[PartitionAlloc] fix cookie tracking for large no-op reallocs

The new test case will trip an assert under debug builds
prior to the patch: *cookie_ptr == kCookieValue[i] because
a new cookie is not written, and the old location now is part
of the space made available to the caller.

Bug: 897585
Change-Id: I9cb0a0378bd692445580f7b8b796200154bc15c6
Reviewed-on: https://chromium-review.googlesource.com/c/1294724Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602042}

base/allocator/partition_allocator/partition_alloc.cc

@@ -207,12 +207,10 @@ bool PartitionReallocDirectMappedInPlace(PartitionRootGeneric* root,
 
   // bucket->slot_size is the current size of the allocation.
   size_t current_size = page->bucket->slot_size;
-  if (new_size == current_size)
-    return true;
-
   char* char_ptr = static_cast<char*>(internal::PartitionPage::ToPointer(page));
-
-  if (new_size < current_size) {
+  if (new_size == current_size) {
+    // No need to move any memory around, but update size and cookie below.
+  } else if (new_size < current_size) {
     size_t map_size =
         internal::PartitionDirectMapExtent::FromPage(page)->map_size;
 

base/allocator/partition_allocator/partition_alloc_unittest.cc

@@ -2211,6 +2211,24 @@ TEST_F(PartitionAllocTest, ZeroFill) {
   }
 }
 
+TEST_F(PartitionAllocTest, Bug_897585) {
+  // Need sizes big enough to be direct mapped and a delta small enough to
+  // allow re-use of the page when cookied. These numbers fall out of the
+  // test case in the indicated bug.
+  size_t kInitialSize = 983040;
+  size_t kDesiredSize = 983100;
+  void* ptr = PartitionAllocGenericFlags(generic_allocator.root(),
+                                         PartitionAllocReturnNull, kInitialSize,
+                                         nullptr);
+  ASSERT_NE(nullptr, ptr);
+  ptr = PartitionReallocGenericFlags(generic_allocator.root(),
+                                     PartitionAllocReturnNull, ptr,
+                                     kDesiredSize, nullptr);
+  ASSERT_NE(nullptr, ptr);
+  memset(ptr, 0xbd, kDesiredSize);
+  PartitionFree(ptr);
+}
+
 }  // namespace internal
 }  // namespace base