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net, xsk: Avoid taking multiple skbuff references 

Commit 642e450b6b ("xsk: Do not discard packet when NETDEV_TX_BUSY")
addressed the problem that packets were discarded from the Tx AF_XDP
ring, when the driver returned NETDEV_TX_BUSY. Part of the fix was
bumping the skbuff reference count, so that the buffer would not be
freed by dev_direct_xmit(). A reference count larger than one means
that the skbuff is "shared", which is not the case.

If the "shared" skbuff is sent to the generic XDP receive path,
netif_receive_generic_xdp(), and pskb_expand_head() is entered the
BUG_ON(skb_shared(skb)) will trigger.

This patch adds a variant to dev_direct_xmit(), __dev_direct_xmit(),
where a user can select the skbuff free policy. This allows AF_XDP to
avoid bumping the reference count, but still keep the NETDEV_TX_BUSY
behavior.

Fixes: 642e450b6b ("xsk: Do not discard packet when NETDEV_TX_BUSY")
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201123175600.146255-1-bjorn.topel@gmail.com
This commit is contained in:
Björn Töpel 2020-11-23 18:56:00 +01:00 committed by Daniel Borkmann
parent 178648916e
commit 36ccdf8582
3 changed files with 16 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
include/linux/netdevice.h
View File

@ -2813,9 +2813,21 @@ u16 dev_pick_tx_zero(struct net_device *dev, struct sk_buff *skb,
struct net_device *sb_dev);
u16 dev_pick_tx_cpu_id(struct net_device *dev, struct sk_buff *skb,
struct net_device *sb_dev);
int dev_queue_xmit(struct sk_buff *skb);
int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev);
int dev_direct_xmit(struct sk_buff *skb, u16 queue_id);
int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id);
static inline int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
{
int ret;
ret = __dev_direct_xmit(skb, queue_id);
if (!dev_xmit_complete(ret))
kfree_skb(skb);
return ret;
}
int register_netdevice(struct net_device *dev);
void unregister_netdevice_queue(struct net_device *dev, struct list_head *head);
void unregister_netdevice_many(struct list_head *head);

8
net/core/dev.c
View File

@ -4180,7 +4180,7 @@ int dev_queue_xmit_accel(struct sk_buff *skb, struct net_device *sb_dev)
}
EXPORT_SYMBOL(dev_queue_xmit_accel);
int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
int __dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
{
struct net_device *dev = skb->dev;
struct sk_buff *orig_skb = skb;
@ -4210,17 +4210,13 @@ int dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
dev_xmit_recursion_dec();
local_bh_enable();
if (!dev_xmit_complete(ret))
kfree_skb(skb);
return ret;
drop:
atomic_long_inc(&dev->tx_dropped);
kfree_skb_list(skb);
return NET_XMIT_DROP;
}
EXPORT_SYMBOL(dev_direct_xmit);
EXPORT_SYMBOL(__dev_direct_xmit);
/*************************************************************************
* Receiver routines

8
net/xdp/xsk.c
View File

@ -411,11 +411,7 @@ static int xsk_generic_xmit(struct sock *sk)
skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
skb->destructor = xsk_destruct_skb;
/* Hinder dev_direct_xmit from freeing the packet and
* therefore completing it in the destructor
*/
refcount_inc(&skb->users);
err = dev_direct_xmit(skb, xs->queue_id);
err = __dev_direct_xmit(skb, xs->queue_id);
if (err == NETDEV_TX_BUSY) {
/* Tell user-space to retry the send */
skb->destructor = sock_wfree;
@ -429,12 +425,10 @@ static int xsk_generic_xmit(struct sock *sk)
/* Ignore NET_XMIT_CN as packet might have been sent */
if (err == NET_XMIT_DROP) {
/* SKB completed but not sent */
kfree_skb(skb);
err = -EBUSY;
goto out;
}
consume_skb(skb);
sent_frame = true;
}