FEX-Emu--FEX/Source/Tools/FEXMountDaemon/Main.cpp

#include <atomic>
#include <bits/types/siginfo_t.h>
#include <chrono>
#include <condition_variable>
#include <cstdlib>
#include <cstdint>
#include <errno.h>
#include <fcntl.h>
#include <filesystem>
#include <limits.h>
#include <linux/in.h>
#include <mutex>
#include <poll.h>
#include <stdio.h>
#include <string.h>
#include <string>
#include <unistd.h>
#include <signal.h>
#include <sys/epoll.h>
#include <sys/prctl.h>
#include <sys/wait.h>
#include <sys/inotify.h>
#include <sys/un.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/utsname.h>
#include <thread>

namespace {
std::atomic<bool> ShuttingDown{};

void SignalShutdown();
namespace EPollWatcher {
  static int epoll_fd{};
  static std::thread EPollThread{};
  static std::atomic<pid_t> EPollThreadTID{};
  std::atomic<uint64_t> NumPipesWatched{};
  std::atomic<bool> EPollWatcherShutdown {false};
  std::chrono::time_point<std::chrono::system_clock> TimeWhileZeroFDs{};
  // Timeout is ten seconds
  constexpr std::chrono::duration<size_t> TimeoutPeriod = std::chrono::seconds(10);

  uint64_t NumPipesRemaining() {
    return NumPipesWatched.load();
  }

  void AddPipeToWatch(int pipe) {
    struct epoll_event evt{};
    evt.events = EPOLLERR; // This event will return when the read end of a pipe is closed
    evt.data.fd = pipe; // Just return the pipe in the user data
    int Result = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, pipe, &evt);
    if (Result == -1) {
      fprintf(stderr, "[FEXMountDaemon] epoll_ctl returned error %d %s\n", errno, strerror(errno));
    }
    else {
      ++NumPipesWatched;
    }
  }

  void RemovePipeToWatch(int pipe) {
    int Result = epoll_ctl(epoll_fd, EPOLL_CTL_DEL, pipe, nullptr);
    if (Result == -1) {
      fprintf(stderr, "[FEXMountDaemon] epoll_ctl returned error %d %s\n", errno, strerror(errno));
    }
    else {
      --NumPipesWatched;
    }

    // If the number of pipes we are watching drops to zero then start a timer so we can exit
    if (NumPipesRemaining() == 0) {
      TimeWhileZeroFDs = std::chrono::system_clock::now();
    }
  }

  void EPollWatch() {
    pthread_setname_np(pthread_self(), "EPollWatcher");
    EPollThreadTID = ::gettid();
    constexpr size_t MAX_EVENTS = 16;
    struct epoll_event Events[MAX_EVENTS]{};

    // Spin while we are not shutting down
    // Also spin while we have pipes to watch
    while (!EPollWatcherShutdown.load() || NumPipesRemaining() != 0) {
      // Loop every ten seconds
      // epoll_pwait2 only available since kernel 5.11...
      int Result = epoll_pwait(epoll_fd, Events, MAX_EVENTS, 10 * 1000, nullptr);
      if (Result == -1) {
        // EINTR is common here
      }
      else {
        for (size_t i = 0; i < Result; ++i) {
          auto &Event = Events[i];
          if (Event.events & EPOLLERR) {
            // This pipe's read end has closed
            // No need to watch it anymore
            RemovePipeToWatch(Event.data.fd);
          }
        }

        // If we are at zero pipes then check our timer if we are past the timeout period to exit
        if (NumPipesRemaining() == 0) {
          auto Now = std::chrono::system_clock::now();
          auto Dur = Now - TimeWhileZeroFDs;
          if (Dur >= TimeoutPeriod) {
            // We need to shutdown now
            ::SignalShutdown();
          }
        }
      }
    }

    // Close the poll_fd
    close(epoll_fd);
  }

  void SetupEPoll() {
    epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    EPollThread = std::thread{EPollWatcher::EPollWatch};
  }

  void SignalShutdown() {
    EPollWatcherShutdown = true;
    // Send a signal to wake up epoll immediately
    tgkill(::getpid(), EPollThreadTID, SIGUSR1);
  }

  void ShutdownEPoll() {
    SignalShutdown();
    EPollThread.join();
  }
}

namespace SocketWatcher {
  static int socket_fd{};
  static std::thread SocketThread{};
  static std::atomic<pid_t> SocketThreadTID{};
  const std::string *SocketWatcherPath{};
  std::atomic<bool> SocketShutdown {false};

  void SocketFunction() {
    pthread_setname_np(pthread_self(), "SocketWatcher");
    SocketThreadTID = ::gettid();
    listen(socket_fd, 16);
    while (!SocketShutdown.load()) {
      // Listen for connections
      // Wait for new clients coming in
      struct pollfd pfd{};
      pfd.fd = socket_fd;
      pfd.events = POLLIN;

