4 files changed, 35 insertions, 9 deletions
diff --git a/lib/msan/msan.cc b/lib/msan/msan.cc
index 7b40051d3..12a141bf4 100644
--- a/lib/msan/msan.cc
+++ b/lib/msan/msan.cc
@@ -415,6 +415,8 @@ void __msan_init() {
 
   MsanTSDInit(MsanTSDDtor);
 
+  MsanAllocatorInit();
+
   MsanThread *main_thread = MsanThread::Create(0, 0);
   SetCurrentThread(main_thread);
   main_thread->ThreadStart();
diff --git a/lib/msan/msan.h b/lib/msan/msan.h
index 703069e98..3776fa9ce 100644
--- a/lib/msan/msan.h
+++ b/lib/msan/msan.h
@@ -189,6 +189,7 @@ bool InitShadow(bool init_origins);
 char *GetProcSelfMaps();
 void InitializeInterceptors();
 
+void MsanAllocatorInit();
 void MsanAllocatorThreadFinish();
 void *MsanCalloc(StackTrace *stack, uptr nmemb, uptr size);
 void *MsanReallocate(StackTrace *stack, void *oldp, uptr size,
diff --git a/lib/msan/msan_allocator.cc b/lib/msan/msan_allocator.cc
index b713c66b9..006d993f0 100644
--- a/lib/msan/msan_allocator.cc
+++ b/lib/msan/msan_allocator.cc
@@ -87,12 +87,7 @@ static Allocator allocator;
 static AllocatorCache fallback_allocator_cache;
 static SpinMutex fallback_mutex;
 
-static int inited = 0;
-
-static inline void Init() {
-  if (inited) return;
-  __msan_init();
-  inited = true;  // this must happen before any threads are created.
+void MsanAllocatorInit() {
   allocator.Init(common_flags()->allocator_may_return_null);
 }
 
@@ -108,7 +103,6 @@ void MsanThreadLocalMallocStorage::CommitBack() {
 
 static void *MsanAllocate(StackTrace *stack, uptr size, uptr alignment,
                           bool zeroise) {
-  Init();
   if (size > kMaxAllowedMallocSize) {
     Report("WARNING: MemorySanitizer failed to allocate %p bytes\n",
            (void *)size);
@@ -143,7 +137,6 @@ static void *MsanAllocate(StackTrace *stack, uptr size, uptr alignment,
 
 void MsanDeallocate(StackTrace *stack, void *p) {
   CHECK(p);
-  Init();
   MSAN_FREE_HOOK(p);
   Metadata *meta = reinterpret_cast<Metadata *>(allocator.GetMetaData(p));
   uptr size = meta->requested_size;
@@ -170,7 +163,6 @@ void MsanDeallocate(StackTrace *stack, void *p) {
 }
 
 void *MsanCalloc(StackTrace *stack, uptr nmemb, uptr size) {
-  Init();
   if (CallocShouldReturnNullDueToOverflow(size, nmemb))
     return allocator.ReturnNullOrDie();
   return MsanReallocate(stack, 0, nmemb * size, sizeof(u64), true);
diff --git a/test/msan/allocator_mapping.cc b/test/msan/allocator_mapping.cc
new file mode 100644
index 000000000..954b8dfef
--- /dev/null
+++ b/test/msan/allocator_mapping.cc
@@ -0,0 +1,31 @@
+// Test that a module constructor can not map memory over the MSan heap
+// (without MAP_FIXED, of course). Current implementation ensures this by
+// mapping the heap early, in __msan_init.
+//
+// RUN: %clangxx_msan -O0 %s -o %t_1
+// RUN: %clangxx_msan -O0 -DHEAP_ADDRESS=$(%run %t_1) %s -o %t_2 && %run %t_2
+
+#include <assert.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <stdlib.h>
+
+#ifdef HEAP_ADDRESS
+struct A {
+  A() {
+    void *const hint = reinterpret_cast<void *>(HEAP_ADDRESS);
+    void *p = mmap(hint, 4096, PROT_READ | PROT_WRITE,
+                   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    // This address must be already mapped. Check that mmap() succeeds, but at a
+    // different address.
+    assert(p != reinterpret_cast<void *>(-1));
+    assert(p != hint);
+  }
+} a;
+#endif
+
+int main() {
+  void *p = malloc(10);
+  printf("0x%zx\n", reinterpret_cast<size_t>(p) & (~0xfff));
+  free(p);
+}