Add documentation for the dynamic code modifications.

src/trusted/validator_ragel/unreviewed/dfa_validate_64.c

Index: src/trusted/validator_ragel/unreviewed/dfa_validate_64.c
===================================================================
--- src/trusted/validator_ragel/unreviewed/dfa_validate_64.c	(revision 9570)
+++ src/trusted/validator_ragel/unreviewed/dfa_validate_64.c	(working copy)
@@ -34,7 +34,7 @@
   UNREFERENCED_PARAMETER(info);
 
   /* Instruction is unsupported by CPU, but otherwise valid.  */
-  if ((info & VALIDATION_ERRORS) == CPUID_UNSUPPORTED_INSTRUCTION) {
+  if ((info & VALIDATION_ERRORS_MASK) == CPUID_UNSUPPORTED_INSTRUCTION) {
     *((enum NaClValidationStatus*)callback_data) =
                                             NaClValidationFailedCpuNotSupported;
   }
@@ -46,7 +46,7 @@
   UNREFERENCED_PARAMETER(callback_data);
 
   /* Stubout unsupported by CPU, but otherwise valid instructions.  */
-  if ((info & VALIDATION_ERRORS) == CPUID_UNSUPPORTED_INSTRUCTION) {
+  if ((info & VALIDATION_ERRORS_MASK) == CPUID_UNSUPPORTED_INSTRUCTION) {
     memset((uint8_t *)begin, NACL_HALT_OPCODE, end - begin + 1);
     return TRUE;
   } else {
@@ -111,7 +111,7 @@
 
   return data->copy_func(
       (uint8_t *)begin_new + data->delta,
-      (info & VALIDATION_ERRORS) == CPUID_UNSUPPORTED_INSTRUCTION ?
+      (info & VALIDATION_ERRORS_MASK) == CPUID_UNSUPPORTED_INSTRUCTION ?
         (uint8_t *)kStubOutMem :
         (uint8_t *)begin_new,
       (uint8_t)(end_new - begin_new + 1));
@@ -157,7 +157,7 @@
   CHECK(instruction_length <= MAX_INSTRUCTION_LENGTH);
 
   /* Unsupported instruction must have been replaced with HLTs.  */
-  if ((info & VALIDATION_ERRORS) == CPUID_UNSUPPORTED_INSTRUCTION) {
+  if ((info & VALIDATION_ERRORS_MASK) == CPUID_UNSUPPORTED_INSTRUCTION) {
     if (memcmp(kStubOutMem, begin_old, instruction_length) == 0) {
       return TRUE;
     } else {
@@ -166,11 +166,11 @@
   /* If we have jump which jumps out of it's range...  */
   } else if (info & DIRECT_JUMP_OUT_OF_RANGE) {
     /* then everything is fine if it's the only error and jump is unchanged!  */
-    if ((info & (VALIDATION_ERRORS & ~DIRECT_JUMP_OUT_OF_RANGE)) ||
+    if ((info & (VALIDATION_ERRORS_MASK & ~DIRECT_JUMP_OUT_OF_RANGE)) ||
         memcmp(begin_new, begin_old, instruction_length) != 0)
       return FALSE;
   /* If instruction is not accepted then we have nothing to do here.  */
-  } else if (info & (VALIDATION_ERRORS | BAD_JUMP_TARGET)) {
+  } else if (info & (VALIDATION_ERRORS_MASK | BAD_JUMP_TARGET)) {
     return FALSE;
   /* Instruction is untouched: we are done.  */
   } else if (memcmp(begin_new, begin_old, instruction_length) == 0) {
@@ -180,17 +180,21 @@
     return FALSE;
   /* Instruction with two-bit immediate can only change these two bits.  */
   } else if ((info & IMMEDIATE_2BIT) == IMMEDIATE_2BIT) {
-    if (memcmp(begin_new, begin_old, instruction_length - 1) != 0 ||
+    if (memcmp(begin_new, begin_old,
+                 instruction_length - (info & IMMEDIATES_SIZE_MASK) - 1) != 0 ||
         (*end_new & 0xfc) != (*end_old & 0xfc)) {
       return FALSE;
     }
+  /* Instruction's last byte is not immediate, thus it must be unchanged.  */
   } else if (info & LAST_BYTE_IS_NOT_IMMEDIATE) {
-    if (memcmp(begin_new, begin_old, instruction_length - 1) != 0) {
+    if (memcmp(begin_new, begin_old,
+                 instruction_length - (info & IMMEDIATES_SIZE_MASK) - 1) != 0 ||
+        (*end_new) != (*end_old)) {
       return FALSE;
     }
   /* Normal instruction can only change an immediate.  */
   } else if (memcmp(begin_new, begin_old,
-                          instruction_length - (info & IMMEDIATES_SIZE)) != 0) {
+                     instruction_length - (info & IMMEDIATES_SIZE_MASK)) != 0) {
     return FALSE;
   }
   return TRUE;