      // Wait for ten seconds
      struct timespec ts{};
      ts.tv_sec = 10;

      int Result = ppoll(&pfd, 1, &ts, nullptr);

      if (Result > 0) {
        struct sockaddr_in from{};
        socklen_t addrlen = sizeof(from);
        int new_client = accept(socket_fd, reinterpret_cast<struct sockaddr *>(&from), &addrlen);

        // Wait for data coming in
        struct pollfd pfd{};
        pfd.fd = new_client;
        pfd.events = POLLIN;

        // Wait for ten seconds
        struct timespec ts{};
        ts.tv_sec = 10;

        int Result = ppoll(&pfd, 1, &ts, nullptr);
        if (Result == -1) {
          // EINTR is common here
        }
        else if (Result > 0) {
          // We got data to grab
          struct msghdr msg{};
          struct iovec iov{};
          char iov_data{};

          // Setup the ancillary buffer. This is where we will be getting pipe FDs
          // We only need 4 bytes for the FD
          constexpr size_t CMSG_SIZE = CMSG_SPACE(sizeof(int));
          union AncillaryBuffer {
            struct cmsghdr Header;
            uint8_t Buffer[CMSG_SIZE];
          };
          AncillaryBuffer AncBuf{};

          // Set up message header
          msg.msg_name = nullptr;
          msg.msg_namelen = 0;
          msg.msg_iov = &iov;
          msg.msg_iovlen = 1;

          // Setup iov. We won't be receiving any real data here
          iov.iov_base = &iov_data;
          iov.iov_len = sizeof(iov_data);

          // Now link to our ancilllary buffer
          msg.msg_control = AncBuf.Buffer;
          msg.msg_controllen = CMSG_SIZE;

          ssize_t DataResult = recvmsg(new_client, &msg, 0);
          if (DataResult == -1) {
            // Sometimes get a spurious read?
          }
          else {
            // Now that we have the data, we can extract the FD from the ancillary buffer
            struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);

            // Do some error checking
            if (cmsg == nullptr ||
                cmsg->cmsg_len != CMSG_LEN(sizeof(int)) ||
                cmsg->cmsg_level != SOL_SOCKET ||
                cmsg->cmsg_type != SCM_RIGHTS) {
              fprintf(stderr, "[FEXMountDaemon:SocketWatcher] cmsg data was incorrect\n");
            }
            else {
              // Now that we know the cmsg is sane, read the FD
              int NewFD{};
              memcpy(&NewFD, CMSG_DATA(cmsg), sizeof(NewFD));

              // Add to to the epoll watcher
              EPollWatcher::AddPipeToWatch(NewFD);
              // If we received an event while in the process of shutting down then we have raced

              // Send back an ack message
              uint8_t c = 0;
              send(new_client, &c, sizeof(c), 0);
            }
          }
        }

        // Now that we have received data on this socket we don't need to keep it around
        close(new_client);
      }
      else {
        // EINTR common here
      }
    }
  }

  bool CreateServerSocket(std::string &SocketPath) {
    // Unlink the socket file if it exists
    // We are being asked to create a daemon, not error check
    // We don't care if this failed or not
    remove(SocketPath.c_str());

    // Create the initial unix socket
    socket_fd = socket(AF_UNIX, SOCK_STREAM, 0);
    if (socket_fd == -1) {
      fprintf(stderr, "[FEXMountDaemon:SocketWatcher] Couldn't create AF_UNIX socket: %d %s\n", errno, strerror(errno));
      return false;
    }

    struct sockaddr_un addr{};
    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, SocketPath.data(), sizeof(addr.sun_path));

