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

From: Vlad Buslov <[email-masked]>
Date: Mon, 9 Nov 2020 17:51:03 +0200

On Mon 09 Nov 2020 at 16:54, wenxu <[email-masked]> wrote:
> 在 2020/11/9 21:24, Vlad Buslov 写道:
>> On Sun 08 Nov 2020 at 01:30, [email-masked] wrote:
>>> From: wenxu <[email-masked]>
>>>
>>> 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 <[email-masked]>
>>> ---
>>> 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.

Okay. I have no strong opinion regarding this so it is better for
maintainers to decide.

>
>>>  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?

Agree. See my response to Marcelo.

>
>>
>>> +		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 <[email-masked]>");
>>> +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);
>>