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@xxxxxxxxxx> --- Changes in v4: None Changes in v3: - Make get_can_len() return u8. - Make the skb const. Reference: https://lkml.org/lkml/2020/9/30/883 Changes in v2: None --- 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..d90890172d2a 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); +/* + * 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 u8 get_can_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); +} + 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) \ -- 2.26.2
