target: Fix handling of small allocation lengths in REPORT LUNS

REPORT LUNS should not fail just because the allocation length is less
than 16.  The relevant section of SPC-4 is:

  4.2.5.6 Allocation length

  The ALLOCATION LENGTH field specifies the maximum number of bytes or
  blocks that an application client has allocated in the Data-In
  Buffer. The ALLOCATION LENGTH field specifies bytes unless a
  different requirement is stated in the command definition.

  An allocation length of zero specifies that no data shall be
  transferred. This condition shall not be considered an error.

So we should just truncate our response rather than return an error.
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Spencer Baugh <sbaugh@catern.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_spc.c

@@ -1203,17 +1203,13 @@ sense_reason_t spc_emulate_report_luns(struct se_cmd *cmd)
 	struct se_dev_entry *deve;
 	struct se_session *sess = cmd->se_sess;
 	struct se_node_acl *nacl;
+	struct scsi_lun slun;
 	unsigned char *buf;
 	u32 lun_count = 0, offset = 8;
-
-	if (cmd->data_length < 16) {
-		pr_warn("REPORT LUNS allocation length %u too small\n",
-			cmd->data_length);
-		return TCM_INVALID_CDB_FIELD;
-	}
+	__be32 len;
 
 	buf = transport_kmap_data_sg(cmd);
-	if (!buf)
+	if (cmd->data_length && !buf)
 		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
 
 	/*
@@ -1234,10 +1230,12 @@ sense_reason_t spc_emulate_report_luns(struct se_cmd *cmd)
 		 * See SPC2-R20 7.19.
 		 */
 		lun_count++;
-		if ((offset + 8) > cmd->data_length)
+		if (offset >= cmd->data_length)
 			continue;
 
-		int_to_scsilun(deve->mapped_lun, (struct scsi_lun *)&buf[offset]);
+		int_to_scsilun(deve->mapped_lun, &slun);
+		memcpy(buf + offset, &slun,
+		       min(8u, cmd->data_length - offset));
 		offset += 8;
 	}
 	rcu_read_unlock();
@@ -1250,16 +1248,18 @@ sense_reason_t spc_emulate_report_luns(struct se_cmd *cmd)
 	 * If no LUNs are accessible, report virtual LUN 0.
 	 */
 	if (lun_count == 0) {
-		int_to_scsilun(0, (struct scsi_lun *)&buf[offset]);
+		int_to_scsilun(0, &slun);
+		if (cmd->data_length > 8)
+			memcpy(buf + offset, &slun,
+			       min(8u, cmd->data_length - offset));
 		lun_count = 1;
 	}
 
-	lun_count *= 8;
-	buf[0] = ((lun_count >> 24) & 0xff);
-	buf[1] = ((lun_count >> 16) & 0xff);
-	buf[2] = ((lun_count >> 8) & 0xff);
-	buf[3] = (lun_count & 0xff);
-	transport_kunmap_data_sg(cmd);
+	if (buf) {
+		len = cpu_to_be32(lun_count * 8);
+		memcpy(buf, &len, min_t(int, sizeof len, cmd->data_length));
+		transport_kunmap_data_sg(cmd);
+	}
 
 	target_complete_cmd_with_length(cmd, GOOD, 8 + lun_count * 8);
 	return 0;