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net, xsk: Avoid taking multiple skbuff references

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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
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8
net/core/dev.c
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@@ -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