:613)
[ 5723.180168] __x64_sys_read (fs/read_write.c:621)
[ 5723.180170] do_syscall_64 (arch/x86/entry/common.c:?)
[ 5723.180174] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
[ 5723.180177] RIP: 0033:0x7fa455677d2a

Fixes: 09eed119 ("neighbour: switch to standard rcu, instead of rcu_bh")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230510154646.370659-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/nexthop.h

@@ -497,29 +497,6 @@ static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
 	return NULL;
 }
 
-/* Variant of nexthop_fib6_nh().
- * Caller should either hold rcu_read_lock(), or RTNL.
- */
-static inline struct fib6_nh *nexthop_fib6_nh_bh(struct nexthop *nh)
-{
-	struct nh_info *nhi;
-
-	if (nh->is_group) {
-		struct nh_group *nh_grp;
-
-		nh_grp = rcu_dereference_rtnl(nh->nh_grp);
-		nh = nexthop_mpath_select(nh_grp, 0);
-		if (!nh)
-			return NULL;
-	}
-
-	nhi = rcu_dereference_rtnl(nh->nh_info);
-	if (nhi->family == AF_INET6)
-		return &nhi->fib6_nh;
-
-	return NULL;
-}
-
 static inline struct net_device *fib6_info_nh_dev(struct fib6_info *f6i)
 {
 	struct fib6_nh *fib6_nh;

net/ipv6/ip6_fib.c

@@ -2491,7 +2491,7 @@ static int ipv6_route_native_seq_show(struct seq_file *seq, void *v)
 	const struct net_device *dev;
 
 	if (rt->nh)
-		fib6_nh = nexthop_fib6_nh_bh(rt->nh);
+		fib6_nh = nexthop_fib6_nh(rt->nh);
 
 	seq_printf(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
 
@@ -2556,14 +2556,14 @@ static struct fib6_table *ipv6_route_seq_next_table(struct fib6_table *tbl,
 
 	if (tbl) {
 		h = (tbl->tb6_id & (FIB6_TABLE_HASHSZ - 1)) + 1;
-		node = rcu_dereference_bh(hlist_next_rcu(&tbl->tb6_hlist));
+		node = rcu_dereference(hlist_next_rcu(&tbl->tb6_hlist));
 	} else {
 		h = 0;
 		node = NULL;
 	}
 
 	while (!node && h < FIB6_TABLE_HASHSZ) {
-		node = rcu_dereference_bh(
+		node = rcu_dereference(
 			hlist_first_rcu(&net->ipv6.fib_table_hash[h++]));
 	}
 	return hlist_entry_safe(node, struct fib6_table, tb6_hlist);
@@ -2593,7 +2593,7 @@ static void *ipv6_route_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 	if (!v)
 		goto iter_table;
 
-	n = rcu_dereference_bh(((struct fib6_info *)v)->fib6_next);
+	n = rcu_dereference(((struct fib6_info *)v)->fib6_next);
 	if (n)
 		return n;
 
@@ -2619,12 +2619,12 @@ static void *ipv6_route_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 }
 
 static void *ipv6_route_seq_start(struct seq_file *seq, loff_t *pos)
-	__acquires(RCU_BH)
+	__acquires(RCU)
 {
 	struct net *net = seq_file_net(seq);
 	struct ipv6_route_iter *iter = seq->private;
 
-	rcu_read_lock_bh();
+	rcu_read_lock();
 	iter->tbl = ipv6_route_seq_next_table(NULL, net);
 	iter->skip = *pos;
 
@@ -2645,7 +2645,7 @@ static bool ipv6_route_iter_active(struct ipv6_route_iter *iter)
 }
 
 static void ipv6_route_native_seq_stop(struct seq_file *seq, void *v)
-	__releases(RCU_BH)
+	__releases(RCU)
 {
 	struct net *net = seq_file_net(seq);
 	struct ipv6_route_iter *iter = seq->private;
@@ -2653,7 +2653,7 @@ static void ipv6_route_native_seq_stop(struct seq_file *seq, void *v)
 	if (ipv6_route_iter_active(iter))
 		fib6_walker_unlink(net, &iter->w);
 
-	rcu_read_unlock_bh();
+	rcu_read_unlock();
 }
 
 #if IS_BUILTIN(CONFIG_IPV6) && defined(CONFIG_BPF_SYSCALL)