- Do not access the first character in zero-length strings.

runtime/vm/symbols.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/symbols.h"
 
 #include "vm/handles.h"
 #include "vm/handles_impl.h"
 #include "vm/hash_table.h"
 #include "vm/isolate.h"
@@ skipping 407 matching lines @@
   }
   const bool is_one_byte_string = char_size == kOneByteChar;
 
   Zone* zone = Thread::Current()->zone();
   if (is_one_byte_string) {
     uint8_t* buffer = zone->Alloc<uint8_t>(len_sum);
     const uint8_t* const orig_buffer = buffer;
     for (intptr_t i = 0; i < strs_length; i++) {
       NoSafepointScope no_safepoint;
       intptr_t str_len = lengths[i];
-      const String& str = strs[i];
-      ASSERT(str.IsOneByteString() || str.IsExternalOneByteString());
-      const uint8_t* src_p = str.IsOneByteString() ?
-          OneByteString::CharAddr(str, 0) :
-          ExternalOneByteString::CharAddr(str, 0);
-      memmove(buffer, src_p, str_len);
-      buffer += str_len;
+      if (str_len > 0) {
+        const String& str = strs[i];
+        ASSERT(str.IsOneByteString() || str.IsExternalOneByteString());
+        const uint8_t* src_p = str.IsOneByteString() ?
+            OneByteString::CharAddr(str, 0) :
+            ExternalOneByteString::CharAddr(str, 0);
+        memmove(buffer, src_p, str_len);
+        buffer += str_len;
+      }
     }
     ASSERT(len_sum == buffer - orig_buffer);
     return Symbols::FromLatin1(orig_buffer, len_sum);
   } else {
     uint16_t* buffer = zone->Alloc<uint16_t>(len_sum);
     const uint16_t* const orig_buffer = buffer;
     for (intptr_t i = 0; i < strs_length; i++) {
       NoSafepointScope no_safepoint;
       intptr_t str_len = lengths[i];
-      const String& str = strs[i];
-      if (str.IsTwoByteString()) {
-        memmove(buffer, TwoByteString::CharAddr(str, 0), str_len * 2);
-      } else if (str.IsExternalTwoByteString()) {
-        memmove(buffer, ExternalTwoByteString::CharAddr(str, 0), str_len * 2);
-      } else {
-        // One-byte to two-byte string copy.
-        ASSERT(str.IsOneByteString() || str.IsExternalOneByteString());
-        const uint8_t* src_p = str.IsOneByteString() ?
-            OneByteString::CharAddr(str, 0) :
-            ExternalOneByteString::CharAddr(str, 0);
-        for (int n = 0; n < str_len; n++) {
-          buffer[n] = src_p[n];
+      if (str_len > 0) {
+        const String& str = strs[i];
+        if (str.IsTwoByteString()) {
+          memmove(buffer, TwoByteString::CharAddr(str, 0), str_len * 2);
+        } else if (str.IsExternalTwoByteString()) {
+          memmove(buffer, ExternalTwoByteString::CharAddr(str, 0), str_len * 2);
+        } else {
+          // One-byte to two-byte string copy.
+          ASSERT(str.IsOneByteString() || str.IsExternalOneByteString());
+          const uint8_t* src_p = str.IsOneByteString() ?
+              OneByteString::CharAddr(str, 0) :
+              ExternalOneByteString::CharAddr(str, 0);
+          for (int n = 0; n < str_len; n++) {
+            buffer[n] = src_p[n];
+          }
         }
+        buffer += str_len;
       }
-      buffer += str_len;
     }
     ASSERT(len_sum == buffer - orig_buffer);
     return Symbols::FromUTF16(orig_buffer, len_sum);
   }
 }
 
 
 // StringType can be StringSlice, ConcatString, or {Latin1,UTF16,UTF32}Array.
 template<typename StringType>
 RawString* Symbols::NewSymbol(const StringType& str) {
@@ skipping 70 matching lines @@
 RawObject* Symbols::GetVMSymbol(intptr_t object_id) {
   ASSERT(IsVMSymbolId(object_id));
   intptr_t i = (object_id - kMaxPredefinedObjectIds);
   if ((i > kIllegal) && (i < Symbols::kMaxPredefinedId)) {
     return symbol_handles_[i]->raw();
   }
   return Object::null();
 }
 
 }  // namespace dart