suspicious RCU usage (was: Re: [PATCHv3 next 1/3] loopback: create blackhole net device similar to loopack.)
From: Geert Uytterhoeven
Date: Tue Jul 02 2019 - 08:54:59 EST
Hi Mahesh,
On Mon, 1 Jul 2019, Mahesh Bandewar wrote:
Create a blackhole net device that can be used for "dead"
dst entries instead of loopback device. This blackhole device differs
from loopback in few aspects: (a) It's not per-ns. (b) MTU on this
device is ETH_MIN_MTU (c) The xmit function is essentially kfree_skb().
and (d) since it's not registered it won't have ifindex.
Lower MTU effectively make the device not pass the MTU check during
the route check when a dst associated with the skb is dead.
Signed-off-by: Mahesh Bandewar <maheshb@xxxxxxxxxx>
This is now commit 4de83b88c66a1e4d ("loopback: create blackhole net
device similar to loopack.") in net-next, and causes the following
warning on arm64:
WARNING: suspicious RCU usage
5.2.0-rc6-arm64-renesas-01699-g4de83b88c66a1e4d #263 Not tainted
-----------------------------
include/linux/rtnetlink.h:85 suspicious rcu_dereference_protected() usage!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1
no locks held by swapper/0/1.
stack backtrace:
CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.2.0-rc6-arm64-renesas-01699-g4de83b88c66a1e4d #263
Hardware name: Renesas Salvator-X 2nd version board based on r8a7795 ES2.0+ (DT)
Call trace:
dump_backtrace+0x0/0x148
show_stack+0x14/0x20
dump_stack+0xd4/0x11c
lockdep_rcu_suspicious+0xcc/0x110
dev_init_scheduler+0x114/0x150
blackhole_netdev_init+0x40/0x80
do_one_initcall+0x178/0x37c
kernel_init_freeable+0x490/0x530
kernel_init+0x10/0x100
ret_from_fork+0x10/0x1c
---
v1->v2->v3
no change
drivers/net/loopback.c | 76 ++++++++++++++++++++++++++++++++++-----
include/linux/netdevice.h | 2 ++
2 files changed, 69 insertions(+), 9 deletions(-)
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 87d361666cdd..3b39def5471e 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -55,6 +55,13 @@
#include <net/net_namespace.h>
#include <linux/u64_stats_sync.h>
+/* blackhole_netdev - a device used for dsts that are marked expired!
+ * This is global device (instead of per-net-ns) since it's not needed
+ * to be per-ns and gets initialized at boot time.
+ */
+struct net_device *blackhole_netdev;
+EXPORT_SYMBOL(blackhole_netdev);
+
/* The higher levels take care of making this non-reentrant (it's
* called with bh's disabled).
*/
@@ -150,12 +157,14 @@ static const struct net_device_ops loopback_ops = {
.ndo_set_mac_address = eth_mac_addr,
};
-/* The loopback device is special. There is only one instance
- * per network namespace.
- */
-static void loopback_setup(struct net_device *dev)
+static void gen_lo_setup(struct net_device *dev,
+ unsigned int mtu,
+ const struct ethtool_ops *eth_ops,
+ const struct header_ops *hdr_ops,
+ const struct net_device_ops *dev_ops,
+ void (*dev_destructor)(struct net_device *dev))
{
- dev->mtu = 64 * 1024;
+ dev->mtu = mtu;
dev->hard_header_len = ETH_HLEN; /* 14 */
dev->min_header_len = ETH_HLEN; /* 14 */
dev->addr_len = ETH_ALEN; /* 6 */
@@ -174,11 +183,20 @@ static void loopback_setup(struct net_device *dev)
| NETIF_F_NETNS_LOCAL
| NETIF_F_VLAN_CHALLENGED
| NETIF_F_LOOPBACK;
- dev->ethtool_ops = &loopback_ethtool_ops;
- dev->header_ops = ð_header_ops;
- dev->netdev_ops = &loopback_ops;
+ dev->ethtool_ops = eth_ops;
+ dev->header_ops = hdr_ops;
+ dev->netdev_ops = dev_ops;
dev->needs_free_netdev = true;
- dev->priv_destructor = loopback_dev_free;
+ dev->priv_destructor = dev_destructor;
+}
+
+/* The loopback device is special. There is only one instance
+ * per network namespace.
+ */
+static void loopback_setup(struct net_device *dev)
+{
+ gen_lo_setup(dev, (64 * 1024), &loopback_ethtool_ops, ð_header_ops,
+ &loopback_ops, loopback_dev_free);
}
/* Setup and register the loopback device. */
@@ -213,3 +231,43 @@ static __net_init int loopback_net_init(struct net *net)
struct pernet_operations __net_initdata loopback_net_ops = {
.init = loopback_net_init,
};
+
+/* blackhole netdevice */
+static netdev_tx_t blackhole_netdev_xmit(struct sk_buff *skb,
+ struct net_device *dev)
+{
+ kfree_skb(skb);
+ net_warn_ratelimited("%s(): Dropping skb.\n", __func__);
+ return NETDEV_TX_OK;
+}
+
+static const struct net_device_ops blackhole_netdev_ops = {
+ .ndo_start_xmit = blackhole_netdev_xmit,
+};
+
+/* This is a dst-dummy device used specifically for invalidated
+ * DSTs and unlike loopback, this is not per-ns.
+ */
+static void blackhole_netdev_setup(struct net_device *dev)
+{
+ gen_lo_setup(dev, ETH_MIN_MTU, NULL, NULL, &blackhole_netdev_ops, NULL);
+}
+
+/* Setup and register the blackhole_netdev. */
+static int __init blackhole_netdev_init(void)
+{
+ blackhole_netdev = alloc_netdev(0, "blackhole_dev", NET_NAME_UNKNOWN,
+ blackhole_netdev_setup);
+ if (!blackhole_netdev)
+ return -ENOMEM;
+
+ dev_init_scheduler(blackhole_netdev);
+ dev_activate(blackhole_netdev);
+
+ blackhole_netdev->flags |= IFF_UP | IFF_RUNNING;
+ dev_net_set(blackhole_netdev, &init_net);
+
+ return 0;
+}
+
+device_initcall(blackhole_netdev_init);
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index eeacebd7debb..88292953aa6f 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4870,4 +4870,6 @@ do { \
#define PTYPE_HASH_SIZE (16)
#define PTYPE_HASH_MASK (PTYPE_HASH_SIZE - 1)
+extern struct net_device *blackhole_netdev;
+
#endif /* _LINUX_NETDEVICE_H */
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds