ICFS/src/sourcefs.c

/*
  ICFS: Interactively Controlled File System
  Copyright (C) 2024-2025  Fedir Kovalov

  This program can be distributed under the terms of the GNU GPLv2.
  See the file LICENSE.
*/

#define _GNU_SOURCE

#include "sourcefs.h"
#include <dirent.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/**
 * @brief Global handle for the source filesystem implementation
 *
 * Stores the root directory information to enable relative path operations.
 */
static struct source_files_handle {
  char *mountpoint; // Absolute path to the mounted filesystem root
  int root_fd;      // File descriptor for the root directory (O_PATH)
} handle;

/**
 * @brief Translate FUSE paths to real paths in the underlying filesystem
 *
 * FUSE passes paths starting with '/' (e.g., "/dir/file"), but we need to
 * operate relative to our root_fd. This function converts:
 * - "/" -> "." (current directory)
 * - "/dir/file" -> "dir/file"
 *
 * @param filename Path from FUSE (always starts with '/')
 * @return Relative path suitable for use with root_fd
 */
const char *source_filename_translate(const char *filename) {
  if (strcmp("/", filename) == 0) {
    return ".";
  }
  return filename + 1; // Skip leading slash
}

/**
 * @brief Initialize the source filesystem with a root directory
 *
 * Sets up the mount point and opens a protected file descriptor to the
 * root directory for safe relative operations.
 *
 * @param root_path Absolute path to the physical root directory
 * @return 0 on success, -1 on failure
 */
int source_init(const char *root_path) {
  // Allocate memory for mount point path
  handle.mountpoint = malloc(strlen(root_path) + 1);
  if (handle.mountpoint == NULL) {
    perror("[ICFS] Malloc failed");
    return -1;
  }

  strcpy(handle.mountpoint, root_path);

  // Open root directory with O_PATH to prevent accidental reads/writes
  // while maintaining a valid descriptor for relative operations
  int root_fd = open(root_path, O_PATH);
  if (root_fd == -1) {
    fprintf(stderr, "[ICFS] Could not initialize source file system at %s",
            root_path);
    perror("");
    return -1;
  }

  handle.root_fd = root_fd;
  return 0;
}

/**
 * @brief Clean up resources used by the source filesystem
 */
void source_destroy(void) {
  free(handle.mountpoint); // Free allocated mount point path
}

/**
 * @brief Get the current mount point path
 * @return Absolute path to the filesystem root
 */
const char *get_mountpoint(void) { return handle.mountpoint; }

/**
 * @brief Build a real path to the file.
 *
 * @param filename Relative path within the filesystem
 * @return Newly allocated absolute path (caller must free)
 */
const char *real_filename(const char *filename) {
  const char *mountpoint = get_mountpoint();
  size_t len1 = strlen(mountpoint);
  size_t len2 = strlen(filename);

  // Allocate space for combined path + null terminator
  char *result = malloc(len1 + len2 + 1);
  if (result == NULL) {
    fprintf(stderr, "[ICFS] Memory allocation failed");
    perror("");
    return NULL;
  }

  strcpy(result, mountpoint);
  strcat(result, filename);
  return result;
}

/**
 * @brief Create a directory in the filesystem
 *
 * Uses mkdirat() to safely create directories relative to root_fd,
 * preventing race conditions from concurrent path modifications.
 */
int source_mkdir(const char *filename, mode_t mode) {
  const char *relative_filename = source_filename_translate(filename);
  return mkdirat(handle.root_fd, relative_filename, mode);
}

/**
 * @brief Remove a file from the filesystem
 *
 * Uses unlinkat() to safely remove files relative to the root directory.
 */
int source_unlink(const char *filename) {
  const char *relative_filename = source_filename_translate(filename);
  return unlinkat(handle.root_fd, relative_filename, 0);
}

/**
 * @brief Get file status information
 *
 * Uses fstatat() to retrieve metadata relative to root_fd. Follows symlinks
 * by default (flags=0).
 */
int source_stat(const char *restrict filename, struct stat *restrict statbuf) {
  const char *relative_filename = source_filename_translate(filename);
  return fstatat(handle.root_fd, relative_filename, statbuf, 0);
}

/**
 * @brief Remove an empty directory
 *
 * Uses unlinkat() with AT_REMOVEDIR flag to safely remove directories.
 */
int source_rmdir(const char *filename) {
  const char *relative_filename = source_filename_translate(filename);
  return unlinkat(handle.root_fd, relative_filename, AT_REMOVEDIR);
}

