base/posix/global_descriptors.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_POSIX_GLOBAL_DESCRIPTORS_H_
   6 #define BASE_POSIX_GLOBAL_DESCRIPTORS_H_
   7
   8 #include "build/build_config.h"
   9
  10 #include <vector>
  11 #include <utility>
  12
  13 #include <stdint.h>
  14
  15 #include "base/files/memory_mapped_file.h"
  16 #include "base/memory/singleton.h"
  17
  18 namespace base {
  19
  20 // It's common practice to install file descriptors into well known slot
  21 // numbers before execing a child; stdin, stdout and stderr are ubiqutous
  22 // examples.
  23 //
  24 // However, when using a zygote model, this becomes troublesome. Since the
  25 // descriptors which need to be in these slots generally aren't known, any code
  26 // could open a resource and take one of the reserved descriptors. Simply
  27 // overwriting the slot isn't a viable solution.
  28 //
  29 // We could try to fill the reserved slots as soon as possible, but this is a
  30 // fragile solution since global constructors etc are able to open files.
  31 //
  32 // Instead, we retreat from the idea of installing descriptors in specific
  33 // slots and add a layer of indirection in the form of this singleton object.
  34 // It maps from an abstract key to a descriptor. If independent modules each
  35 // need to define keys, then values should be chosen randomly so as not to
  36 // collide.
  37 class BASE_EXPORT GlobalDescriptors {
  38  public:
  39   typedef uint32_t Key;
  40   struct Descriptor {
  41     Descriptor(Key key, int fd);
  42     Descriptor(Key key, int fd, base::MemoryMappedFile::Region region);
  43
  44     // Globally unique key.
  45     Key key;
  46     // Actual FD.
  47     int fd;
  48     // Optional region, defaults to kWholeFile.
  49     base::MemoryMappedFile::Region region;
  50   };
  51   typedef std::vector<Descriptor> Mapping;
  52
  53   // Often we want a canonical descriptor for a given Key. In this case, we add
  54   // the following constant to the key value:
  55 #if !defined(OS_ANDROID)
  56   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
  57 #else
  58   static const int kBaseDescriptor = 4;  // 3 used by __android_log_write().
  59 #endif
  60
  61   // Return the singleton instance of GlobalDescriptors.
  62   static GlobalDescriptors* GetInstance();
  63
  64   // Get a descriptor given a key. It is a fatal error if the key is not known.
  65   int Get(Key key) const;
  66
  67   // Get a descriptor given a key. Returns -1 on error.
  68   int MaybeGet(Key key) const;
  69
  70   // Get a region given a key. It is a fatal error if the key is not known.
  71   base::MemoryMappedFile::Region GetRegion(Key key) const;
  72
  73   // Set the descriptor for the given |key|. This sets the region associated
  74   // with |key| to kWholeFile.
  75   void Set(Key key, int fd);
  76
  77   // Set the descriptor and |region| for the given |key|.
  78   void Set(Key key, int fd, base::MemoryMappedFile::Region region);
  79
  80   void Reset(const Mapping& mapping);
  81
  82  private:
  83   friend struct DefaultSingletonTraits<GlobalDescriptors>;
  84   GlobalDescriptors();
  85   ~GlobalDescriptors();
  86
  87   Mapping descriptors_;
  88 };
  89
  90 }  // namespace base
  91
  92 #endif  // BASE_POSIX_GLOBAL_DESCRIPTORS_H_