xbmc/platform/posix/utils/SharedMemory.cpp

   1 /*
   2  *  Copyright (C) 2017-2018 Team Kodi
   3  *  This file is part of Kodi - https://kodi.tv
   4  *
   5  *  SPDX-License-Identifier: GPL-2.0-or-later
   6  *  See LICENSES/README.md for more information.
   7  */
   8
   9 #include "SharedMemory.h"
  10
  11 #include <fcntl.h>
  12 #include <sys/mman.h>
  13 #include <unistd.h>
  14
  15 #if defined(HAVE_LINUX_MEMFD)
  16 #include <linux/memfd.h>
  17 #include <sys/syscall.h>
  18 #endif
  19
  20 #include <cerrno>
  21 #include <cstdlib>
  22 #include <system_error>
  23
  24 #include "utils/log.h"
  25
  26 using namespace KODI::UTILS::POSIX;
  27
  28 CSharedMemory::CSharedMemory(std::size_t size)
  29 : m_size{size}, m_fd{Open()}, m_mmap(nullptr, m_size, PROT_READ | PROT_WRITE, MAP_SHARED, m_fd, 0)
  30 {
  31 }
  32
  33 CFileHandle CSharedMemory::Open()
  34 {
  35   CFileHandle fd;
  36   try
  37   {
  38     fd = OpenMemfd();
  39   }
  40   catch (std::system_error const& error)
  41   {
  42     if (error.code() == std::errc::function_not_supported)
  43     {
  44       CLog::Log(LOGDEBUG, "Kernel does not support memfd, falling back to plain shm");
  45       fd = OpenShm();
  46     }
  47     else
  48     {
  49       throw;
  50     }
  51   }
  52
  53   if (ftruncate(fd, m_size) < 0)
  54   {
  55     throw std::system_error(errno, std::generic_category(), "ftruncate");
  56   }
  57
  58   return fd;
  59 }
  60
  61 CFileHandle CSharedMemory::OpenMemfd()
  62 {
  63 #if defined(SYS_memfd_create) && defined(HAVE_LINUX_MEMFD)
  64   // This is specific to Linux >= 3.17, but preferred over shm_create if available
  65   // because it is race-free
  66   int fd = syscall(SYS_memfd_create, "kodi", MFD_CLOEXEC);
  67   if (fd < 0)
  68   {
  69     throw std::system_error(errno, std::generic_category(), "memfd_create");
  70   }
  71
  72   return CFileHandle(fd);
  73 #else
  74   throw std::system_error(std::make_error_code(std::errc::function_not_supported), "memfd_create");
  75 #endif
  76 }
  77
  78 CFileHandle CSharedMemory::OpenShm()
  79 {
  80   char* xdgRuntimeDir = std::getenv("XDG_RUNTIME_DIR");
  81   if (!xdgRuntimeDir)
  82   {
  83     throw std::runtime_error("XDG_RUNTIME_DIR environment variable must be set");
  84   }
  85
  86   std::string tmpFilename(xdgRuntimeDir);
  87   tmpFilename.append("/kodi-shared-XXXXXX");
  88
  89   int rawFd;
  90 #if defined(HAVE_MKOSTEMP)
  91   // Opening the file with O_CLOEXEC is preferred since it avoids races where
  92   // other threads might exec() before setting the CLOEXEC flag
  93   rawFd = mkostemp(&tmpFilename[0], O_CLOEXEC);
  94 #else
  95   rawFd = mkstemp(&tmpFilename[0]);
  96 #endif
  97
  98   if (rawFd < 0)
  99   {
 100     throw std::system_error(errno, std::generic_category(), "mkstemp");
 101   }
 102   CFileHandle fd(rawFd);
 103
 104   int flags = fcntl(fd, F_GETFD);
 105   if (flags < 0)
 106   {
 107     throw std::system_error(errno, std::generic_category(), "fcntl F_GETFD");
 108   }
 109   // Set FD_CLOEXEC if unset
 110   if (!(flags & FD_CLOEXEC) && fcntl(fd, F_SETFD, flags | FD_CLOEXEC) == -1)
 111   {
 112     throw std::system_error(errno, std::generic_category(), "fcntl F_SETFD FD_CLOEXEC");
 113   }
 114
 115   unlink(tmpFilename.c_str());
 116
 117   return fd;
 118 }