    // Bind the socket to the path
    int Result = bind(socket_fd, reinterpret_cast<struct sockaddr*>(&addr), sizeof(addr));
    if (Result == -1) {
      fprintf(stderr, "[FEXMountDaemon:SocketWatcher] Couldn't bind AF_UNIX socket '%s': %d %s\n", addr.sun_path, errno, strerror(errno));
      return false;
    }

    return true;
  }

  bool SetupSocketWatcher(std::string &SocketPath) {
    SocketWatcherPath = &SocketPath;
    if (!CreateServerSocket(SocketPath)) {
      return false;
    }

    SocketThread = std::thread{SocketWatcher::SocketFunction};
    return true;
  }

  void SignalShutdown() {
    SocketShutdown = true;
    // Send a signal to wake up ppoll immediately
    tgkill(::getpid(), SocketThreadTID, SIGUSR1);
  }

  void ShutdownSocketWatcher() {
    SignalShutdown();
    SocketThread.join();

    // Close the socket and delete the socket file
    close(socket_fd);
    remove(SocketWatcherPath->c_str());
  }
}

namespace INotifyWatcher {
  static int lock_fd {-1};
  static std::condition_variable WaitCV{};
  static std::mutex WaitMutex{};
  static std::atomic<bool> INotifyShutdown {false};
  const std::string *INotifyLockPath{};
  enum LockFailure {
    LOCK_FAIL_FATAL,
    LOCK_FAIL_EXISTS,
    LOCK_FAIL_CREATION_RACE,
    LOCK_FAIL_CREATED,
  };

  constexpr int USER_PERMS = S_IRWXU | S_IRWXG | S_IRWXO;

  void RemoveLock();

  LockFailure CreateINotifyLock(const std::string &LockPath, const char *MountPath) {
    INotifyLockPath = &LockPath;
    lock_fd = open(LockPath.c_str(), O_RDONLY, USER_PERMS);
    if (lock_fd != -1) {
      // LockFD already existed!
      // This will have now refcounted the existing daemon!
      close(lock_fd);
      return LOCK_FAIL_EXISTS;
    }

    lock_fd = open(LockPath.c_str(), O_CREAT | O_RDWR, USER_PERMS);
    if (lock_fd == -1) {
      // Couldn't open lock file for some reason
      // Likely read only file system
      return LOCK_FAIL_FATAL;
    }

    LockFailure Failure = LOCK_FAIL_FATAL;

    // Set up the write lock to ensure this doesn't race
    {
      // Attempt to open a write lease on the lock file
      // First thing, mask the signal from the least interface
      // By default it is SIGIO
      {
        sigset_t set;
        sigemptyset(&set);
        sigaddset(&set, SIGIO);
        if (sigprocmask(SIG_BLOCK, &set, nullptr) == -1) {
          goto err;
        }
      }

      // Set the file's lease signal
      // Even if we set it to default, this is necessary
      {
        int Res = fcntl(lock_fd, F_SETSIG, SIGIO);
        if (Res == -1) {
          // Shouldn't fail
          goto err;
        }
      }

      // Now attempt to get a write lock on this file
      {
        int Res = fcntl(lock_fd, F_SETLEASE, F_WRLCK);
        if (Res == -1) {
          // Couldn't get a write lock
          // This means another FEXMountDaemon is in the process of setting up a rootfs
          // Early exit, this will be mounted in another process
          Failure = LOCK_FAIL_CREATION_RACE;
          goto err;
        }
      }

      // Now that we have a lock on the file.
      // Write where we are going to be mounting
      // Nothing else can currently open the file for reads yet to see this
      write(lock_fd, MountPath, strlen(MountPath));
    }

    return LOCK_FAIL_CREATED;
    err:
    if (lock_fd != -1) {
      close (lock_fd);
    }

    return Failure;
  }

  void WatchLock() {
    pthread_setname_np(pthread_self(), "INotifyWatcher");

    // Let go of the file lease file to allow FEX to continue
    fcntl(lock_fd, F_SETLEASE, F_UNLCK);

    // Wait in a mutex to shutdown
    std::unique_lock<std::mutex> lk (WaitMutex);
    WaitCV.wait(lk, [] { return INotifyShutdown.load(); });

    // Remove the locks immediately after shutdown
    INotifyWatcher::RemoveLock();
  }

  void RemoveLock() {
    if (lock_fd != -1) {
      // Grab the file lock so FEX can't open it
      int Res = fcntl(lock_fd, F_SETLEASE, F_WRLCK);
      if (Res == -1) {
        fprintf(stderr, "Couldn't lock lock file: %d %s\n", errno, strerror(errno));
      }

