Re: [PATCH v5 net-next 3/3] net/sched: act_frag: add implict packet fragment support.

[wenxu <wenxu@ucloud.cn>]

在 2020/11/9 21:24, Vlad Buslov 写道:
> On Sun 08 Nov 2020 at 01:30, wenxu@ucloud.cn wrote:
>> From: wenxu <wenxu@ucloud.cn>
>>
>> Currently kernel tc subsystem can do conntrack in act_ct. But when several
>> fragment packets go through the act_ct, function tcf_ct_handle_fragments
>> will defrag the packets to a big one. But the last action will redirect
>> mirred to a device which maybe lead the reassembly big packet over the mtu
>> of target device.
>>
>> This patch add support for a xmit hook to mirred, that gets executed before
>> xmiting the packet. Then, when act_ct gets loaded, it configs that hook.
>> The frag xmit hook maybe reused by other modules.
>>
>> Signed-off-by: wenxu <wenxu@ucloud.cn>
>> ---
>> v2: Fix the crash for act_frag module without load
>> v3: modify the kconfig describe and put tcf_xmit_hook_is_enabled
>> in the tcf_dev_queue_xmit, and xchg atomic for tcf_xmit_hook
>> v4: using skb_protocol and fix line length exceeds 80 columns
>> v5: no change
>>
>>  include/net/act_api.h  |  16 +++++
>>  net/sched/Kconfig      |  13 ++++
>>  net/sched/Makefile     |   1 +
>>  net/sched/act_api.c    |  51 +++++++++++++++
>>  net/sched/act_ct.c     |   7 +++
>>  net/sched/act_frag.c   | 164 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  net/sched/act_mirred.c |   2 +-
>>  7 files changed, 253 insertions(+), 1 deletion(-)
>>  create mode 100644 net/sched/act_frag.c
>>
>> diff --git a/include/net/act_api.h b/include/net/act_api.h
>> index 8721492..403a618 100644
>> --- a/include/net/act_api.h
>> +++ b/include/net/act_api.h
>> @@ -239,6 +239,22 @@ int tcf_action_check_ctrlact(int action, struct tcf_proto *tp,
>>  			     struct netlink_ext_ack *newchain);
>>  struct tcf_chain *tcf_action_set_ctrlact(struct tc_action *a, int action,
>>  					 struct tcf_chain *newchain);
>> +
>> +int tcf_dev_queue_xmit(struct sk_buff *skb, int (*xmit)(struct sk_buff *skb));
>> +int tcf_set_xmit_hook(int (*xmit_hook)(struct sk_buff *skb,
>> +				       int (*xmit)(struct sk_buff *skb)));
>> +void tcf_clear_xmit_hook(void);
>> +
>> +#if IS_ENABLED(CONFIG_NET_ACT_FRAG)
>> +int tcf_frag_xmit_hook(struct sk_buff *skb, int (*xmit)(struct sk_buff *skb));
>> +#else
>> +static inline int tcf_frag_xmit_hook(struct sk_buff *skb,
>> +				     int (*xmit)(struct sk_buff *skb))
>> +{
>> +	return 0;
>> +}
>> +#endif
>> +
>>  #endif /* CONFIG_NET_CLS_ACT */
>>  
>>  static inline void tcf_action_stats_update(struct tc_action *a, u64 bytes,
>> diff --git a/net/sched/Kconfig b/net/sched/Kconfig
>> index a3b37d8..9a240c7 100644
>> --- a/net/sched/Kconfig
>> +++ b/net/sched/Kconfig
>> @@ -974,9 +974,22 @@ config NET_ACT_TUNNEL_KEY
>>  	  To compile this code as a module, choose M here: the
>>  	  module will be called act_tunnel_key.
>>  
>> +config NET_ACT_FRAG
>> +	tristate "Packet fragmentation"
>> +	depends on NET_CLS_ACT
>> +	help
>> +         Say Y here to allow fragmenting big packets when outputting
>> +         with the mirred action.
>> +
>> +	  If unsure, say N.
>> +
>> +	  To compile this code as a module, choose M here: the
>> +	  module will be called act_frag.
>> +
> Just wondering, what is the motivation for putting the frag code into
> standalone module? It doesn't implement usual act_* interface and is not
> user-configurable. To me it looks like functionality that belongs to
> act_api. Am I missing something?

The fragment operation is an single L3 action. 

So we put in an single modules. Maybe it is not proper to put in the act_api directly.

