ART: Resolve MAP_32BIT limitation in x86_64

Add checks that ensure when low4gb is set and an expected pointer
is given the requested memory fits into 4GB.

On x86_64, only use MAP_32BIT when there is no expected pointer.
This avoids a limitation in mmap (only 2GB visible).

Add tests to check behavior.

Original Author: Qiming Shi <qiming.shi@intel.com>
Change-Id: Ia2e3e0a46764ef70126b0c264f1fae681622d3cb

1 files changed, 15 insertions, 1 deletions

diff --git a/runtime/mem_map.cc b/runtime/mem_map.cc
index 1594338df3..98b0bbf125 100644
--- a/runtime/mem_map.cc
+++ b/runtime/mem_map.cc
@@ -128,6 +128,20 @@ MemMap* MemMap::MapAnonymous(const char* name, byte* expected, size_t byte_count
   // We need to store and potentially set an error number for pretty printing of errors
   int saved_errno = 0;
 
+#ifdef __LP64__
+  // When requesting low_4g memory and having an expectation, the requested range should fit into
+  // 4GB.
+  if (low_4gb && (
+      // Start out of bounds.
+      (reinterpret_cast<uintptr_t>(expected) >> 32) != 0 ||
+      // End out of bounds. For simplicity, this will fail for the last page of memory.
+      (reinterpret_cast<uintptr_t>(expected + page_aligned_byte_count) >> 32) != 0)) {
+    *error_msg = StringPrintf("The requested address space (%p, %p) cannot fit in low_4gb",
+                              expected, expected + page_aligned_byte_count);
+    return nullptr;
+  }
+#endif
+
   // TODO:
   // A page allocator would be a useful abstraction here, as
   // 1) It is doubtful that MAP_32BIT on x86_64 is doing the right job for us
@@ -192,7 +206,7 @@ MemMap* MemMap::MapAnonymous(const char* name, byte* expected, size_t byte_count
 
 #else
 #ifdef __x86_64__
-  if (low_4gb) {
+  if (low_4gb && expected == nullptr) {
     flags |= MAP_32BIT;
   }
 #endif