refactoring: introduce MappedFileFragment

Change-Id: I97b0c1131711ed0ac13ab33e3357e99e0b6b1a4e

linker/linker_mapped_file_fragment.cpp

@@ -0,0 +1,82 @@
+/*
+ * Copyright (C) 2015 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "linker_mapped_file_fragment.h"
+#include "linker_debug.h"
+
+#include <inttypes.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+constexpr off64_t kPageMask = ~static_cast<off64_t>(PAGE_SIZE-1);
+
+static off64_t page_start(off64_t offset) {
+  return offset & kPageMask;
+}
+
+static bool safe_add(off64_t* out, off64_t a, size_t b) {
+  CHECK(a >= 0);
+  if (static_cast<uint64_t>(INT64_MAX - a) < b) {
+    return false;
+  }
+
+  *out = a + b;
+  return true;
+}
+
+static size_t page_offset(off64_t offset) {
+  return static_cast<size_t>(offset & (PAGE_SIZE-1));
+}
+
+MappedFileFragment::MappedFileFragment() : map_start_(nullptr), map_size_(0),
+                                           data_(nullptr), size_ (0)
+{ }
+
+MappedFileFragment::~MappedFileFragment() {
+  if (map_start_ != nullptr) {
+    munmap(map_start_, map_size_);
+  }
+}
+
+bool MappedFileFragment::Map(int fd, off64_t base_offset, size_t elf_offset, size_t size) {
+  off64_t offset;
+  CHECK(safe_add(&offset, base_offset, elf_offset));
+
+  off64_t page_min = page_start(offset);
+  off64_t end_offset;
+
+  CHECK(safe_add(&end_offset, offset, size));
+  CHECK(safe_add(&end_offset, end_offset, page_offset(offset)));
+
+  size_t map_size = static_cast<size_t>(end_offset - page_min);
+  CHECK(map_size >= size);
+
+  uint8_t* map_start = static_cast<uint8_t*>(
+                          mmap64(nullptr, map_size, PROT_READ, MAP_PRIVATE, fd, page_min));
+
+  if (map_start == MAP_FAILED) {
+    return false;
+  }
+
+  map_start_ = map_start;
+  map_size_ = map_size;
+
+  data_ = map_start + page_offset(offset);
+  size_ = size;
+
+  return true;
+}