[RFC PATCH 13/14] can: netlink: add userland error messages

[Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>]

Use NL_SET_ERR_MSG_FMT() to return meaningfull error messages to the
userland whenever the validation of the CAN netlink arguments fails.

Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
See this one as a WIP. I wrote this quickly. I will revisit and
rephrase when dropping the RFC tag of this series.

Also, do not try to imagine that I wrote this because I forgot how to
provide to can bittiming arguments to the ip tool. This patch is only
meant to help the newcommers, nothing else :D
---
 drivers/net/can/dev/netlink.c | 40 ++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 8 deletions(-)

diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
index 3c89b304c5b8..0a6700194ab0 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/drivers/net/can/dev/netlink.c
@@ -124,8 +124,11 @@ static int can_validate(struct nlattr *tb[], struct nlattr *data[],
 				       cm->flags & CAN_CTRLMODE_TDC_AUTO,
 				       cm->flags & CAN_CTRLMODE_TDC_MANUAL,
 				       extack);
-		if (err)
+		if (err) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "TDC parameters are incorrect");
 			return err;
+		}
 	}
 
 	if (data[IFLA_CAN_BITTIMING]) {
@@ -133,26 +136,41 @@ static int can_validate(struct nlattr *tb[], struct nlattr *data[],
 
 		memcpy(&bt, nla_data(data[IFLA_CAN_BITTIMING]), sizeof(bt));
 		err = can_validate_bittiming(&bt, extack);
-		if (err)
+		if (err) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "Nominal bittiming parameters are incorrect");
 			return err;
+		}
 	}
 
 	if (is_can_fd) {
-		if (!data[IFLA_CAN_BITTIMING] || !data[IFLA_CAN_DATA_BITTIMING])
+		if (!data[IFLA_CAN_BITTIMING] || !data[IFLA_CAN_DATA_BITTIMING]) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "Provide both nominal and FD data bittiming");
 			return -EOPNOTSUPP;
+		}
 	}
 	if (is_can_xl) {
-		if (!data[IFLA_CAN_BITTIMING] || !data[IFLA_CAN_XL_DATA_BITTIMING])
+		if (!data[IFLA_CAN_BITTIMING] || !data[IFLA_CAN_XL_DATA_BITTIMING]) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "Provide both nominal and XL data bittiming");
 			return -EOPNOTSUPP;
+		}
 	}
 
 	if (data[IFLA_CAN_DATA_BITTIMING] || data[IFLA_CAN_TDC]) {
-		if (!is_can_fd)
+		if (!is_can_fd) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "CAN FD is required to use FD data bittiming or FD TDC");
 			return -EOPNOTSUPP;
+		}
 	}
 	if (data[IFLA_CAN_XL_DATA_BITTIMING] || data[IFLA_CAN_XL_TDC]) {
-		if (!is_can_xl)
+		if (!is_can_xl) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "CAN XL is required to use XL data bittiming or XL TDC");
 			return -EOPNOTSUPP;
+		}
 	}
 
 	if (data[IFLA_CAN_DATA_BITTIMING]) {
@@ -160,16 +178,22 @@ static int can_validate(struct nlattr *tb[], struct nlattr *data[],
 
 		memcpy(&bt, nla_data(data[IFLA_CAN_DATA_BITTIMING]), sizeof(bt));
 		err = can_validate_bittiming(&bt, extack);
-		if (err)
+		if (err) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "FD data bittiming parameters are incorrect");
 			return err;
+		}
 	}
 	if (data[IFLA_CAN_XL_DATA_BITTIMING]) {
 		struct can_bittiming bt;
 
 		memcpy(&bt, nla_data(data[IFLA_CAN_XL_DATA_BITTIMING]), sizeof(bt));
 		err = can_validate_bittiming(&bt, extack);
-		if (err)
+		if (err) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "FD data bittiming parameters are incorrect");
 			return err;
+		}
 	}
 
 	return 0;
-- 
2.45.2