Write protect executable pages in the VM.

runtime/vm/code_patcher.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/code_patcher.h"
 #include "vm/cpu.h"
 #include "vm/instructions.h"
 #include "vm/object.h"
+#include "vm/virtual_memory.h"
 
 namespace dart {
 
+WritableInstructionsScope::WritableInstructionsScope(uword address,
+                                                     intptr_t size)
+    : address_(address), size_(size) {
+  bool status =
+      VirtualMemory::Protect(reinterpret_cast<void*>(address),
+                             size,
+                             VirtualMemory::kReadWriteExecute);
+  ASSERT(status);
+}
+
+
+WritableInstructionsScope::~WritableInstructionsScope() {
+  bool status =
+      VirtualMemory::Protect(reinterpret_cast<void*>(address_),
+                             size_,
+                             VirtualMemory::kReadExecute);
+  ASSERT(status);
+}
+
+
 static void SwapCode(intptr_t num_bytes, char* code, char* buffer) {
   uword code_address = reinterpret_cast<uword>(code);
   for (intptr_t i = 0; i < num_bytes; i++) {
     char tmp = *code;
     *code = *buffer;
     *buffer = tmp;
     code++;
     buffer++;
   }
   CPU::FlushICache(code_address, num_bytes);
   // The buffer is not executed. No need to flush.
 }
 
 
 // The patch code buffer contains the jmp code which will be inserted at
 // entry point.
 void CodePatcher::PatchEntry(const Code& code) {
   const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
                                                 PcDescriptors::kEntryPatch);
   ASSERT(patch_addr != 0);
   JumpPattern jmp_entry(patch_addr, code);
   ASSERT(!jmp_entry.IsValid());
   const uword patch_buffer = code.GetPatchCodePc();
   ASSERT(patch_buffer != 0);
   JumpPattern jmp_patch(patch_buffer, code);
   ASSERT(jmp_patch.IsValid());
   const uword jump_target = jmp_patch.TargetAddress();
-  SwapCode(jmp_patch.pattern_length_in_bytes(),
-           reinterpret_cast<char*>(patch_addr),
-           reinterpret_cast<char*>(patch_buffer));
-  jmp_entry.SetTargetAddress(jump_target);
+  intptr_t length = jmp_patch.pattern_length_in_bytes();
+  {
+    WritableInstructionsScope writable_code(patch_addr, length);
+    WritableInstructionsScope writable_buffer(patch_buffer, length);
+    SwapCode(jmp_patch.pattern_length_in_bytes(),
+             reinterpret_cast<char*>(patch_addr),
+             reinterpret_cast<char*>(patch_buffer));
+    jmp_entry.SetTargetAddress(jump_target);
+  }
 }
 
 
 // The entry point is a jmp instruction, the patch code buffer contains
 // original code, the entry point contains the jump instruction.
 void CodePatcher::RestoreEntry(const Code& code) {
   const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
                                                 PcDescriptors::kEntryPatch);
   ASSERT(patch_addr != 0);
   JumpPattern jmp_entry(patch_addr, code);
   ASSERT(jmp_entry.IsValid());
   const uword jump_target = jmp_entry.TargetAddress();
   const uword patch_buffer = code.GetPatchCodePc();
   ASSERT(patch_buffer != 0);
   // 'patch_buffer' contains original entry code.
   JumpPattern jmp_patch(patch_buffer, code);
   ASSERT(!jmp_patch.IsValid());
-  SwapCode(jmp_patch.pattern_length_in_bytes(),
-           reinterpret_cast<char*>(patch_addr),
-           reinterpret_cast<char*>(patch_buffer));
-  ASSERT(jmp_patch.IsValid());
-  jmp_patch.SetTargetAddress(jump_target);
+  intptr_t length = jmp_patch.pattern_length_in_bytes();
+  {
+    WritableInstructionsScope writable_code(patch_addr, length);
+    WritableInstructionsScope writable_buffer(patch_buffer, length);
+    SwapCode(jmp_patch.pattern_length_in_bytes(),
+             reinterpret_cast<char*>(patch_addr),
+             reinterpret_cast<char*>(patch_buffer));
+    ASSERT(jmp_patch.IsValid());
+    jmp_patch.SetTargetAddress(jump_target);
+  }
 }
 
 
 bool CodePatcher::CodeIsPatchable(const Code& code) {
   const uword patch_addr = code.GetPcForDeoptId(Isolate::kNoDeoptId,
                                                 PcDescriptors::kEntryPatch);
   // kEntryPatch may not exist which means the function is not patchable.
   if (patch_addr == 0) {
     return true;
   }
   JumpPattern jmp_entry(patch_addr, code);
   if (code.Size() < (jmp_entry.pattern_length_in_bytes() * 2)) {
     return false;
   }
   const uword limit = patch_addr + jmp_entry.pattern_length_in_bytes();
   // Check no object stored between patch_addr .. limit.
   for (intptr_t i = 0; i < code.pointer_offsets_length(); i++) {
     const uword obj_start = code.GetPointerOffsetAt(i) + code.EntryPoint();
     const uword obj_end  = obj_start + kWordSize;
     if ((obj_start < limit) && (obj_end > patch_addr)) {
       return false;
     }
   }
   return true;
 }
 
 }  // namespace dart