Date:   Sat, 14 May 2022 11:59:09 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-can@...r.kernel.org,
        kernel@...gutronix.de, David Jander <david@...tonic.nl>,
        Devid Antonio Filoni <devid.filoni@...uetechnologies.com>
Subject: Re: [PATCH v2] can: skb: add extended skb support



On 12.05.22 19:46, Oleksij Rempel wrote:
> Hi Oliver,
> 
> On Thu, May 12, 2022 at 06:54:46PM +0200, Oliver Hartkopp wrote:
>> Hi Oleksij,
>>
>> On 12.05.22 14:59, Oleksij Rempel wrote:
>>> Add CAN specific skb extension support and add first currently needed
>>> local_origin variable.
>>>
>>> On the CAN stack we push same skb data in different direction depending
>>> on the interface type:
>>> - to the HW egress and at same time back to the stack as echo
>>> - over virtual vcan/vxcan interfaces as egress on one side and ingress on other
>>>     side of the vxcan tunnel.
>>> We can't use skb->sk as marker of the origin, because not all packets
>>> not all packets with local_origin are assigned to some socket. Some of
>>> them are generate from the kernel, for example like J1939 control messages.
>>> So, to properly detect flow direction is is better to store this information
>>> as part of the SKB.
>>>
>>> The advantage of using skb_ext is that it is options and extendable
>>> without affecting other skb users. It can be shared between cloned skbs and
>>> duplicated only if needed.
>>>
>>> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
>>> Cc: Devid Antonio Filoni <devid.filoni@...uetechnologies.com>
>>> ---
>>> changes v2:
>>> - migrate it to SKB_EXT
>>
>> The use of SKB_EXT seems to be very costly to just store a boolean value.
>>
>> What I could see from some of the other SKB_EXT users this extension (which
>> performs alloc & COW) is used in special circumstances.
>>
>> With your suggestion this additional effort is needed for every CAN related
>> skb.
>>
>> So at least for this use-case extending struct can_skb_priv seems to be more
>> efficient.
>>
>> https://elixir.bootlin.com/linux/latest/source/include/linux/can/skb.h#L44
>>
>> We might get into problems with PF_PACKET sockets when extending the
>> can_skb_priv length beyond HH_DATA_MOD, see:
>>
>> https://elixir.bootlin.com/linux/latest/source/include/linux/can/skb.h#L99
>>
>> But for now I'm not sure that SKB_EXT isn't too heavy to store that single
>> flag.
> 
> Yes, I was thinking about potential overkill for, currently, just one
> bit of storage. But here is my motivation:
> CAN frameworks is currently using two ways to store metaadata (expecpt
> of SKB):
> 1. skb->cb. This variant is not extendable and can be used only insight
>     of one driver.
> 2. can_skb_priv as part of skb->data->head. Is potentially extendable
>     but we will need to use skb_copy instead of skb_clone. Because we
>     can't modify head for clone only. IMO, this will add potentially more
>     overhead than SKB_EXT.
> 
> In long term, as soon as we will need to extend can specific meta
> data, we will have same situation: it will be not big enough to migrate
> to SKB_EXT. Maybe we need to move can_skb_priv to SKB_EXT as well?

I wonder if our current issue with the correct attribution of host 
generated CAN skbs in j1939 could be solved by additionally setting 
skb->redirected = 1 when the CAN skb is echo'ed back.

Or what about creating a single socket/sk instance for the j1939 
protocol which makes j1939 created skbs assigned to the j1939 system?