>>  config NET_ACT_CT
>>  	tristate "connection tracking tc action"
>>  	depends on NET_CLS_ACT && NF_CONNTRACK && NF_NAT && NF_FLOW_TABLE
>> +	depends on NET_ACT_FRAG
>>  	help
>>  	  Say Y here to allow sending the packets to conntrack module.
>>  
>> diff --git a/net/sched/Makefile b/net/sched/Makefile
>> index 66bbf9a..c146186 100644
>> --- a/net/sched/Makefile
>> +++ b/net/sched/Makefile
>> @@ -29,6 +29,7 @@ obj-$(CONFIG_NET_IFE_SKBMARK)	+= act_meta_mark.o
>>  obj-$(CONFIG_NET_IFE_SKBPRIO)	+= act_meta_skbprio.o
>>  obj-$(CONFIG_NET_IFE_SKBTCINDEX)	+= act_meta_skbtcindex.o
>>  obj-$(CONFIG_NET_ACT_TUNNEL_KEY)+= act_tunnel_key.o
>> +obj-$(CONFIG_NET_ACT_FRAG)	+= act_frag.o
>>  obj-$(CONFIG_NET_ACT_CT)	+= act_ct.o
>>  obj-$(CONFIG_NET_ACT_GATE)	+= act_gate.o
>>  obj-$(CONFIG_NET_SCH_FIFO)	+= sch_fifo.o
>> diff --git a/net/sched/act_api.c b/net/sched/act_api.c
>> index f66417d..e7b501c 100644
>> --- a/net/sched/act_api.c
>> +++ b/net/sched/act_api.c
>> @@ -22,6 +22,57 @@
>>  #include <net/act_api.h>
>>  #include <net/netlink.h>
>>  
>> +static int (*tcf_xmit_hook)(struct sk_buff *skb,
>> +			    int (*xmit)(struct sk_buff *skb));
>> +static DEFINE_STATIC_KEY_FALSE(tcf_xmit_hook_in_use);
>> +
>> +static void tcf_inc_xmit_hook(void)
>> +{
>> +	static_branch_inc(&tcf_xmit_hook_in_use);
>> +}
>> +
>> +static void tcf_dec_xmit_hook(void)
>> +{
>> +	static_branch_dec(&tcf_xmit_hook_in_use);
>> +}
>> +
>> +static bool tcf_xmit_hook_enabled(void)
>> +{
>> +	return static_branch_unlikely(&tcf_xmit_hook_in_use);
>> +}
>> +
>> +int tcf_set_xmit_hook(int (*xmit_hook)(struct sk_buff *skb,
>> +				       int (*xmit)(struct sk_buff *skb)))
>> +{
>> +	if (!tcf_xmit_hook_enabled())
>> +		xchg(&tcf_xmit_hook, xmit_hook);
> Marcelo, why did you suggest to use atomic operations to change
> tcf_xmit_hook variable? It is not obvious to me after reading the code.
>
>> +	else if (xmit_hook != tcf_xmit_hook)
>> +		return -EBUSY;
>> +
>> +	tcf_inc_xmit_hook();
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(tcf_set_xmit_hook);
>> +
>> +void tcf_clear_xmit_hook(void)
>> +{
>> +	tcf_dec_xmit_hook();
>> +
>> +	if (!tcf_xmit_hook_enabled())
>> +		xchg(&tcf_xmit_hook, NULL);
>> +}
>> +EXPORT_SYMBOL_GPL(tcf_clear_xmit_hook);
>> +
>> +int tcf_dev_queue_xmit(struct sk_buff *skb, int (*xmit)(struct sk_buff *skb))
>> +{
>> +	if (tcf_xmit_hook_enabled())
> Okay, so what happens here if tcf_xmit_hook is disabled concurrently? If
> we get here from some rule that doesn't involve act_ct but uses
> act_mirred and act_ct is concurrently removed decrementing last
> reference to static branch and setting tcf_xmit_hook to NULL?


Yes.  Maybe the tcf_xmit_hook is protected by a rcu lock is better?