      // remove the contents in case any client is in the process of opening this
      ftruncate(lock_fd, 0);

      // Remove the lock file itself
      unlink(INotifyLockPath->c_str());

      // Clear the lease on it since we are shutting down
      fcntl(lock_fd, F_SETLEASE, F_UNLCK);

      // Close the lock fd
      close(lock_fd);
      lock_fd = -1;
    }
  }

  void SignalShutdown() {
    INotifyShutdown = true;
    WaitCV.notify_all();
  }
}

void ActionHandler(int sig, siginfo_t *info, void *context) {
  if (sig == SIGCHLD) {
    if (EPollWatcher::NumPipesRemaining() != 0) {
      // Check the pipes we are watching to see if any exist
      // If our child process shutdown while our parent is still running
      // Then the parent loses its rootfs and problems occur
      fprintf(stderr, "FEXMountDaemon child process from squashfuse has closed while FEX instances still exist\n");
      fprintf(stderr, "Expect errors!\n");
    }
  }
  else if (sig == SIGUSR1) {
    // Ignore
  }
  else {
    // Signal sent directly to process
    // Force a shutdown
    fprintf(stderr, "We are being told to shutdown with SIGTERM\n");
    fprintf(stderr, "Watch out! You might get dangling mount points!\n");
    ::SignalShutdown();
  }
}

void SignalShutdown() {
  ::ShuttingDown = true;
  // We need to very carefully deconstruct our state to ensure we don't have a race condition
  // First thing to do is to shutdown the lock thread
  // Race here where the application opens the lock file, reads the mount directory while the file is getting deleted
  // Nothing we can really do on our side
  INotifyWatcher::SignalShutdown();

  // Now that we have shutdown the lock file. No more NEW clients of FEX will find this daemon
  // Next race is that any clients that managed to read the lock file before it was erased can find the socket
  // for this rootfs daemon
  // Depending on when the socket closes either the client will fail to send an FD to us OR
  // we receive the FD and start watching it
  //
  // On failure to send FD, the client needs to retry FEXMountDaemon mounting
  SocketWatcher::SignalShutdown();

  // Time to handle the tail end of the last race condition
  // In the case of FEX sending an FD while SocketWatcher is shutting down
  //
  // Shutdown()
  // -> INotifyWatcher::Shutdown()
  // -> SocketWatcher:: Receives FD
  // -> EPollWatcher::SignalShutdown
  // -> EPollWatcher::EPollWatch keeps running until all FDs leave
  EPollWatcher::SignalShutdown();
}

}

int main(int argc, char **argv, char **envp) {
  if (argc < 4) {
    fprintf(stderr, "usage: %s <SquashFS> <MountPoint> <pipe wr>\n", argv[0]);
    return -1;
  }

  pid_t pid = fork();
  if (pid != 0) {
    // Parent is leaving to force this process to deparent itself
    // This lets this process become the child of whatever the reaper parent is
    // Do this up front so a FEXInterptreter instance doesn't erroneously pick up a SIGCHLD
    return 0;
  }

  const char *SquashFSPath = argv[1];
  const char *MountPath = argv[2];
  int pipe_wr = std::atoi(argv[3]);

  // Setup our signal handlers now so we can capture some events
  struct sigaction act{};
  act.sa_sigaction = ActionHandler;
  act.sa_flags = SA_SIGINFO;

  // SIGCHLD if squashfuse exits early
  sigaction(SIGCHLD, &act, nullptr);
  // SIGTERM if something is trying to terminate us
  sigaction(SIGTERM, &act, nullptr);
  // SIGUSR1 just to interrupt syscalls
  sigaction(SIGUSR1, &act, nullptr);

  // Ignore SIGPIPE, we will be checking for pipe closure which could send this signal
  signal(SIGPIPE, SIG_IGN);

  // Start the epoll watcher
  EPollWatcher::SetupEPoll();

  // Switch this process over to a new session id
  // Probably not required but allows this to become the process group leader of its session
  ::setsid();

  // Create local FDs so our internal forks can communicate
  int localfds[2];
  pipe2(localfds, 0);

  ::prctl(PR_SET_CHILD_SUBREAPER, 1);

  struct utsname uts{};
  uname (&uts);

  std::string LockPath = "/tmp/.FEX-";
  LockPath += std::filesystem::path(SquashFSPath).filename();
  LockPath += ".lock.";
  LockPath += uts.nodename;

  std::string SocketPath = MountPath;
  SocketPath += ".socket";

  if (!SocketWatcher::SetupSocketWatcher(SocketPath)) {
    EPollWatcher::ShutdownEPoll();
    return -1;
  }

