iscsi-target: Do not reject non-immediate CmdSNs exceeding MaxCmdSN

This patch changes iscsit_sequence_cmd() logic to no longer reject
non-immediate CmdSNs that exceed MaxCmdSN with a protocol error,
but instead silently ignore them.

This is done to correctly follow RFC-3720 Section 3.2.2.1:

For non-immediate commands, the CmdSN field can take any
value from ExpCmdSN to MaxCmdSN inclusive. The target MUST silently
ignore any non-immediate command outside of this range or non-
immediate duplicates within the range.

Reported-by: Santosh Kulkarni <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/iscsi/iscsi_target_core.h

···
 	CMDSN_NORMAL_OPERATION		= 0,
 	CMDSN_LOWER_THAN_EXP		= 1,
 	CMDSN_HIGHER_THAN_EXP		= 2,
+	CMDSN_MAXCMDSN_OVERRUN		= 3,
 };
 
 /* Used for iscsi_handle_immediate_data() return values */

drivers/target/iscsi/iscsi_target_util.c

···
 	 */
 	if (iscsi_sna_gt(cmdsn, sess->max_cmd_sn)) {
 		pr_err("Received CmdSN: 0x%08x is greater than"
-		       " MaxCmdSN: 0x%08x, protocol error.\n", cmdsn,
+		       " MaxCmdSN: 0x%08x, ignoring.\n", cmdsn,
 		       sess->max_cmd_sn);
-		ret = CMDSN_ERROR_CANNOT_RECOVER;
+		ret = CMDSN_MAXCMDSN_OVERRUN;
 
 	} else if (cmdsn == sess->exp_cmd_sn) {
 		sess->exp_cmd_sn++;
···
 		ret = CMDSN_HIGHER_THAN_EXP;
 		break;
 	case CMDSN_LOWER_THAN_EXP:
+	case CMDSN_MAXCMDSN_OVERRUN:
+	default:
 		cmd->i_state = ISTATE_REMOVE;
 		iscsit_add_cmd_to_immediate_queue(cmd, conn, cmd->i_state);
-		ret = cmdsn_ret;
-		break;
-	default:
-		reason = ISCSI_REASON_PROTOCOL_ERROR;
-		reject = true;
-		ret = cmdsn_ret;
+		/*
+		 * Existing callers for iscsit_sequence_cmd() will silently
+		 * ignore commands with CMDSN_LOWER_THAN_EXP, so force this
+		 * return for CMDSN_MAXCMDSN_OVERRUN as well..
+		 */
+		ret = CMDSN_LOWER_THAN_EXP;
 		break;
 	}
 	mutex_unlock(&conn->sess->cmdsn_mutex);