>
>> +		return tcf_xmit_hook(skb, xmit);
>> +	else
>> +		return xmit(skb);
>> +}
>> +EXPORT_SYMBOL_GPL(tcf_dev_queue_xmit);
>> +
>>  static void tcf_action_goto_chain_exec(const struct tc_action *a,
>>  				       struct tcf_result *res)
>>  {
>> diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
>> index 9c79fb9..dff3c40 100644
>> --- a/net/sched/act_ct.c
>> +++ b/net/sched/act_ct.c
>> @@ -1541,8 +1541,14 @@ static int __init ct_init_module(void)
>>  	if (err)
>>  		goto err_register;
>>  
>> +	err = tcf_set_xmit_hook(tcf_frag_xmit_hook);
>> +	if (err)
>> +		goto err_action;
>> +
>>  	return 0;
>>  
>> +err_action:
>> +	tcf_unregister_action(&act_ct_ops, &ct_net_ops);
>>  err_register:
>>  	tcf_ct_flow_tables_uninit();
>>  err_tbl_init:
>> @@ -1552,6 +1558,7 @@ static int __init ct_init_module(void)
>>  
>>  static void __exit ct_cleanup_module(void)
>>  {
>> +	tcf_clear_xmit_hook();
>>  	tcf_unregister_action(&act_ct_ops, &ct_net_ops);
>>  	tcf_ct_flow_tables_uninit();
>>  	destroy_workqueue(act_ct_wq);
>> diff --git a/net/sched/act_frag.c b/net/sched/act_frag.c
>> new file mode 100644
>> index 0000000..3a7ab92
>> --- /dev/null
>> +++ b/net/sched/act_frag.c
>> @@ -0,0 +1,164 @@
>> +// SPDX-License-Identifier: GPL-2.0 OR Linux-OpenIB
>> +#include <net/netlink.h>
>> +#include <net/act_api.h>
>> +#include <net/dst.h>
>> +#include <net/ip.h>
>> +#include <net/ip6_fib.h>
>> +
>> +struct tcf_frag_data {
>> +	unsigned long dst;
>> +	struct qdisc_skb_cb cb;
>> +	__be16 inner_protocol;
>> +	u16 vlan_tci;
>> +	__be16 vlan_proto;
>> +	unsigned int l2_len;
>> +	u8 l2_data[VLAN_ETH_HLEN];
>> +	int (*xmit)(struct sk_buff *skb);
>> +};
>> +
>> +static DEFINE_PER_CPU(struct tcf_frag_data, tcf_frag_data_storage);
>> +
>> +static int tcf_frag_xmit(struct net *net, struct sock *sk, struct sk_buff *skb)
>> +{
>> +	struct tcf_frag_data *data = this_cpu_ptr(&tcf_frag_data_storage);
>> +
>> +	if (skb_cow_head(skb, data->l2_len) < 0) {
>> +		kfree_skb(skb);
>> +		return -ENOMEM;
>> +	}
>> +
>> +	__skb_dst_copy(skb, data->dst);
>> +	*qdisc_skb_cb(skb) = data->cb;
>> +	skb->inner_protocol = data->inner_protocol;
>> +	if (data->vlan_tci & VLAN_CFI_MASK)
>> +		__vlan_hwaccel_put_tag(skb, data->vlan_proto,
>> +				       data->vlan_tci & ~VLAN_CFI_MASK);
>> +	else
>> +		__vlan_hwaccel_clear_tag(skb);
>> +
>> +	/* Reconstruct the MAC header.  */
>> +	skb_push(skb, data->l2_len);
>> +	memcpy(skb->data, &data->l2_data, data->l2_len);
>> +	skb_postpush_rcsum(skb, skb->data, data->l2_len);
>> +	skb_reset_mac_header(skb);
>> +
>> +	data->xmit(skb);
>> +
>> +	return 0;
>> +}
>> +
>> +static void tcf_frag_prepare_frag(struct sk_buff *skb,
>> +				  int (*xmit)(struct sk_buff *skb))
>> +{
>> +	unsigned int hlen = skb_network_offset(skb);
>> +	struct tcf_frag_data *data;
>> +
>> +	data = this_cpu_ptr(&tcf_frag_data_storage);
>> +	data->dst = skb->_skb_refdst;
>> +	data->cb = *qdisc_skb_cb(skb);
>> +	data->xmit = xmit;
>> +	data->inner_protocol = skb->inner_protocol;
>> +	if (skb_vlan_tag_present(skb))
>> +		data->vlan_tci = skb_vlan_tag_get(skb) | VLAN_CFI_MASK;
>> +	else
>> +		data->vlan_tci = 0;
>> +	data->vlan_proto = skb->vlan_proto;
>> +	data->l2_len = hlen;
