FEX-Emu--FEX/Source/Tools/FEXServer/SquashFS.cpp

#include "Common/FEXServerClient.h"
#include "Common/FileFormatCheck.h"

#include <FEXCore/Config/Config.h>
#include <FEXCore/Utils/LogManager.h>

#include <fcntl.h>
#include <filesystem>
#include <sys/poll.h>
#include <sys/wait.h>
#include <thread>

namespace SquashFS {

  constexpr int USER_PERMS = S_IRWXU | S_IRWXG | S_IRWXO;
  int ServerRootFSLockFD {-1};
  std::string MountFolder{};

  bool InitializeSquashFSPipe() {
    std::string RootFSLockFile = FEXServerClient::GetServerRootFSLockFile();

    int Ret = open(RootFSLockFile.c_str(), O_CREAT | O_RDWR | O_TRUNC | O_EXCL, USER_PERMS);
    ServerRootFSLockFD = Ret;
    if (Ret == -1 && errno == EEXIST) {
      // If the fifo exists then it might be a stale connection.
      // Check the lock status to see if another process is still alive.
      ServerRootFSLockFD = open(RootFSLockFile.c_str(), O_RDWR, USER_PERMS);
      if (ServerRootFSLockFD != -1) {
        // Now that we have opened the file, try to get a write lock.
        flock lk {
          .l_type = F_WRLCK,
          .l_whence = SEEK_SET,
          .l_start = 0,
          .l_len = 0,
        };
        Ret = fcntl(ServerRootFSLockFD, F_SETLK, &lk);

        if (Ret != -1) {
          // Write lock was gained, we can now continue onward.
        }
        else {
          // We couldn't get a write lock, this means that another process already owns a lock on the fifo
          close(ServerRootFSLockFD);
          ServerRootFSLockFD = -1;
          return false;
        }
      }
      else {
        // File couldn't get opened even though it existed?
        // Must have raced something here.
        return false;
      }
    }
    else if (Ret == -1) {
      // Unhandled error.
      LogMan::Msg::EFmt("[FEXServer] Unable to create FEXServer RootFS lock file at: {} {} {}", RootFSLockFile, errno, strerror(errno));
      return false;
    }
    else {
      // FIFO file was created. Try to get a write lock
      flock lk {
        .l_type = F_WRLCK,
        .l_whence = SEEK_SET,
        .l_start = 0,
        .l_len = 0,
      };
      Ret = fcntl(ServerRootFSLockFD, F_SETLK, &lk);

      if (Ret == -1) {
        // Couldn't get a write lock, something else must have got it
        close(ServerRootFSLockFD);
        ServerRootFSLockFD = -1;
        return false;
      }
    }

    return true;
  }

  bool DowngradeRootFSPipeToReadLock() {
    flock lk {
      .l_type = F_RDLCK,
      .l_whence = SEEK_SET,
      .l_start = 0,
      .l_len = 0,
    };
    int Ret = fcntl(ServerRootFSLockFD, F_SETLK, &lk);

    if (Ret == -1) {
      // This shouldn't occur
      LogMan::Msg::EFmt("[FEXServer] Unable to downgrade a rootfs write lock to a read lock {} {}", errno, strerror(errno));
      close(ServerRootFSLockFD);
      ServerRootFSLockFD = -1;
      return false;
    }

    return true;
  }

  bool MountSquashFSPath(std::string SquashFS) {
    pid_t ParentTID = ::getpid();
    MountFolder = fmt::format("{}/.FEXMount{}-XXXXXX", std::filesystem::temp_directory_path().string(), ParentTID);
    char *MountFolderStr = MountFolder.data();

    // Make the temporary mount folder
    if (mkdtemp(MountFolderStr) == nullptr) {
      LogMan::Msg::EFmt("[FEXServer] Couldn't create temporary mount name: {}", MountFolder);
      return false;
    }

    // Change the permissions
    if (chmod(MountFolderStr, 0777) != 0) {
      LogMan::Msg::EFmt("[FEXServer] Couldn't change permissions on temporary mount: {}", MountFolder);
      rmdir(MountFolderStr);
      return false;
    }

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

    int pid = fork();
    if (pid == 0) {
      // Child
      close(fds[0]); // Close read side
      const char *argv[4];
      argv[0] = "squashfuse";
      argv[1] = SquashFS.c_str();
      argv[2] = MountFolder.c_str();
      argv[3] = nullptr;

      // Try and execute squashfuse to mount our rootfs
      if (execvpe(argv[0], (char * const*)argv, environ) == -1) {
        // Give a hopefully helpful error message for users
        LogMan::Msg::EFmt("[FEXServer] '{}' Couldn't execute for some reason: {} {}\n", argv[0], errno, strerror(errno));
        LogMan::Msg::EFmt("[FEXServer] To mount squashfs rootfs files you need squashfuse installed\n");
        LogMan::Msg::EFmt("[FEXServer] Check your FUSE setup.\n");

        // Let the parent know that we couldn't execute for some reason
        uint64_t error{1};
        write(fds[1], &error, sizeof(error));

        // End the child
        exit(1);
      }
    }
    else {
      // Parent
      // Wait for the child to exit
      // This will happen with execvpe of squashmount or exit on failure
      waitpid(pid, nullptr, 0);

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

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

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

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

        LogMan::Msg::EFmt("[FEXServer] Couldn't mount squashfs\n");
        return false;
      }

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

    // Write to the lock file where we are mounted
    write(ServerRootFSLockFD, MountFolder.c_str(), MountFolder.size());
    fdatasync(ServerRootFSLockFD);

    return true;
  }

  void UnmountRootFS() {
    FEX_CONFIG_OPT(LDPath, ROOTFS);
    if (!FEX::FormatCheck::IsSquashFS(LDPath())) {
      return;
    }

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

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

      if (execvp(argv[0], (char * const*)argv) == -1) {
        fprintf(stderr, "fusermount failed to execute. You may have an mount living at '%s' to clean up now\n", MountFolder.c_str());
        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
      rmdir(MountFolder.c_str());

      // Remove the rootfs lock file
      std::string RootFSLockFile = FEXServerClient::GetServerRootFSLockFile();
      unlink(RootFSLockFile.c_str());
    }
  }

  bool InitializeSquashFS() {
    FEX_CONFIG_OPT(LDPath, ROOTFS);

    MountFolder = LDPath();
    if (!FEX::FormatCheck::IsSquashFS(MountFolder)) {
      // If this isn't a squashfs mount then we have nothing to do here
      return true;
    }

    if (!InitializeSquashFSPipe()) {
      LogMan::Msg::EFmt("[FEXServer] Couldn't initialize SquashFSPipe");
      return false;
    }

    // Setup rootfs here
    if (!MountSquashFSPath(LDPath())) {
      LogMan::Msg::EFmt("[FEXServer] Couldn't mount squashfs path");
      return false;
    }

    if (!DowngradeRootFSPipeToReadLock()) {
      LogMan::Msg::EFmt("[FEXServer] Couldn't downgrade read lock");
      return false;
    }

    return true;
  }

  std::string GetMountFolder() {
    return MountFolder;
  }
}