libdisasm: disassembly of strex may cause SIGSEGV
To make things confusing enough, the names of bitfields between ldrex and
strex are not totally consistent so this commit makes it easier on future
readers.

@@ -1274,32 +1274,56 @@
  */
 static int
 arm_dis_lsexcl(uint32_t in, char *buf, size_t buflen)
 {
         arm_cond_code_t cc;
-        arm_reg_t rn, rd, rm;
+        arm_reg_t rx, ry, rz;
         int lbit;
         size_t len;
 
+        /*
+         * To make things confusing enough, the names of bitfields between
+         * ldrex and strex are not totally consistent.  Specifically,
+         *    STREX rd, rt, [rn]
+         *       rn = 19:16
+         *       rd = 15:12
+         *       rt = 3:0
+         *
+         *    LDREX rt, [rn]
+         *       rn = 19:16
+         *       rt = 15:12
+         *
+         * To avoid having to do too many mental gymnastics, let's just
+         * think of the bitfields as:
+         *
+         *       rx = 19:16
+         *       ry = 15:12
+         *       rz = 3:0
+         *
+         * And so we print the instructions as:
+         *    STREX ry, rz, [rx]
+         *    LDREX ry, [rx]
+         */
+
         cc = (in & ARM_CC_MASK) >> ARM_CC_SHIFT;
-        rn = (in & ARM_ELS_RN_MASK) >> ARM_ELS_RN_SHIFT;
-        rd = (in & ARM_ELS_RD_MASK) >> ARM_ELS_RD_SHIFT;
-        rm = in & ARM_ELS_RN_MASK;
+        rx = (in & ARM_ELS_RN_MASK) >> ARM_ELS_RN_SHIFT;
+        ry = (in & ARM_ELS_RD_MASK) >> ARM_ELS_RD_SHIFT;
+        rz = (in & ARM_ELS_LOW_AM_MASK);
         lbit = in & ARM_ELS_LBIT_MASK;
 
         len = snprintf(buf, buflen, "%s%sex %s, ",
             lbit != 0 ? "ldr" : "str",
-            arm_cond_names[cc], arm_reg_names[rd]);
+            arm_cond_names[cc], arm_reg_names[ry]);
         if (len >= buflen)
                 return (-1);
 
         if (lbit)
                 len += snprintf(buf + len, buflen - len, "[%s]",
-                    arm_reg_names[rn]);
+                    arm_reg_names[rx]);
         else
                 len += snprintf(buf + len, buflen - len, "%s, [%s]",
-                    arm_reg_names[rm], arm_reg_names[rn]);
+                    arm_reg_names[rz], arm_reg_names[rx]);
         return (len >= buflen ? -1 : 0);
 }
 
 /*
  * This is designed to handle the multiplication instruction extension space.