Prevent the madvise(MADV_MERGEABLE) mmap hack from affecting errno.

Bug: 9889616
Change-Id: I4a7323e0ae5aeb5cbe0da1b2bc7501d83b3a2aa4

libc/bionic/mmap.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <errno.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "private/ErrnoRestorer.h"
+
+// mmap2(2) is like mmap(2), but the offset is in 4096-byte blocks, not bytes.
+extern "C" void*  __mmap2(void*, size_t, int, int, int, size_t);
+
+#define MMAP2_SHIFT 12 // 2**12 == 4096
+
+void* mmap(void* addr, size_t size, int prot, int flags, int fd, long offset) {
+  if (offset & ((1UL << MMAP2_SHIFT)-1)) {
+    errno = EINVAL;
+    return MAP_FAILED;
+  }
+
+  void* result = __mmap2(addr, size, prot, flags, fd, offset >> MMAP2_SHIFT);
+
+  if (result != MAP_FAILED && (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) != 0) {
+    ErrnoRestorer errno_restorer;
+    madvise(result, size, MADV_MERGEABLE);
+  }
+
+  return result;
+}