>> +	memcpy(&data->l2_data, skb->data, hlen);
>> +
>> +	memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
>> +	skb_pull(skb, hlen);
>> +}
>> +
>> +static unsigned int
>> +tcf_frag_dst_get_mtu(const struct dst_entry *dst)
>> +{
>> +	return dst->dev->mtu;
>> +}
>> +
>> +static struct dst_ops tcf_frag_dst_ops = {
>> +	.family = AF_UNSPEC,
>> +	.mtu = tcf_frag_dst_get_mtu,
>> +};
>> +
>> +static int tcf_fragment(struct net *net, struct sk_buff *skb,
>> +			u16 mru, int (*xmit)(struct sk_buff *skb))
>> +{
>> +	if (skb_network_offset(skb) > VLAN_ETH_HLEN) {
>> +		net_warn_ratelimited("L2 header too long to fragment\n");
>> +		goto err;
>> +	}
>> +
>> +	if (skb_protocol(skb, true) == htons(ETH_P_IP)) {
>> +		struct dst_entry tcf_frag_dst;
>> +		unsigned long orig_dst;
>> +
>> +		tcf_frag_prepare_frag(skb, xmit);
>> +		dst_init(&tcf_frag_dst, &tcf_frag_dst_ops, NULL, 1,
>> +			 DST_OBSOLETE_NONE, DST_NOCOUNT);
>> +		tcf_frag_dst.dev = skb->dev;
>> +
>> +		orig_dst = skb->_skb_refdst;
>> +		skb_dst_set_noref(skb, &tcf_frag_dst);
>> +		IPCB(skb)->frag_max_size = mru;
>> +
>> +		ip_do_fragment(net, skb->sk, skb, tcf_frag_xmit);
>> +		refdst_drop(orig_dst);
>> +	} else if (skb_protocol(skb, true) == htons(ETH_P_IPV6)) {
>> +		unsigned long orig_dst;
>> +		struct rt6_info tcf_frag_rt;
>> +
>> +		tcf_frag_prepare_frag(skb, xmit);
>> +		memset(&tcf_frag_rt, 0, sizeof(tcf_frag_rt));
>> +		dst_init(&tcf_frag_rt.dst, &tcf_frag_dst_ops, NULL, 1,
>> +			 DST_OBSOLETE_NONE, DST_NOCOUNT);
>> +		tcf_frag_rt.dst.dev = skb->dev;
>> +
>> +		orig_dst = skb->_skb_refdst;
>> +		skb_dst_set_noref(skb, &tcf_frag_rt.dst);
>> +		IP6CB(skb)->frag_max_size = mru;
>> +
>> +		ipv6_stub->ipv6_fragment(net, skb->sk, skb, tcf_frag_xmit);
>> +		refdst_drop(orig_dst);
>> +	} else {
>> +		net_warn_ratelimited("Fail frag %s: eth=%x, MRU=%d, MTU=%d\n",
>> +				     netdev_name(skb->dev),
>> +				     ntohs(skb_protocol(skb, true)), mru,
>> +				     skb->dev->mtu);
>> +		goto err;
>> +	}
>> +
>> +	qdisc_skb_cb(skb)->mru = 0;
>> +	return 0;
>> +err:
>> +	kfree_skb(skb);
>> +	return -1;
>> +}
>> +
>> +int tcf_frag_xmit_hook(struct sk_buff *skb, int (*xmit)(struct sk_buff *skb))
>> +{
>> +	u16 mru = qdisc_skb_cb(skb)->mru;
>> +	int err;
>> +
>> +	if (mru && skb->len > mru + skb->dev->hard_header_len)
>> +		err = tcf_fragment(dev_net(skb->dev), skb, mru, xmit);
>> +	else
>> +		err = xmit(skb);
>> +
>> +	return err;
>> +}
>> +EXPORT_SYMBOL_GPL(tcf_frag_xmit_hook);
>> +
>> +static int __init frag_init_module(void)
>> +{
>> +	return 0;
>> +}
>> +
>> +static void __exit frag_cleanup_module(void)
>> +{
>> +}
>> +
>> +module_init(frag_init_module);
>> +module_exit(frag_cleanup_module);
>> +MODULE_AUTHOR("wenxu <wenxu@ucloud.cn>");
>> +MODULE_LICENSE("GPL v2");
>> diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
>> index 17d0095..7153c67 100644
>> --- a/net/sched/act_mirred.c
>> +++ b/net/sched/act_mirred.c
>> @@ -210,7 +210,7 @@ static int tcf_mirred_forward(bool want_ingress, struct sk_buff *skb)
>>  	int err;
>>  
>>  	if (!want_ingress)
>> -		err = dev_queue_xmit(skb);
>> +		err = tcf_dev_queue_xmit(skb, dev_queue_xmit);
>>  	else
>>  		err = netif_receive_skb(skb);
>