[PATCH] net: orphan queued skbs if device tx can stall

From: Michael S. Tsirkin
Date: Sun Apr 08 2012 - 13:13:36 EST


commit 0110d6f22f392f976e84ab49da1b42f85b64a3c5
tun: orphan an skb on tx
Fixed a configuration where skbs get queued
at the tun device forever, blocking senders.

However this fix isn't waterproof:
userspace can control whether the interface
is stopped, and if it is, packets
get queued in the qdisc, again potentially forever.

Complete the fix by setting a private flag and orphaning
at the qdisc level.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
drivers/net/tun.c | 3 +++
include/linux/if.h | 1 +
net/core/dev.c | 5 +++++
net/sched/sch_generic.c | 5 +++++
4 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index bb8c72c..15c5bb8 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -535,6 +535,9 @@ static void tun_net_init(struct net_device *dev)
dev->tx_queue_len = TUN_READQ_SIZE; /* We prefer our own queue length */
break;
}
+ /* Once queue becomes full, we stop tx until userspace
+ * dequeues some packets, that is potentially forever. */
+ dev->priv_flags |= IFF_TX_CAN_STALL;
}

/* Character device part */
diff --git a/include/linux/if.h b/include/linux/if.h
index f995c66..dd2c7f7 100644
--- a/include/linux/if.h
+++ b/include/linux/if.h
@@ -81,6 +81,7 @@
#define IFF_UNICAST_FLT 0x20000 /* Supports unicast filtering */
#define IFF_TEAM_PORT 0x40000 /* device used as team port */
#define IFF_SUPP_NOFCS 0x80000 /* device supports sending custom FCS */
+#define IFF_TX_CAN_STALL 0x100000 /* Device can stop tx forever */


#define IF_GET_IFACE 0x0001 /* for querying only */
diff --git a/net/core/dev.c b/net/core/dev.c
index 5d59155..e812706 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2516,6 +2516,11 @@ int dev_queue_xmit(struct sk_buff *skb)
struct Qdisc *q;
int rc = -ENOMEM;

+ /* Orphan the skb - required if we might hang on to it
+ * for indefinite time. */
+ if (dev->priv_flags & IFF_TX_CAN_STALL)
+ skb_orphan(skb);
+
/* Disable soft irqs for various locks below. Also
* stops preemption for RCU.
*/
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 67fc573..27883d1 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -120,6 +120,11 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
/* And release qdisc */
spin_unlock(root_lock);

+ /* Orphan the skb - required if we might hang on to it
+ * for indefinite time. */
+ if (dev->priv_flags & IFF_TX_CAN_STALL)
+ skb_orphan(skb);
+
HARD_TX_LOCK(dev, txq, smp_processor_id());
if (!netif_xmit_frozen_or_stopped(txq))
ret = dev_hard_start_xmit(skb, dev, txq);
--
1.7.9.111.gf3fb0
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