blob: edb299de5c957cc36f5fbd0ae85a056051012ba9 [file] [log] [blame]
// Copyright (c) 2012 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 "build/build_config.h"
#include <vector>
#include <utility>
#include <stdint.h>
#include "base/files/memory_mapped_file.h"
#include "base/memory/singleton.h"
namespace base {
// It's common practice to install file descriptors into well known slot
// numbers before execing a child; stdin, stdout and stderr are ubiqutous
// examples.
// However, when using a zygote model, this becomes troublesome. Since the
// descriptors which need to be in these slots generally aren't known, any code
// could open a resource and take one of the reserved descriptors. Simply
// overwriting the slot isn't a viable solution.
// We could try to fill the reserved slots as soon as possible, but this is a
// fragile solution since global constructors etc are able to open files.
// Instead, we retreat from the idea of installing descriptors in specific
// slots and add a layer of indirection in the form of this singleton object.
// It maps from an abstract key to a descriptor. If independent modules each
// need to define keys, then values should be chosen randomly so as not to
// collide.
class BASE_EXPORT GlobalDescriptors {
typedef uint32_t Key;
struct Descriptor {
Descriptor(Key key, int fd);
Descriptor(Key key, int fd, base::MemoryMappedFile::Region region);
// Globally unique key.
Key key;
// Actual FD.
int fd;
// Optional region, defaults to kWholeFile.
base::MemoryMappedFile::Region region;
typedef std::vector<Descriptor> Mapping;
// Often we want a canonical descriptor for a given Key. In this case, we add
// the following constant to the key value:
#if !defined(OS_ANDROID)
static const int kBaseDescriptor = 3; // 0, 1, 2 are already taken.
// 3 used by __android_log_write().
// 4 used by... something important on Android M.
// 5 used by... something important on Android L... on low-end devices.
// TODO(amistry): An Android, this mechanism is only used for tests since the
// content child launcher spawns a process by creating a new Activity using
// the Android APIs. For tests, come up with a way that doesn't require using
// a pre-defined fd.
static const int kBaseDescriptor = 6;
// Return the singleton instance of GlobalDescriptors.
static GlobalDescriptors* GetInstance();
// Get a descriptor given a key. It is a fatal error if the key is not known.
int Get(Key key) const;
// Get a descriptor given a key. Returns -1 on error.
int MaybeGet(Key key) const;
// Get a region given a key. It is a fatal error if the key is not known.
base::MemoryMappedFile::Region GetRegion(Key key) const;
// Set the descriptor for the given |key|. This sets the region associated
// with |key| to kWholeFile.
void Set(Key key, int fd);
// Set the descriptor and |region| for the given |key|.
void Set(Key key, int fd, base::MemoryMappedFile::Region region);
void Reset(const Mapping& mapping);
friend struct DefaultSingletonTraits<GlobalDescriptors>;
Mapping descriptors_;
} // namespace base