extlinux.c

linux内核

  if ( !ioctl(devfd, HDIO_GETGEO, &geo) ) {
    return 0;
  } else if ( !ioctl(devfd, FDGETPRM, &fd_str) ) {
    geo->heads     = fd_str.head;
    geo->sectors   = fd_str.sect;
    geo->cylinders = fd_str.track;
    geo->start     = 0;
    return 0;
  } 

  /* Didn't work.  Let's see if this is one of the standard geometries */
  for ( gp = standard_geometries ; gp->bytes ; gp++ ) {
    if ( gp->bytes == totalbytes ) {
      memcpy(geo, &gp->g, sizeof *geo);
      return 0;
    }
  }

  /* Didn't work either... assign a geometry of 64 heads, 32 sectors; this is
     what zipdisks use, so this would help if someone has a USB key that
     they're booting in USB-ZIP mode. */

  geo->heads     = opt.heads ?: 64;
  geo->sectors   = opt.sectors ?: 32;
  geo->cylinders = totalbytes/(geo->heads*geo->sectors << SECTOR_SHIFT);
  geo->start     = 0;

  if ( !opt.sectors && !opt.heads )
    fprintf(stderr, "Warning: unable to obtain device geometry (defaulting to %d heads, %d sectors)\n"
	    "         (on hard disks, this is usually harmless.)\n",
	    geo->heads, geo->sectors);

  return 1;
}

/*
 * Query the device geometry and put it into the boot sector.
 * Map the file and put the map in the boot sector and file.
 * Stick the "current directory" inode number into the file.
 */
void
patch_file_and_bootblock(int fd, int dirfd, int devfd)
{
  struct stat dirst;
  struct hd_geometry geo;
  uint32_t *sectp;
  uint64_t totalbytes, totalsectors;
  int nsect;
  unsigned char *p, *patcharea;
  int i, dw;
  uint32_t csum;

  if ( fstat(dirfd, &dirst) ) {
    perror("fstat dirfd");
    exit(255);			/* This should never happen */
  }

  totalbytes = get_size(devfd);
  get_geometry(devfd, totalbytes, &geo);

  if ( opt.heads )
    geo.heads = opt.heads;
  if ( opt.sectors )
    geo.sectors = opt.sectors;

  /* Patch this into a fake FAT superblock.  This isn't because
     FAT is a good format in any way, it's because it lets the
     early bootstrap share code with the FAT version. */
  dprintf("heads = %u, sect = %u\n", geo.heads, geo.sectors);

  totalsectors = totalbytes >> SECTOR_SHIFT;
  if ( totalsectors >= 65536 ) {
    set_16(boot_block+bsSectors, 0);
  } else {
    set_16(boot_block+bsSectors, totalsectors);
  }
  set_32(boot_block+bsHugeSectors, totalsectors);

  set_16(boot_block+bsBytesPerSec, SECTOR_SIZE);
  set_16(boot_block+bsSecPerTrack, geo.sectors);
  set_16(boot_block+bsHeads, geo.heads);
  set_32(boot_block+bsHiddenSecs, geo.start);

  /* Construct the boot file */

  dprintf("directory inode = %lu\n", (unsigned long) dirst.st_ino);
  nsect = (boot_image_len+SECTOR_SIZE-1) >> SECTOR_SHIFT;
  sectp = alloca(sizeof(uint32_t)*nsect);
  if ( sectmap(fd, sectp, nsect) ) {
    perror("bmap");
    exit(1);
  }

  /* First sector need pointer in boot sector */
  set_32(boot_block+0x1F8, *sectp++);
  nsect--;
  
  /* Search for LDLINUX_MAGIC to find the patch area */
  for ( p = boot_image ; get_32(p) != LDLINUX_MAGIC ; p += 4 );
  patcharea = p+8;
  
  /* Set up the totals */
  dw = boot_image_len >> 2; /* COMPLETE dwords! */
  set_16(patcharea, dw);
  set_16(patcharea+2, nsect);	/* Does not include the first sector! */
  set_32(patcharea+8, dirst.st_ino); /* "Current" directory */
  
