target: reject COMPARE_AND_WRITE if emulate_caw is not set

In struct se_dev_attrib, there is a field emulate_caw exposed
as a /sys/kernel/config/target/core/$HBA/$DEV/attrib/.

If this field is set zero, it means the corresponding struct se_device
does not support the scsi cmd COMPARE_AND_WRITE

In function sbc_parse_cdb(), go ahead and reject scsi COMPARE_AND_WRITE
if emulate_caw is not set, because it has been explicitly disabled
from user-space.

(Make pr_err ratelimited - nab)

Signed-off-by: Jiang Yi <jiangyilism@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_sbc.c

@@ -1005,6 +1005,12 @@ sbc_parse_cdb(struct se_cmd *cmd, struct sbc_ops *ops)
 		break;
 	}
 	case COMPARE_AND_WRITE:
+		if (!dev->dev_attrib.emulate_caw) {
+			pr_err_ratelimited("se_device %s/%s (vpd_unit_serial %s) reject"
+				" COMPARE_AND_WRITE\n", dev->se_hba->backend->ops->name,
+				dev->dev_group.cg_item.ci_name, dev->t10_wwn.unit_serial);
+			return TCM_UNSUPPORTED_SCSI_OPCODE;
+		}
 		sectors = cdb[13];
 		/*
 		 * Currently enforce COMPARE_AND_WRITE for a single sector