diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
index 2cff8a7349a7..c5f9b348b438 100644
--- a/drivers/scsi/st.c
+++ b/drivers/scsi/st.c
@@ -45,6 +45,7 @@ static const char *verstr = "20160209";
 
 #include <linux/uaccess.h>
 #include <asm/dma.h>
+#include <asm/unaligned.h>
 
 #include <scsi/scsi.h>
 #include <scsi/scsi_dbg.h>
@@ -2680,8 +2681,7 @@ static void deb_space_print(struct scsi_tape *STp, int direction, char *units, u
 	if (!debugging)
 		return;
 
-	sc = cmd[2] & 0x80 ? 0xff000000 : 0;
-	sc |= (cmd[2] << 16) | (cmd[3] << 8) | cmd[4];
+	sc = sign_extend32(get_unaligned_be24(&cmd[2]), 23);
 	if (direction)
 		sc = -sc;
 	st_printk(ST_DEB_MSG, STp, "Spacing tape %s over %d %s.\n",