diff mbox series

[v2,2/6] can: dev: add a helper function to get the correct length of Classical frames

Message ID 20200930144602.10290-3-mailhol.vincent@wanadoo.fr
State Superseded
Headers show
Series [v2,1/6] can: dev: can_get_echo_skb(): prevent call to kfree_skb() in hard IRQ context | expand

Commit Message

Vincent MAILHOL Sept. 30, 2020, 2:45 p.m. UTC
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>
---
 include/linux/can/dev.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

Comments

Marc Kleine-Budde Sept. 30, 2020, 3:21 p.m. UTC | #1
On 9/30/20 4:45 PM, Vincent Mailhol wrote:
> 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>
> ---
>  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 5e3d45525bd3..72a8a60c0094 100644
> --- a/include/linux/can/dev.h
> +++ b/include/linux/can/dev.h
> @@ -177,6 +177,29 @@ u8 can_dlc2len(u8 can_dlc);
>  /* map the sanitized data length to an appropriate data length code */
>  u8 can_len2dlc(u8 len);
>  
> +/*
> + * get_can_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 int get_can_len(struct sk_buff *skb)

make this return an u8
make the skb const

> +{
> +	struct canfd_frame *cf = (struct canfd_frame *)skb->data;

const

> +
> +	if (can_is_canfd_skb(skb))
> +		return min_t(__u8, cf->len, CANFD_MAX_DLEN);

u8

> +	else if (cf->can_id & CAN_RTR_FLAG)
> +		return 0;
> +	else
> +		return min_t(__u8, cf->len, CAN_MAX_DLEN);

u8

> +}
> +
>  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) \
> 

Marc
diff mbox series

Patch

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 5e3d45525bd3..72a8a60c0094 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -177,6 +177,29 @@  u8 can_dlc2len(u8 can_dlc);
 /* map the sanitized data length to an appropriate data length code */
 u8 can_len2dlc(u8 len);
 
+/*
+ * get_can_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 int get_can_len(struct sk_buff *skb)
+{
+	struct canfd_frame *cf = (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);
+}
+
 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) \