GWP-ASan: Change alloc_offset to alloc_ptr

Currently, the SlotMetadata records the allocation offset in the
returned page; however, it's only ever used to calculate the final
allocation address. Instead, just store the allocation address.

Bug: 896019
Change-Id: Id14a15e4a7c9afdeb4a517b85a90ddb9f30f7a9f
Reviewed-on: https://chromium-review.googlesource.com/c/1318404
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605548}

components/gwp_asan/common/guarded_page_allocator.cc

@@ -82,10 +82,12 @@ void* GuardedPageAllocator::Allocate(size_t size, size_t align) {
     // Return left-aligned allocation to detect underflows.
     offset = 0;
 
+  void* alloc = reinterpret_cast<void*>(free_page + offset);
+
   // Initialize slot metadata.
-  data_[free_slot].RecordAllocation(size, offset);
+  data_[free_slot].RecordAllocation(size, alloc);
 
-  return reinterpret_cast<void*>(free_page + offset);
+  return alloc;
 }
 
 void GuardedPageAllocator::Deallocate(void* ptr) {
@@ -95,7 +97,7 @@ void GuardedPageAllocator::Deallocate(void* ptr) {
   MarkPageInaccessible(reinterpret_cast<void*>(GetPageAddr(addr)));
 
   size_t slot = AddrToSlot(GetPageAddr(addr));
-  DCHECK_EQ(addr, GetPageAddr(addr) + data_[slot].alloc_offset);
+  DCHECK_EQ(ptr, data_[slot].alloc_ptr);
   // Check for double free.
   if (data_[slot].dealloc_trace_addr) {
     double_free_detected_ = true;
@@ -113,7 +115,7 @@ size_t GuardedPageAllocator::GetRequestedSize(const void* ptr) const {
   DCHECK(PointerIsMine(ptr));
   const uintptr_t addr = reinterpret_cast<uintptr_t>(ptr);
   size_t slot = AddrToSlot(GetPageAddr(addr));
-  DCHECK_EQ(addr, GetPageAddr(addr) + data_[slot].alloc_offset);
+  DCHECK_EQ(ptr, data_[slot].alloc_ptr);
   return data_[slot].alloc_size;
 }
 
@@ -257,11 +259,11 @@ void GuardedPageAllocator::SlotMetadata::Reset() {
 }
 
 void GuardedPageAllocator::SlotMetadata::RecordAllocation(size_t size,
-                                                          size_t offset) {
+                                                          void* ptr) {
   Reset();
 
   alloc_size = size;
-  alloc_offset = offset;
+  alloc_ptr = ptr;
 
   alloc_tid = base::PlatformThread::CurrentId();
   new (stacktrace_alloc) StackTrace();

components/gwp_asan/common/guarded_page_allocator.h

@@ -83,16 +83,16 @@ class GWP_ASAN_EXPORT GuardedPageAllocator {
     // having them be statically allocated in the SlotMetadata itself.)
     void Init();
 
-    // Update slot metadata on an allocation with the given size and offset.
-    void RecordAllocation(size_t size, size_t offset);
+    // Update slot metadata on an allocation with the given size and pointer.
+    void RecordAllocation(size_t size, void* ptr);
 
     // Update slot metadata on a deallocation.
     void RecordDeallocation();
 
     // Size of the allocation
     size_t alloc_size = 0;
-    // How far into the page is the returned allocation.
-    size_t alloc_offset = 0;
+    // The allocation address.
+    void* alloc_ptr = nullptr;
 
     // (De)allocation thread id or base::kInvalidThreadId if no (de)allocation
     // occurred.