Support read/write memory-mapped files.

base/files/memory_mapped_file_posix.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/memory_mapped_file.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 #include "base/logging.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
 
 namespace base {
 
 MemoryMappedFile::MemoryMappedFile() : data_(NULL), length_(0) {
 }
 
 #if !defined(OS_NACL)
 bool MemoryMappedFile::MapFileRegionToMemory(
-    const MemoryMappedFile::Region& region) {
+    const MemoryMappedFile::Region& region,
+    Access access) {
   ThreadRestrictions::AssertIOAllowed();
 
   off_t map_start = 0;
   size_t map_size = 0;
   int32_t data_offset = 0;
 
   if (region == MemoryMappedFile::Region::kWholeFile) {
     int64_t file_len = file_.GetLength();
     if (file_len == -1) {
       DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
@@ skipping 23 matching lines @@
             std::numeric_limits<size_t>::max()) {
       DLOG(ERROR) << "Region bounds are not valid for mmap";
       return false;
     }
 
     map_start = static_cast<off_t>(aligned_start);
     map_size = static_cast<size_t>(aligned_size);
     length_ = static_cast<size_t>(region.size);
   }
 
-  data_ = static_cast<uint8_t*>(mmap(NULL, map_size, PROT_READ, MAP_SHARED,
+  int flags = 0;
+  switch (access) {
+    case READ_ONLY:
+      flags |= PROT_READ;
+      break;
+    case READ_WRITE:
+      flags |= PROT_READ | PROT_WRITE;
+      break;
+    case READ_WRITE_EXTEND:
+      // POSIX won't auto-extend the file when it is written so it must first

[Mark Mentovai, 2016/05/19 14:57:06]

What kinds of sizes are we talking about here? HFS+, the OS X filesystem,
doesn’t support sparse files, so all of these zeroes will actually be written to
disk.

[bcwhite, 2016/05/19 15:40:25]

That's annoying.

For my immediate use (sharing SyzyASAN metrics with the browser on some fraction
of Canary users), it is a couple MB.

+      // be explicitly extended to the maximum size. Zeros will fill the new
+      // space.
+      file_.SetLength(std::max(file_.GetLength(), region.offset + region.size));
+      flags |= PROT_READ | PROT_WRITE;
+      break;
+  }
+  data_ = static_cast<uint8_t*>(mmap(NULL, map_size, flags, MAP_SHARED,
                                      file_.GetPlatformFile(), map_start));
   if (data_ == MAP_FAILED) {
     DPLOG(ERROR) << "mmap " << file_.GetPlatformFile();
     return false;
   }
 
   data_ += data_offset;
   return true;
 }
 #endif
 
 void MemoryMappedFile::CloseHandles() {
   ThreadRestrictions::AssertIOAllowed();
 
   if (data_ != NULL)
     munmap(data_, length_);
   file_.Close();
 
   data_ = NULL;
   length_ = 0;
 }
 
 }  // namespace base