/**
 * @brief Create a symbolic link
 *
 * Creates symlinks relative to the root_fd directory for safety.
 */
int source_symlink(const char *target, const char *linkpath) {
  const char *relative_linkpath = source_filename_translate(linkpath);
  return symlinkat(target, handle.root_fd, relative_linkpath);
}

/**
 * @brief Check file access permissions
 *
 * Uses faccessat() to check access rights relative to root_fd.
 */
int source_access(const char *filename, int mode) {
  const char *relative_filename = source_filename_translate(filename);
  return faccessat(handle.root_fd, relative_filename, mode, 0);
}

/**
 * @brief Open a directory for reading
 *
 * Combines openat() and fdopendir() to safely access directories relative
 * to the root_fd.
 */
DIR *source_opendir(const char *filename) {
  const char *relative_filename = source_filename_translate(filename);
  int fd = openat(handle.root_fd, relative_filename, 0);
  if (fd < 0) {
    perror("[ICFS] Openat failed");
    return NULL;
  }
  DIR *dir_pointer = fdopendir(fd);
  return dir_pointer;
}

/**
 * @brief Rename a file or directory
 *
 * Uses renameat() to safely rename within the same root_fd namespace.
 */
int source_rename(const char *oldpath, const char *newpath) {
  const char *relative_oldpath = source_filename_translate(oldpath);
  const char *relative_newpath = source_filename_translate(newpath);
  return renameat(handle.root_fd, relative_oldpath, handle.root_fd,
                  relative_newpath);
}

/**
 * @brief Create a hard link
 *
 * Uses linkat() with flags=0 (default behavior). May need AT_SYMLINK_NOFOLLOW
 * if symlink handling should be modified.
 */
int source_link(const char *oldpath, const char *newpath) {
  const char *relative_oldpath = source_filename_translate(oldpath);
  const char *relative_newpath = source_filename_translate(newpath);
  return linkat(handle.root_fd, relative_oldpath, handle.root_fd,
                relative_newpath, 0);
}

/**
 * @brief Change file access mode
 *
 * Uses fchmodat() with flags=0 (follow symlinks). Consider using
 * AT_SYMLINK_NOFOLLOW if symlink metadata should be modified directly.
 */
int source_chmod(const char *filename, mode_t mode) {
  const char *relative_filename = source_filename_translate(filename);
  return fchmodat(handle.root_fd, relative_filename, mode, 0);
}

/**
 * @brief Change file owner and group
 *
 * Uses fchownat() with AT_SYMLINK_NOFOLLOW to modify symlink metadata
 * rather than its target.
 */
int source_chown(const char *filename, uid_t owner, gid_t group) {
  const char *relative_filename = source_filename_translate(filename);
  return fchownat(handle.root_fd, filename, owner, group, AT_SYMLINK_NOFOLLOW);
}

/**
 * @brief Truncate a file to a specified length
 *
 * Opens the file with read-only access then truncates it. This may fail
 * if the file wasn't opened with write permissions. Consider changing
 * openat() flags to O_WRONLY for reliability.
 */
int source_truncate(const char *filename, off_t length) {
  const char *relative_filename = source_filename_translate(filename);
  int fd = openat(handle.root_fd, relative_filename, 0);
  if (fd < 0) {
    perror("[ICFS] Openat failed");
    return -1;
  }
  return ftruncate(fd, length);
}

/**
 * @brief Open a file with specified flags
 *
 * Uses openat() to safely access files relative to root_fd.
 */
int source_open(const char *filename, int flags) {
  const char *relative_filename = source_filename_translate(filename);
  return openat(handle.root_fd, relative_filename, flags);
}

/**
 * @brief Create and open a new file
 *
 * Uses openat() with O_CREAT to create files relative to root_fd.
 */
int source_create(const char *filename, int flags, mode_t mode) {
  const char *relative_filename = source_filename_translate(filename);
  return openat(handle.root_fd, relative_filename, flags | O_CREAT, mode);
}

int source_utimens(const char *filename, const struct timespec ts[2],
                   int flags) {
  const char *relative_filename = source_filename_translate(filename);
  return utimensat(handle.root_fd, relative_filename, ts, AT_SYMLINK_NOFOLLOW);
}