[SyzyAsan] Fix overflow error in ShadowWalker for 4GB 32-bit processes.

syzygy/agent/asan/runtime.cc

 // Copyright 2012 Google Inc. All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 925 matching lines @@
     error_info->corrupt_ranges[i] = corrupt_ranges[i];
 
     // Allocate space for the first block of this range on the stack.
     // TODO(sebmarchand): Report more blocks if necessary.
     AsanBlockInfo* asan_block_info = block_infos;
     error_info->corrupt_ranges[i].block_info = block_infos;
     error_info->corrupt_ranges[i].block_info_count = 1;
     ++block_infos;
 
     // Use a shadow walker to find the first corrupt block in this range and
-    // copy its metadata.
+    // copy its metadata. It's fine for the upper bound address calculation to
+    // overflow to 0, as this represents the end of the 4GB address space.
     ShadowWalker shadow_walker(
         shadow(), false,
         reinterpret_cast<const uint8_t*>(corrupt_ranges[i].address),
         reinterpret_cast<const uint8_t*>(corrupt_ranges[i].address) +
             corrupt_ranges[i].length);
     BlockInfo block_info = {};
     CHECK(shadow_walker.Next(&block_info));
     // The heap checker removes block protections as it goes, so this block
     // should be readable. However, remove page protections just to be sure.
     // They are left turned off so that the minidump generation can introspect
@@ skipping 307 matching lines @@
   if (crash_reporter_.get() != nullptr &&
       crash_reporter_->GetName() == reporters::CrashpadReporter::kName) {
     enabled_features |= ASAN_FEATURE_ENABLE_CRASHPAD;
   }
 
   return enabled_features;
 }
 
 }  // namespace asan
 }  // namespace agent