can: slcan: allow reception of short error messages

Allows slcan to receive short messages (typically errors) from the serial
interface.

When error support was added to slcan protocol in
b32ff4668544e1333b694fcc7812b2d7397b4d6a ("can: slcan: extend the protocol
with error info") the minimum valid message size changed from 5 (minimum
standard can frame tIII0) to 3 ("e1a" is a valid protocol message, it is
one of the examples given in the comments for slcan_bump_err() ), but the
check for minimum message length prodicating all decoding was not adjusted.
This makes short error messages discarded and error frames not being
generated.

This patch changes the minimum length to the new minimum (3 characters,
excluding terminator, is now a valid message).

Signed-off-by: Carlos Sanchez <[e-mail]>
Fixes: b32ff4668544 ("can: slcan: extend the protocol with error info")
Reviewed-by: Vincent Mailhol <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Cc: [e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/slcan/slcan-core.c

···
 #define SLCAN_CMD_LEN 1
 #define SLCAN_SFF_ID_LEN 3
 #define SLCAN_EFF_ID_LEN 8
+#define SLCAN_DATA_LENGTH_LEN 1
+#define SLCAN_ERROR_LEN 1
 #define SLCAN_STATE_LEN 1
 #define SLCAN_STATE_BE_RXCNT_LEN 3
 #define SLCAN_STATE_BE_TXCNT_LEN 3
-#define SLCAN_STATE_FRAME_LEN       (1 + SLCAN_CMD_LEN + \
-				     SLCAN_STATE_BE_RXCNT_LEN + \
-				     SLCAN_STATE_BE_TXCNT_LEN)
+#define SLCAN_STATE_MSG_LEN     (SLCAN_CMD_LEN +		\
+                                 SLCAN_STATE_LEN +		\
+                                 SLCAN_STATE_BE_RXCNT_LEN +	\
+                                 SLCAN_STATE_BE_TXCNT_LEN)
+#define SLCAN_ERROR_MSG_LEN_MIN (SLCAN_CMD_LEN +	\
+                                 SLCAN_ERROR_LEN +	\
+                                 SLCAN_DATA_LENGTH_LEN)
+#define SLCAN_FRAME_MSG_LEN_MIN (SLCAN_CMD_LEN +	\
+                                 SLCAN_SFF_ID_LEN +	\
+                                 SLCAN_DATA_LENGTH_LEN)
 struct slcan {
 	struct can_priv         can;
 
···
 	int i, tmp;
 	u32 tmpid;
 	char *cmd = sl->rbuff;
+
+	if (sl->rcount < SLCAN_FRAME_MSG_LEN_MIN)
+		return;
 
 	skb = alloc_can_skb(sl->dev, &cf);
 	if (unlikely(!skb)) {
···
 		return;
 	}
 
-	if (state == sl->can.state || sl->rcount < SLCAN_STATE_FRAME_LEN)
+	if (state == sl->can.state || sl->rcount != SLCAN_STATE_MSG_LEN)
 		return;
 
 	cmd += SLCAN_STATE_BE_RXCNT_LEN + SLCAN_CMD_LEN + 1;
···
 	char *cmd = sl->rbuff;
 	bool rx_errors = false, tx_errors = false, rx_over_errors = false;
 	int i, len;
+
+	if (sl->rcount < SLCAN_ERROR_MSG_LEN_MIN)
+		return;
 
 	/* get len from sanitized ASCII value */
 	len = cmd[1];
···
 static void slcan_unesc(struct slcan *sl, unsigned char s)
 {
 	if ((s == '\r') || (s == '\a')) { /* CR or BEL ends the pdu */
-		if (!test_and_clear_bit(SLF_ERROR, &sl->flags) &&
-		    sl->rcount > 4)
+		if (!test_and_clear_bit(SLF_ERROR, &sl->flags))
 			slcan_bump(sl);
 
 		sl->rcount = 0;