target: go through normal processing for zero-length PSCSI commands

Right now, commands with a zero-size payload are skipped completely.
This is wrong; such commands should be passed down to the device and
processed normally.

For physical backends, this ignores completely things such as START
STOP UNIT.  For virtual backends, we have a hack in place to clear a
unit attention state on a zero-size REQUEST SENSE, but we still do
not report errors properly on zero-length commands---out-of-bounds
0-block reads and writes, too small parameter list lengths, etc.

This patch fixes this for PSCSI.  Uses of transport_kmap_data_sg are
guarded with a check for non-zero cmd->data_length; for all other
commands a zero length is handled properly in pscsi_execute_cmd.
The sole exception will be for now REPORT LUNS, which is handled
through the normal SPC emulation.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_pscsi.c

@@ -688,11 +688,11 @@ static void pscsi_transport_complete(struct se_cmd *cmd, struct scatterlist *sg,
 	 * Hack to make sure that Write-Protect modepage is set if R/O mode is
 	 * forced.
 	 */
-	if (((cdb[0] == MODE_SENSE) || (cdb[0] == MODE_SENSE_10)) &&
-	     (status_byte(result) << 1) == SAM_STAT_GOOD) {
-		if (!cmd->se_deve)
+	if (!cmd->se_deve || !cmd->data_length)
 		goto after_mode_sense;
 
+	if (((cdb[0] == MODE_SENSE) || (cdb[0] == MODE_SENSE_10)) &&
+	     (status_byte(result) << 1) == SAM_STAT_GOOD) {
 		if (cmd->se_deve->lun_flags & TRANSPORT_LUNFLAGS_READ_ONLY) {
 			unsigned char *buf = transport_kmap_data_sg(cmd);
 
@@ -709,7 +709,7 @@ static void pscsi_transport_complete(struct se_cmd *cmd, struct scatterlist *sg,
 	}
 after_mode_sense:
 
-	if (sd->type != TYPE_TAPE)
+	if (sd->type != TYPE_TAPE || !cmd->data_length)
 		goto after_mode_select;
 
 	/*

drivers/target/target_core_transport.c

@@ -2300,7 +2300,9 @@ int transport_generic_new_cmd(struct se_cmd *cmd)
 	 * into the fabric for data transfers, go ahead and complete it right
 	 * away.
 	 */
-	if (!cmd->data_length) {
+	if (!cmd->data_length &&
+	    (cmd->se_dev->transport->transport_type != TRANSPORT_PLUGIN_PHBA_PDEV ||
+	     cmd->t_task_cdb[0] == REPORT_LUNS) {
 		spin_lock_irq(&cmd->t_state_lock);
 		cmd->t_state = TRANSPORT_COMPLETE;
 		cmd->transport_state |= CMD_T_ACTIVE;