Index: lib/scudo/standalone/combined.h
===================================================================
--- lib/scudo/standalone/combined.h
+++ lib/scudo/standalone/combined.h
@@ -375,7 +375,7 @@
     const uptr From = Base;
     const uptr To = Base + Size;
     auto Lambda = [this, From, To, Callback, Arg](uptr Block) {
-      if (Block < From || Block > To)
+      if (Block < From || Block >= To)
         return;
       uptr ChunkSize;
       const uptr ChunkBase = getChunkFromBlock(Block, &ChunkSize);
Index: lib/scudo/standalone/primary64.h
===================================================================
--- lib/scudo/standalone/primary64.h
+++ lib/scudo/standalone/primary64.h
@@ -36,7 +36,7 @@
 // freelist to the thread specific freelist, and back.
 //
 // The memory used by this allocator is never unmapped, but can be partially
-// released it the platform allows for it.
+// released if the platform allows for it.
 
 template <class SizeClassMapT, uptr RegionSizeLog> class SizeClassAllocator64 {
 public:
@@ -135,7 +135,9 @@
   }
 
   template <typename F> void iterateOverBlocks(F Callback) const {
-    for (uptr I = 1; I < NumClasses; I++) {
+    for (uptr I = 0; I < NumClasses; I++) {
+      if (I == SizeClassMap::BatchClassId)
+        continue;
       const RegionInfo *Region = getRegionInfo(I);
       const uptr BlockSize = getSizeByClassId(I);
       const uptr From = Region->RegionBeg;
Index: lib/scudo/standalone/tests/wrappers_c_test.cpp
===================================================================
--- lib/scudo/standalone/tests/wrappers_c_test.cpp
+++ lib/scudo/standalone/tests/wrappers_c_test.cpp
@@ -14,6 +14,14 @@
 #include <malloc.h>
 #include <unistd.h>
 
+extern "C" {
+void malloc_enable(void);
+void malloc_disable(void);
+int malloc_iterate(uintptr_t base, size_t size,
+                   void (*callback)(uintptr_t base, size_t size, void *arg),
+                   void *arg);
+}
+
 // Note that every C allocation function in the test binary will be fulfilled
 // by Scudo (this includes the gtest APIs, etc.), which is a test by itself.
 // But this might also lead to unexpected side-effects, since the allocation and
@@ -239,3 +247,36 @@
   MI = mallinfo();
   EXPECT_GE(static_cast<size_t>(MI.fordblks), Free + BypassQuarantineSize);
 }
+
+static uintptr_t BoundaryP;
+static size_t Count;
+
+static void callback(uintptr_t Base, size_t Size, void *Arg) {
+  if (Base == BoundaryP)
+    Count++;
+}
+
+// Verify that a block located on an iteration boundary is not mis-accounted.
+// To achieve this, we allocate a chunk for which the backing block will be
+// aligned on a page, then run the malloc_iterate on both the pages that the
+// block is a boundary for. It must only be seen once by the callback function.
+TEST(ScudoWrappersCTest, MallocIterateBoundary) {
+  const size_t PageSize = sysconf(_SC_PAGESIZE);
+  const size_t BlockDelta = FIRST_32_SECOND_64(8U, 16U);
+  const size_t SpecialSize = PageSize - BlockDelta;
+
+  void *P = malloc(SpecialSize);
+  EXPECT_NE(P, nullptr);
+  BoundaryP = reinterpret_cast<uintptr_t>(P);
+  const uintptr_t Block = BoundaryP - BlockDelta;
+  EXPECT_EQ((Block & (PageSize - 1)), 0U);
+
+  Count = 0U;
+  malloc_disable();
+  malloc_iterate(Block - PageSize, PageSize, callback, nullptr);
+  malloc_iterate(Block, PageSize, callback, nullptr);
+  malloc_enable();
+  EXPECT_EQ(Count, 1U);
+
+  free(P);
+}