Best regards,
Oliver

> 
> 
>>>
>>>    drivers/net/can/vxcan.c |  4 ++++
>>>    include/linux/can/skb.h |  4 ++++
>>>    include/linux/skbuff.h  |  3 +++
>>>    net/can/Kconfig         |  1 +
>>>    net/can/af_can.c        |  5 +++++
>>>    net/can/raw.c           | 10 ++++++++--
>>>    net/core/skbuff.c       |  7 +++++++
>>>    7 files changed, 32 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/can/vxcan.c b/drivers/net/can/vxcan.c
>>> index 577a80300514..93701a698008 100644
>>> --- a/drivers/net/can/vxcan.c
>>> +++ b/drivers/net/can/vxcan.c
>>> @@ -39,6 +39,7 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
>>>    	struct net_device *peer;
>>>    	struct canfd_frame *cfd = (struct canfd_frame *)oskb->data;
>>>    	struct net_device_stats *peerstats, *srcstats = &dev->stats;
>>> +	struct can_skb_ext *can_ext;
>>>    	struct sk_buff *skb;
>>>    	u8 len;
>>> @@ -66,6 +67,9 @@ static netdev_tx_t vxcan_xmit(struct sk_buff *oskb, struct net_device *dev)
>>>    	skb->pkt_type   = PACKET_BROADCAST;
>>>    	skb->dev        = peer;
>>>    	skb->ip_summed  = CHECKSUM_UNNECESSARY;
>>> +	can_ext = skb_ext_add(skb, SKB_EXT_CAN);
>>> +	if (can_ext)
>>> +		can_ext->local_origin = false;
>>>    	len = cfd->can_id & CAN_RTR_FLAG ? 0 : cfd->len;
>>>    	if (netif_rx(skb) == NET_RX_SUCCESS) {
>>> diff --git a/include/linux/can/skb.h b/include/linux/can/skb.h
>>> index fdb22b00674a..401b08890d74 100644
>>> --- a/include/linux/can/skb.h
>>> +++ b/include/linux/can/skb.h
>>> @@ -55,6 +55,10 @@ struct can_skb_priv {
>>>    	struct can_frame cf[];
>>>    };
>>> +struct can_skb_ext {
>>> +	bool local_origin;
>>> +};
>>> +
>>>    static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
>>>    {
>>>    	return (struct can_skb_priv *)(skb->head);
>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>> index 3270cb72e4d8..d39e70e5f7f2 100644
>>> --- a/include/linux/skbuff.h
>>> +++ b/include/linux/skbuff.h
>>> @@ -4563,6 +4563,9 @@ enum skb_ext_id {
>>>    #endif
>>>    #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>>>    	SKB_EXT_MCTP,
>>> +#endif
>>> +#if IS_ENABLED(CONFIG_CAN)
>>> +	SKB_EXT_CAN,
>>>    #endif
>>>    	SKB_EXT_NUM, /* must be last */
>>>    };
>>> diff --git a/net/can/Kconfig b/net/can/Kconfig
>>> index a9ac5ffab286..eb826e3771fe 100644
>>> --- a/net/can/Kconfig
>>> +++ b/net/can/Kconfig
>>> @@ -5,6 +5,7 @@
>>>    menuconfig CAN
>>>    	tristate "CAN bus subsystem support"
>>> +	select SKB_EXTENSIONS
>>>    	help
>>>    	  Controller Area Network (CAN) is a slow (up to 1Mbit/s) serial
>>>    	  communications protocol. Development of the CAN bus started in
>>> diff --git a/net/can/af_can.c b/net/can/af_can.c
>>> index 1fb49d51b25d..329c540d3ddf 100644
>>> --- a/net/can/af_can.c
>>> +++ b/net/can/af_can.c
>>> @@ -201,6 +201,7 @@ int can_send(struct sk_buff *skb, int loop)
>>>    	struct sk_buff *newskb = NULL;
>>>    	struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
>>>    	struct can_pkg_stats *pkg_stats = dev_net(skb->dev)->can.pkg_stats;
>>> +	struct can_skb_ext *can_ext;
>>>    	int err = -EINVAL;
>>>    	if (skb->len == CAN_MTU) {
>>> @@ -240,6 +241,10 @@ int can_send(struct sk_buff *skb, int loop)
>>>    	skb_reset_network_header(skb);
>>>    	skb_reset_transport_header(skb);
>>> +	can_ext = skb_ext_add(skb, SKB_EXT_CAN);
>>> +	if (can_ext)
>>> +		can_ext->local_origin = true;
>>> +
>>>    	if (loop) {
>>>    		/* local loopback of sent CAN frames */
>>> diff --git a/net/can/raw.c b/net/can/raw.c
>>> index b7dbb57557f3..cba18cdf017f 100644
>>> --- a/net/can/raw.c
>>> +++ b/net/can/raw.c
>>> @@ -121,6 +121,7 @@ static void raw_rcv(struct sk_buff *oskb, void *data)
>>>    {
>>>    	struct sock *sk = (struct sock *)data;
>>>    	struct raw_sock *ro = raw_sk(sk);
>>> +	struct can_skb_ext *can_ext;
>>>    	struct sockaddr_can *addr;
>>>    	struct sk_buff *skb;
>>>    	unsigned int *pflags;
>>> @@ -173,8 +174,13 @@ static void raw_rcv(struct sk_buff *oskb, void *data)
>>>    	/* add CAN specific message flags for raw_recvmsg() */
>>>    	pflags = raw_flags(skb);
>>>    	*pflags = 0;
>>> -	if (oskb->sk)
>>> -		*pflags |= MSG_DONTROUTE;
>>> +
>>> +	can_ext = skb_ext_find(oskb, SKB_EXT_CAN);
>>> +	if (can_ext) {
>>> +		if (can_ext->local_origin)
>>> +			*pflags |= MSG_DONTROUTE;
>>> +	}
>>> +
>>>    	if (oskb->sk == sk)
>>>    		*pflags |= MSG_CONFIRM;
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index 475183f37891..5a5409ccb767 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -61,6 +61,7 @@
>>>    #include <linux/if_vlan.h>
>>>    #include <linux/mpls.h>
>>>    #include <linux/kcov.h>
>>> +#include <linux/can/skb.h>
>>>    #include <net/protocol.h>
>>>    #include <net/dst.h>
>>> @@ -4338,6 +4339,9 @@ static const u8 skb_ext_type_len[] = {
>>>    #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>>>    	[SKB_EXT_MCTP] = SKB_EXT_CHUNKSIZEOF(struct mctp_flow),
>>>    #endif
>>> +#if IS_ENABLED(CONFIG_CAN)
>>> +	[SKB_EXT_CAN] = SKB_EXT_CHUNKSIZEOF(struct can_skb_ext),
>>> +#endif
>>>    };
>>>    static __always_inline unsigned int skb_ext_total_length(void)
>>> @@ -4357,6 +4361,9 @@ static __always_inline unsigned int skb_ext_total_length(void)
>>>    #endif
>>>    #if IS_ENABLED(CONFIG_MCTP_FLOWS)
>>>    		skb_ext_type_len[SKB_EXT_MCTP] +
>>> +#endif
>>> +#if IS_ENABLED(CONFIG_CAN)
>>> +		skb_ext_type_len[SKB_EXT_CAN] +
>>>    #endif
>>>    		0;
>>>    }
>>
>>
> 

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