  /* Set the sector pointers */
  p = patcharea+12;
  
  memset(p, 0, 64*4);
  while ( nsect-- ) {
    set_32(p, *sectp++);
    p += 4;
  }
  
  /* Now produce a checksum */
  set_32(patcharea+4, 0);

  csum = LDLINUX_MAGIC;
  for ( i = 0, p = boot_image ; i < dw ; i++, p += 4 )
    csum -= get_32(p);		/* Negative checksum */
  
  set_32(patcharea+4, csum);
}

/*
 * Install the boot block on the specified device.
 * Must be run AFTER install_file()!
 */
int
install_bootblock(int fd, const char *device)
{
  struct ext2_super_block sb;

  if ( xpread(fd, &sb, sizeof sb, EXT2_SUPER_OFFSET) != sizeof sb ) {
    perror("reading superblock");
    return 1;
  }

  if ( sb.s_magic != EXT2_SUPER_MAGIC ) {
    fprintf(stderr, "no ext2/ext3 superblock found on %s\n", device);
    return 1;
  }
  
  if ( xpwrite(fd, boot_block, boot_block_len, 0) != boot_block_len ) {
    perror("writing bootblock");
    return 1;
  }

  return 0;
}

int
install_file(const char *path, int devfd, struct stat *rst)
{
  char *file;
  int fd = -1, dirfd = -1, flags;
  struct stat st;

  asprintf(&file, "%s%sextlinux.sys",
	   path,
	   path[0] && path[strlen(path)-1] == '/' ? "" : "/");
  if ( !file ) {
    perror(program);
    return 1;
  }

  dirfd = open(path, O_RDONLY|O_DIRECTORY);
  if ( dirfd < 0 ) {
    perror(path);
    goto bail;
  }
    
  fd = open(file, O_RDONLY);
  if ( fd < 0 ) {
    if ( errno != ENOENT ) {
      perror(file);
      goto bail;
    }
  } else {
    /* If file exist, remove the immutable flag and set u+w mode */
    if ( !ioctl(fd, EXT2_IOC_GETFLAGS, &flags) ) {
      flags &= ~EXT2_IMMUTABLE_FL;
      ioctl(fd, EXT2_IOC_SETFLAGS, &flags);
    }
    if ( !fstat(fd, &st) ) {
      fchmod(fd, st.st_mode | S_IWUSR);
    }
  }
  close(fd);

  fd = open(file, O_WRONLY|O_TRUNC|O_CREAT, S_IRUSR|S_IRGRP|S_IROTH);
  if ( fd < 0 ) {
    perror(file);
    goto bail;
  }
  
  /* Write it the first time */
  if ( xpwrite(fd, boot_image, boot_image_len, 0) != boot_image_len ) {
    fprintf(stderr, "%s: write failure on %s\n", program, file);
    goto bail;
  }

  /* Map the file, and patch the initial sector accordingly */
  patch_file_and_bootblock(fd, dirfd, devfd);

  /* Write it again - this relies on the file being overwritten in place! */
  if ( xpwrite(fd, boot_image, boot_image_len, 0) != boot_image_len ) {
    fprintf(stderr, "%s: write failure on %s\n", program, file);
    goto bail;
  }

  /* Attempt to set immutable flag and remove all write access */
  /* Only set immutable flag if file is owned by root */
  if ( !fstat(fd, &st) ) {
    fchmod(fd, st.st_mode & (S_IRUSR|S_IRGRP|S_IROTH));
    if ( st.st_uid == 0 && !ioctl(fd, EXT2_IOC_GETFLAGS, &flags) ) {
      flags |= EXT2_IMMUTABLE_FL;
      ioctl(fd, EXT2_IOC_SETFLAGS, &flags);
    }
  }

  if ( fstat(fd, rst) ) {
    perror(file);
    goto bail;
  }

  close(dirfd);
  close(fd);
  return 0;

 bail:
  if ( dirfd >= 0 )
    close(dirfd);
  if ( fd >= 0 )
    close(fd);

