target: fix truncation of mode data, support zero allocation length

The offset was not bumped back to the full size after writing the
header of the MODE SENSE response, so the last 1 or 2 bytes were
not copied.

On top of this, support zero-length requests by checking for the
return value of transport_kmap_data_sg.

Testcase: sg_raw -r20 /dev/sdb 5a 00 0a 00 00 00 00 00 14 00
    last byte should be 0x1e
    it is 0x00 without the patch
    it is correct with the patch

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_spc.c

@@ -784,7 +784,7 @@ static int spc_emulate_modesense(struct se_cmd *cmd)
 	unsigned char *rbuf;
 	int type = dev->transport->get_device_type(dev);
 	int ten = (cmd->t_task_cdb[0] == MODE_SENSE_10);
-	int offset = ten ? 8 : 4;
+	u32 offset = ten ? 8 : 4;
 	int length = 0;
 	unsigned char buf[SE_MODE_PAGE_BUF];
 
@@ -817,6 +817,7 @@ static int spc_emulate_modesense(struct se_cmd *cmd)
 		offset -= 2;
 		buf[0] = (offset >> 8) & 0xff;
 		buf[1] = offset & 0xff;
+		offset += 2;
 
 		if ((cmd->se_lun->lun_access & TRANSPORT_LUNFLAGS_READ_ONLY) ||
 		    (cmd->se_deve &&
@@ -826,13 +827,10 @@ static int spc_emulate_modesense(struct se_cmd *cmd)
 		if ((dev->se_sub_dev->se_dev_attrib.emulate_write_cache > 0) &&
 		    (dev->se_sub_dev->se_dev_attrib.emulate_fua_write > 0))
 			spc_modesense_dpofua(&buf[3], type);
-
-		if ((offset + 2) > cmd->data_length)
-			offset = cmd->data_length;
-
 	} else {
 		offset -= 1;
 		buf[0] = offset & 0xff;
+		offset += 1;
 
 		if ((cmd->se_lun->lun_access & TRANSPORT_LUNFLAGS_READ_ONLY) ||
 		    (cmd->se_deve &&
@@ -842,14 +840,13 @@ static int spc_emulate_modesense(struct se_cmd *cmd)
 		if ((dev->se_sub_dev->se_dev_attrib.emulate_write_cache > 0) &&
 		    (dev->se_sub_dev->se_dev_attrib.emulate_fua_write > 0))
 			spc_modesense_dpofua(&buf[2], type);
-
-		if ((offset + 1) > cmd->data_length)
-			offset = cmd->data_length;
 	}
 
 	rbuf = transport_kmap_data_sg(cmd);
-	memcpy(rbuf, buf, offset);
-	transport_kunmap_data_sg(cmd);
+	if (rbuf) {
+		memcpy(rbuf, buf, min(offset, cmd->data_length));
+		transport_kunmap_data_sg(cmd);
+	}
 
 	target_complete_cmd(cmd, GOOD);
 	return 0;