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 15:24:37 +0200

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?

>  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?

> +		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);