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From: Oliver Hartkopp <socketcan@hartkopp.net>
To: Marc Kleine-Budde <mkl@pengutronix.de>, linux-can@vger.kernel.org
Cc: kernel@pengutronix.de, Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Subject: Re: [net-rfc 04/16] can: dev: can_get_len(): add a helper function to get the correct length of Classical frames
Date: Mon, 19 Oct 2020 22:35:47 +0200
Message-ID: <fbbe1b80-c012-dc87-1eb0-4878cd08cce1@hartkopp.net> (raw)
In-Reply-To: <20201019190524.1285319-5-mkl@pengutronix.de>



On 19.10.20 21:05, Marc Kleine-Budde wrote:
> From: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> 
> In classical CAN, the length of the data (i.e. CAN payload) is not
> always equal to the DLC! If the frame is a Remote Transmission Request
> (RTR), data length is always zero regardless of DLC value and else, if
> the DLC is greater than 8, the length is 8. Contrary to common belief,
> ISO 11898-1 Chapter 8.4.2.3 (DLC field) do allow DLCs greater than 8
> for Classical Frames and specifies that those DLCs shall indicate that
> the data field is 8 bytes long.
> 
> Above facts are widely unknown and so many developpers uses the "len"
> field of "struct canfd_frame" to get the length of classical CAN
> frames: this is incorrect!
> 
> This patch introduces function get_can_len() which can be used in
> remediation. The function takes the SKB as an input in order to be
> able to determine if the frame is classical or FD.
> 
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> Link: https://lore.kernel.org/r/20201002154219.4887-4-mailhol.vincent@wanadoo.fr
> [mkl: renamed get_can_len() -> can_get_len()]
> Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
> ---
>   include/linux/can/dev.h | 23 +++++++++++++++++++++++
>   1 file changed, 23 insertions(+)
> 
> diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
> index 41ff31795320..2bb132fc6d88 100644
> --- a/include/linux/can/dev.h
> +++ b/include/linux/can/dev.h
> @@ -192,6 +192,29 @@ u8 can_dlc2len(u8 can_dlc);
>   /* map the sanitized data length to an appropriate data length code */
>   u8 can_len2dlc(u8 len);
>   
> +/*
> + * can_get_len(skb) - get the length of the CAN payload.
> + *
> + * In classical CAN, the length of the data (i.e. CAN payload) is not
> + * always equal to the DLC! If the frame is a Remote Transmission
> + * Request (RTR), data length is always zero regardless of DLC value
> + * and else, if the DLC is greater than 8, the length is 8. Contrary
> + * to common belief, ISO 11898-1 Chapter 8.4.2.3 (DLC field) do allow
> + * DLCs greater than 8 for Classical Frames and specifies that those
> + * DLCs shall indicate that the data field is 8 bytes long.
> + */
> +static inline u8 can_get_len(const struct sk_buff *skb)
> +{
> +	const struct canfd_frame *cf = (const struct canfd_frame *)skb->data;
> +
> +	if (can_is_canfd_skb(skb))
> +		return min_t(u8, cf->len, CANFD_MAX_DLEN);
> +	else if (cf->can_id & CAN_RTR_FLAG)
> +		return 0;
> +	else
> +		return min_t(u8, cf->len, CAN_MAX_DLEN);
> +}

The main idea behind this patch and patch 05/16 is to provide a correct 
statistic in the tx bytes, right?

A simple test for the CAN_RTR_FLAG will do the job as all the length 
sanitizing is already done in the tx path by can_dropped_invalid_skb() 
in all known drivers right *before* the skb is stored in the echo skb array.

IMO there's no need for a separate helper function. Maybe a macro which 
should have something with 'payload' in its name - to determine the tx 
byte statistics based on CAN_RTR_FLAG ...

Regards,
Oliver

> +
>   struct net_device *alloc_candev_mqs(int sizeof_priv, unsigned int echo_skb_max,
>   				    unsigned int txqs, unsigned int rxqs);
>   #define alloc_candev(sizeof_priv, echo_skb_max) \
> 

  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-19 19:05 [RFC]: can 2020-10-19 Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 01/16] can: proc: can_remove_proc(): silence remove_proc_entry warning Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 02/16] can: rx-offload: don't call kfree_skb() from IRQ context Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 03/16] can: dev: can_get_echo_skb(): prevent call to kfree_skb() in hard " Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 04/16] can: dev: can_get_len(): add a helper function to get the correct length of Classical frames Marc Kleine-Budde
2020-10-19 20:35   ` Oliver Hartkopp [this message]
2020-10-20  6:35     ` Marc Kleine-Budde
2020-10-20 11:30       ` Vincent Mailhol
2020-10-20 11:48         ` Marc Kleine-Budde
2020-10-20 12:38         ` Oliver Hartkopp
2020-10-20 15:02           ` Marc Kleine-Budde
2020-10-20 16:07           ` Vincent Mailhol
2020-10-20 17:04             ` Oliver Hartkopp
2020-10-20 18:50               ` Marc Kleine-Budde
2020-10-21  0:52               ` Vincent Mailhol
2020-10-21  6:23                 ` Vincent MAILHOL
2020-10-21  7:11                   ` Joakim Zhang
2020-10-21  7:21                     ` Marc Kleine-Budde
2020-10-21  7:48                       ` Joakim Zhang
2020-10-21  9:21                   ` Oliver Hartkopp
2020-10-21  9:48                     ` Oliver Hartkopp
2020-10-21 11:55                     ` Vincent MAILHOL
2020-10-21 17:52                       ` Oliver Hartkopp
2020-10-22  3:30                         ` Vincent MAILHOL
2020-10-22  7:15                           ` Oliver Hartkopp
2020-10-22 12:23                             ` Vincent MAILHOL
2020-10-22 13:28                               ` Oliver Hartkopp
2020-10-22 15:46                                 ` Vincent MAILHOL
2020-10-22 17:06                                   ` Oliver Hartkopp
2020-10-23 10:36                                     ` Vincent MAILHOL
2020-10-23 16:47                                       ` Oliver Hartkopp
2020-10-24  5:25                                         ` Vincent MAILHOL
2020-10-24 11:31                                           ` Oliver Hartkopp
2020-10-19 19:05 ` [net-rfc 05/16] can: dev: __can_get_echo_skb(): fix the returned length of CAN frame Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 06/16] can: can_create_echo_skb(): fix echo skb generation: always use skb_clone() Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 07/16] can: j1939: j1939_sk_bind(): return failure if netdev is down Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 08/16] can: isotp: Explain PDU in CAN_ISOTP help text Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 09/16] can: isotp: enable RX timeout handling in listen-only mode Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 10/16] can: ti_hecc: add missed clk_disable_unprepare() in error path Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 11/16] can: xilinx_can: handle failure cases of pm_runtime_get_sync Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 12/16] can: peak_usb: fix timestamp wrapping Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 13/16] can: peak_canfd: fix echo management when loopback is on Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 14/16] can: mcp251xfd: mcp251xfd_regmap_crc_read(): increase severity of CRC read error messages Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 15/16] can: mcp251xfd: mcp251xfd_regmap_nocrc_read(): fix semicolon.cocci warnings Marc Kleine-Budde
2020-10-19 19:05 ` [net-rfc 16/16] can: mcp251xfd: remove unneeded break Marc Kleine-Budde

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