Testing Apple File System Compression for RJVB/afsctool#62

main.c

#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/xattr.h>
#include <unistd.h>

const unsigned char rfork_data[] = {
    0x08,
    0x00,
    0x00,
    0x00,
    0x56,
    0x00,
    0x00,
    0x00,
    0x62,
    0x76,
    0x78,
    0x6e,
    0x4c,
    0x00,
    0x00,
    0x00,
    0x3e,
    0x00,
    0x00,
    0x00,
    0xe8,
    0x74,
    0x65,
    0x73,
    0x74,
    0x69,
    0x6e,
    0x67,
    0x20,
    0x08,
    0x08,
    0xe0,
    0x12,
    0x79,
    0x20,
    0x64,
    0x61,
    0x74,
    0x61,
    0x20,
    0x77,
    0x68,
    0x69,
    0x63,
    0x68,
    0x20,
    0x73,
    0x68,
    0x6f,
    0x75,
    0x6c,
    0x64,
    0x20,
    0x63,
    0x6f,
    0x6d,
    0x70,
    0x72,
    0x65,
    0x73,
    0x73,
    0x20,
    0x77,
    0x65,
    0x6c,
    0x6c,
    0x2c,
    0x10,
    0x27,
    0x38,
    0x05,
    0xfe,
    0xe1,
    0x74,
    0x06,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x62,
    0x76,
    0x78,
    0x24,
};

const unsigned char decmpfs_rfork_data[] = {
    0x66,
    0x70,
    0x6d,
    0x63,
    0x0c,
    0x00,
    0x00,
    0x00,
    0x4c,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
};

const unsigned char decmpfs_embedded_data[] = {
    0x66,
    0x70,
    0x6d,
    0x63,
    0x0b,
    0x00,
    0x00,
    0x00,
    0x4c,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x62,
    0x76,
    0x78,
    0x6e,
    0x4c,
    0x00,
    0x00,
    0x00,
    0x3e,
    0x00,
    0x00,
    0x00,
    0xe8,
    0x74,
    0x65,
    0x73,
    0x74,
    0x69,
    0x6e,
    0x67,
    0x20,
    0x08,
    0x08,
    0xe0,
    0x12,
    0x79,
    0x20,
    0x64,
    0x61,
    0x74,
    0x61,
    0x20,
    0x77,
    0x68,
    0x69,
    0x63,
    0x68,
    0x20,
    0x73,
    0x68,
    0x6f,
    0x75,
    0x6c,
    0x64,
    0x20,
    0x63,
    0x6f,
    0x6d,
    0x70,
    0x72,
    0x65,
    0x73,
    0x73,
    0x20,
    0x77,
    0x65,
    0x6c,
    0x6c,
    0x2c,
    0x10,
    0x27,
    0x38,
    0x05,
    0xfe,
    0xe1,
    0x74,
    0x06,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x00,
    0x62,
    0x76,
    0x78,
    0x24,
};

#define EXPECTED_BODY "testing testy data which should compress well, test test test test test test"
#define EXPECTED_BODY_LEN (sizeof(EXPECTED_BODY) - 1)

#define TEST_PATH "/tmp/test.txt"

void check_file_contains_body(int fd) {
  lseek(fd, 0, SEEK_SET);
  char buf[EXPECTED_BODY_LEN];

  ssize_t rc = read(fd, buf, sizeof(buf));
  assert(rc >= 0);
  bool matches = rc == EXPECTED_BODY_LEN && (memcmp(buf, EXPECTED_BODY, EXPECTED_BODY_LEN) == 0);
  printf("File contains expected body: %s\n", matches ? "TRUE" : "FALSE");
}

void do_compression(const char *path, bool skip_rfork_compression_finish) {
  printf("Removing %s\n", path);
  unlink(path);

  printf("Creating %s\n", path);
  int fd = open(path, O_CREAT | O_RDWR, 0644);
  assert(fd != -1);

  printf("Writing decmpfs xattr using rfork\n");
  int rc = fsetxattr(fd, "com.apple.decmpfs", decmpfs_rfork_data, sizeof(decmpfs_rfork_data), 0, XATTR_SHOWCOMPRESSION);
  assert(rc == 0);

  printf("Writing resource fork\n");
  rc = fsetxattr(fd, "com.apple.ResourceFork", rfork_data, sizeof(rfork_data), 0, 0);
  assert(rc == 0);

  if (skip_rfork_compression_finish) {
    printf("NOT setting compressed flag\n");
  } else {
    printf("Setting compressed flag\n");
    rc = fchflags(fd, UF_COMPRESSED);
    assert(rc == 0);
    // Uncommenting this causes the later compression to work?
#if 0
    check_file_contains_body(fd);
#endif

    // Performing ANY read seems to cause the later compression to work?
#if 0
    lseek(fd, 0, SEEK_SET);
    char buf[1];
    read(fd, buf, 1);
#endif

    // mmaping the file doesn't cause compression to work
    char *map_result = mmap(NULL, EXPECTED_BODY_LEN, PROT_READ, MAP_SHARED | MAP_NOCACHE, fd, 0);
    // But reading from the mmapped region DOES cause compression to work
#if 0
    bool equal = memcmp(map_result, EXPECTED_BODY, EXPECTED_BODY_LEN) == 0;
    printf("mmap equal: %s\n", equal ? "TRUE" : "FALSE");
#endif
    munmap(map_result, EXPECTED_BODY_LEN);

    printf("Unsetting compressed flag\n");
    rc = fchflags(fd, 0);
    assert(rc == 0);
  }

  printf("Removing resource fork\n");
  rc = fremovexattr(fd, "com.apple.ResourceFork", XATTR_SHOWCOMPRESSION);
  assert(rc == 0);

  printf("Removing decmpfs xattr\n");
  rc = fremovexattr(fd, "com.apple.decmpfs", XATTR_SHOWCOMPRESSION);
  assert(rc == 0);

  printf("Writing decmpfs xattr using embedded data (no rfork)\n");
  rc = fsetxattr(fd, "com.apple.decmpfs", decmpfs_embedded_data, sizeof(decmpfs_embedded_data), 0, XATTR_SHOWCOMPRESSION);
  assert(rc == 0);

  printf("Setting compressed flag\n");
  rc = fchflags(fd, UF_COMPRESSED);
  assert(rc == 0);
  check_file_contains_body(fd);
}

int main() {
  do_compression(TEST_PATH, false);
  // do_compression(TEST_PATH, true);

  return 0;
}