  // Use lock files to ensure we aren't racing to mount multiple FSes
  auto Failure = INotifyWatcher::CreateINotifyLock(LockPath, MountPath);
  if (Failure == INotifyWatcher::LOCK_FAIL_FATAL) {
    EPollWatcher::ShutdownEPoll();
    // Shutdown the signal watcher
    SocketWatcher::ShutdownSocketWatcher();
    return -1;
  }

  if (Failure == INotifyWatcher::LOCK_FAIL_EXISTS ||
      Failure == INotifyWatcher::LOCK_FAIL_CREATION_RACE) {
    EPollWatcher::ShutdownEPoll();
    // Shutdown the signal watcher
    SocketWatcher::ShutdownSocketWatcher();

    // If the lock already exists
    // Then we don't need to spin up the mounts at all
    // Cleanly exit early and let FEX know it can continue
    uint64_t c = 0;
    write(pipe_wr, &c, sizeof(c));
    return 0;
  }

  pid = fork();
  if (pid == 0) {
    // Child
    close(localfds[0]); // Close read side
    const char *argv[4];
    argv[0] = "/usr/bin/squashfuse";
    argv[1] = SquashFSPath;
    argv[2] = MountPath;
    argv[3] = nullptr;

    // Try and execute squashfuse to mount our rootfs
    if (execve(argv[0], (char * const*)argv, envp) == -1) {
      // Let the parent know that we couldn't execute for some reason
      uint64_t error{1};
      write(localfds[1], &error, sizeof(error));

      // Give a hopefully helpful error message for users
      fprintf(stderr, "'%s' Couldn't execute for some reason: %d %s\n", argv[0], errno, strerror(errno));
      fprintf(stderr, "To mount squashfs rootfs files you need squashfuse installed\n");
      fprintf(stderr, "Check your FUSE setup.\n");

      // End the child
      exit(1);
    }
  }
  else {
    // Parent
    close(localfds[1]); // Close the write side
    // Wait for the child to exit
    // This will happen with execve of squashmount or exit on failure
    waitpid(pid, nullptr, 0);

    // Check the child pipe for messages
    pollfd PollFD;
    PollFD.fd = localfds[0];
    PollFD.events = POLLIN;

    int Result = poll(&PollFD, 1, 0);

    if (Result == 1 && PollFD.revents & POLLIN) {
      // Child couldn't execve for whatever reason
      // Remove the mount path and leave Just in case it was created
      rmdir(MountPath);

      // Tell FEX that we encountered an issue
      uint64_t c = 1;
      write(pipe_wr, &c, sizeof(c));
      return 1;
    }

    // Close the pipe now
    close(localfds[0]);

    // Tell FEX that it can continue booting
    // FEX will stall on opening the lock file until we let go of the lease
    uint64_t c = 0;
    write(pipe_wr, &c, sizeof(c));

    // Watch our lock file now for users
    // Blocks here until no more users
    INotifyWatcher::WatchLock();

    // Shutdown the socket watcher fully if it still hasn't
    SocketWatcher::ShutdownSocketWatcher();

    // Shutdown and spin on any epoll FDs remaining
    EPollWatcher::ShutdownEPoll();

    // Handle final mount removal
    // fusermount for unmounting the mountpoint, then the squashfuse will exit automatically
    pid = fork();

    if (pid == 0) {
      const char *argv[5];
      argv[0] = "/bin/fusermount";
      argv[1] = "-u";
      argv[2] = "-q";
      argv[3] = MountPath;
      argv[4] = nullptr;

      if (execve(argv[0], (char * const*)argv, envp) == -1) {
        // Try another location
        argv[0] = "/usr/bin/fusermount";
        if (execve(argv[0], (char * const*)argv, envp) == -1) {
          fprintf(stderr, "fusermount failed to execute. You may have an mount living at '%s' to clean up now\n", SquashFSPath);
          fprintf(stderr, "Try `%s %s %s %s`\n", argv[0], argv[1], argv[2], argv[3]);
          exit(1);
        }
      }
    }
    else {
      // Wait for fusermount to leave
      waitpid(pid, nullptr, 0);

      // Remove the mount path and leave
      rmdir(MountPath);
    }
  }

  return 0;
}