  return 1;
}

/* EXTLINUX installs the string 'EXTLINUX' at offset 3 in the boot
   sector; this is consistent with FAT filesystems. */
int
already_installed(int devfd)
{
  char buffer[8];

  xpread(devfd, buffer, 8, 3);
  return !memcmp(buffer, "EXTLINUX", 8);
}

int
install_loader(const char *path, int update_only)
{
  struct stat st, dst, fst;
  struct mntent *mnt = NULL;
  int devfd, rv;
  FILE *mtab;
  const char *devname = NULL;

  if ( stat(path, &st) || !S_ISDIR(st.st_mode) ) {
    fprintf(stderr, "%s: Not a directory: %s\n", program, path);
    return 1;
  }
  
  devfd = -1;

  if ( (mtab = setmntent("/proc/mounts", "r")) ) {
    while ( (mnt = getmntent(mtab)) ) {
      if ( (!strcmp(mnt->mnt_type, "ext2") ||
	    !strcmp(mnt->mnt_type, "ext3")) &&
	   !stat(mnt->mnt_fsname, &dst) &&
	   dst.st_rdev == st.st_dev ) {
	devname = mnt->mnt_fsname;
	break;
      }
    }
  }

  if ( !devname ) {
    /* Didn't find it in /proc/mounts, try /etc/mtab */
    if ( (mtab = setmntent("/etc/mtab", "r")) ) {
      while ( (mnt = getmntent(mtab)) ) {
	devname = mnt->mnt_fsname;
	break;
      }
    }
  }

  if ( !devname ) {
    fprintf(stderr, "%s: cannot find device for path %s\n", program, path);
    return 1;
  }

  fprintf(stderr, "%s is device %s\n", path, devname);
  if ( (devfd = open(devname, O_RDWR|O_SYNC)) < 0 ) {
    fprintf(stderr, "%s: cannot open device %s\n", program, devname);
    return 1;
  }
  
  if ( update_only && !already_installed(devfd) ) {
    fprintf(stderr, "%s: no previous extlinux boot sector found\n", program);
    return 1;
  }
	    
  install_file(path, devfd, &fst);

  if ( fst.st_dev != st.st_dev ) {
    fprintf(stderr, "%s: file system changed under us - aborting!\n",
	    program);
    return 1;
  }

  sync();
  rv = install_bootblock(devfd, mnt->mnt_fsname);
  close(devfd);
  sync();

  endmntent(mtab);

  if ( rv ) return rv;

  return 0;
}

int
main(int argc, char *argv[])
{
  int o;
  const char *directory;
  int update_only = -1;

  program = argv[0];

  while ( (o = getopt_long(argc, argv, short_options,
			     long_options, NULL)) != EOF ) {
    switch ( o ) {
    case 'z':
      opt.heads = 64;
      opt.sectors = 32;
      break;
    case 'S':
      opt.sectors = strtoul(optarg, NULL, 0);
      if ( opt.sectors < 1 || opt.sectors > 63 ) {
	fprintf(stderr, "%s: invalid number of sectors: %u (must be 1-63)\n",
		program, opt.sectors);
	exit(EX_USAGE);
      }
      break;
    case 'H':
      opt.heads = strtoul(optarg, NULL, 0);
      if ( opt.heads < 1 || opt.heads > 256 ) {
	fprintf(stderr, "%s: invalid number of heads: %u (must be 1-256)\n",
		program, opt.heads);
	exit(EX_USAGE);
      }
      break;
    case 'i':
      update_only = 0;
      break;
    case 'u':
    case 'U':
      update_only = 1;
      break;
    case 'h':
      usage(0);
      break;
    case 'v':
      fputs("extlinux " VERSION "\n", stderr);
      exit(0);
    default:
      fprintf(stderr, "%s: Unknown option: %c\n", program, optopt);
      exit(EX_USAGE);
    }
  }

  directory = argv[optind];

  if ( !directory )
    usage(EX_USAGE);

  if ( update_only == -1 ) {
    fprintf(stderr, "%s: warning: a future version will require --install or --update\n",
	    program);
    update_only = 0;
  }

  return install_loader(directory, update_only);
}