PCScan: Disable unconditionally on 32-bit systems

All partitions on 32-bit systems are marked non-scannable, effectively
disabling PCScan and preventing address space waste for quarantine
bitmaps that would never materialize.

Bug: 1159694
Change-Id: I6fea110633ea0a72146beb917810b8b04d603f23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601894Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840457}

base/allocator/partition_allocator/partition_root.cc

@@ -20,6 +20,10 @@ template <bool thread_safe>
 typename PartitionRoot<thread_safe>::PCScanMode PartitionOptionsToPCScanMode(
     PartitionOptions::PCScan opt) {
   using Root = PartitionRoot<thread_safe>;
+  // PCScan is currently only supported on 64-bit systems.
+  // Mark partitions non-scannable on 32-bit systems unconditionally, so that
+  // address space for quarantine bitmaps doesn't get reserved.
+#if defined(PA_HAS_64_BITS_POINTERS)
   switch (opt) {
     case PartitionOptions::PCScan::kAlwaysDisabled:
       return Root::PCScanMode::kNonScannable;
@@ -28,6 +32,9 @@ typename PartitionRoot<thread_safe>::PCScanMode PartitionOptionsToPCScanMode(
     case PartitionOptions::PCScan::kForcedEnabledForTesting:
       return Root::PCScanMode::kEnabled;
   }
+#else
+  return Root::PCScanMode::kNonScannable;
+#endif
 }
 }  // namespace
 

base/allocator/partition_allocator/pcscan_unittest.cc

@@ -4,6 +4,7 @@
 
 #include "base/allocator/partition_allocator/partition_root.h"
 #if !defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
+#if defined(PA_HAS_64_BITS_POINTERS)
 
 #include "base/allocator/partition_allocator/pcscan.h"
 
@@ -329,4 +330,5 @@ TEST_F(PCScanTest, DanglingInterPartitionReference) {
 }  // namespace internal
 }  // namespace base
 
+#endif  // defined(PA_HAS_64_BITS_POINTERS)
 #endif  // defined(MEMORY_TOOL_REPLACES_ALLOCATOR)