bionic: Add flag to restore legacy mmap behavior

* Pre-lollipop mmap would not care whether offset was signed
  or unsigned.
* Lollipop adds 64-bit support which results in sign extension
  of offset, causing a negative offset when
  a positive offset > 2^31 is given.

Change-Id: I5d19d898fc131cf848217974915d1b466a474f99

2 files changed, 10 insertions, 1 deletions

diff --git a/libc/Android.mk b/libc/Android.mk
index b50717d..d184250 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -610,6 +610,10 @@ ifneq ($(BOARD_MALLOC_ALIGNMENT),)
   libc_common_cflags += -DMALLOC_ALIGNMENT=$(BOARD_MALLOC_ALIGNMENT)
 endif
 
+ifeq ($(BOARD_USES_LEGACY_MMAP),true)
+  libc_common_cflags += -DLEGACY_MMAP
+endif
+
 # Define some common conlyflags
 libc_common_conlyflags := \
     -std=gnu99
diff --git a/libc/bionic/mmap.cpp b/libc/bionic/mmap.cpp
index 8f25a89..53e8b46 100644
--- a/libc/bionic/mmap.cpp
+++ b/libc/bionic/mmap.cpp
@@ -36,6 +36,11 @@
 extern "C" void*  __mmap2(void*, size_t, int, int, int, size_t);
 
 #define MMAP2_SHIFT 12 // 2**12 == 4096
+#ifdef LEGACY_MMAP
+#define TO_64(a) ((a) & 0x00000000ffffffff)
+#else
+#define TO_64(a) (a)
+#endif
 
 static bool kernel_has_MADV_MERGEABLE = true;
 
@@ -60,5 +65,5 @@ void* mmap64(void* addr, size_t size, int prot, int flags, int fd, off64_t offse
 }
 
 void* mmap(void* addr, size_t size, int prot, int flags, int fd, off_t offset) {
-  return mmap64(addr, size, prot, flags, fd, static_cast<off64_t>(offset));
+  return mmap64(addr, size, prot, flags, fd, TO_64(static_cast<off64_t>(offset)));
 }