#!/usr/bin/perl -w
# Change the size of a GPT image's partition array.

# The vast majority of GPT partition tables have an 128-entry partition array.
# However, we get reports of ZFS-related arrays with a mere 9 entries, and
# some others with 140, and parted could not handle either of those because
# it was effectively hard-coding the 128.
# This script takes as input a GPT image that might be created by
# parted itself, and transforms it into one with a different number, N,
# of partition array entries.  That involves the following steps:
# - poke that value of N into the 4-byte "Number of partition entries" slot,
# - compute the crc32 of the N partition table entries,
# - poke the resulting value into its slot,
# - recompute the GPT header's CRC32 checksum and poke that into its slot.
# Do the above for both the primary and the backup GPT headers.

use strict;
use warnings;
use Digest::CRC qw(crc32);
use List::Util qw(max);

(my $ME = $0) =~ s|.*/||;
my $VERSION = '1.0';

# Technically we shouldn't hard-code this, since it's specified
# as the little-endian number in bytes 12..15 of the GPT header.
my $gpt_header_len = 92;

# Size of a partition array entry, in bytes.
# This too is specified in the GPT header, but AFAIK, no one changes it.
my $pe_size = 128;

# Sector size.
my $ss;

# Sector number of the backup GPT header, to be read from the primary header.
my $backup_LBA;

# Given a GPT header $B, extract the my_LBA/backup_LBA sector number.
sub curr_LBA($) { my ($b) = @_; unpack ('Q<', substr ($b, 24, 8)) }
sub backup_LBA($) { my ($b) = @_; unpack ('Q<', substr ($b, 32, 8)) }

# Given a GPT header $B, return its "partition entries starting LBA".
sub pe_start_LBA($) { my ($b) = @_; unpack ('Q<', substr ($b, 72, 8)) }

sub round_up_to_ss ($)
{
  my ($n) = @_;
  return $n + $ss - $n % $ss;
}

# Return the byte offset of the start of the specified partition array.
sub partition_array_start_offset ($$)
{
  my ($pri_or_backup, $n_pe) = @_;
  $pri_or_backup eq 'primary'
    and return 2 * $ss;

  # Backup
  return $backup_LBA * $ss - round_up_to_ss ($n_pe * $pe_size);
}

# Calculate and return the specified partition array crc32 checksum.
sub partition_array_crc ($$$)
{
  my ($pri_or_backup, $n_pe, $in) = @_;
  local *F;
  open F, '<', $in
    or die "$ME: failed to open $in: $!\n";

  # Seek to start of partition array.
  my $off = partition_array_start_offset $pri_or_backup, $n_pe;
  sysseek (F, $off, 0)
    or die "$ME: $in: failed to seek to $off: $!\n";

  # Read the array.
  my $p;
  my $pe_buf;
  my $n = $n_pe * $pe_size;
  ($p = sysread F, $pe_buf, $n) && $p == $n
    or die "$ME: $in: failed to read $pri_or_backup partition array:($p:$n) $!\n";

  return crc32 $pe_buf;
}

# Verify the initial CRC of BUF.
sub check_GPT_header ($$$)
{
  my ($pri_or_backup, $in, $buf) = @_;

  my $curr = curr_LBA $buf;
  my $backup = backup_LBA $buf;
  ($pri_or_backup eq 'primary') == ($curr == 1)
    or die "$ME: $in: invalid curr_LBA($curr) in $pri_or_backup header\n";
  ($pri_or_backup eq 'primary') == (34 < $backup)
    or die "$ME: $in: invalid backup_LBA($backup) in $pri_or_backup header\n";

  $pri_or_backup eq 'backup' && $backup != 1
    and die "$ME: $in: the backup_LBA in the backup header must be 1\n";

  # A primary partition's "partition entries starting LBA" must be 2.
  if ($pri_or_backup eq 'primary')
    {
      my $p = pe_start_LBA $buf;
      $p == 2
	or die "$ME: $in: primary header's PE start LBA is $p (should be 2)\n";
    }

  # Save a copy of the CRC, then zero that field, bytes 16..19:
  my $orig_crc = unpack ('L<', substr ($buf, 16, 4));
  substr ($buf, 16, 4) = "\0" x 4;

  # Compute CRC32 of header: it'd better match.
  my $crc = crc32($buf);
  $orig_crc == $crc
    or die "$ME: $in: cannot reproduce $pri_or_backup GPT header's CRC32\n";
}

# Poke the $N_PE value into $$BUF's number-of-partition-entries slot.
sub poke_n_pe ($$)
{
  my ($buf, $n_pe) = @_;

  # Poke the little-endian value into place.
  substr ($$buf, 80, 4) = pack ('L<', $n_pe);
}

# Compute/set partition-array CRC (given $N_PE), then compute a new
# header-CRC and poke it into its position, too.
sub set_CRCs ($$$$)
{
  my ($pri_or_backup, $buf, $in, $n_pe) = @_;

  # Compute CRC of primary partition array and put it in substr ($pri, 88, 4)
  my $pa_crc = partition_array_crc $pri_or_backup, $n_pe, $in;
  substr ($$buf, 88, 4) = pack ('L<', $pa_crc);

  # In the backup header, we must also set the 8-byte "Partition entries
  # starting LBA number" field to reflect our new value of $n_pe.
  if ($pri_or_backup eq 'backup')
    {
      my $off = partition_array_start_offset $pri_or_backup, $n_pe;
      $off % $ss == 0
	or die "$ME: internal error: starting LBA byte offset($off) is"
	  . " not a multiple of $ss\n";
      my $lba = $off / $ss;
      substr ($$buf, 72, 8) = pack ('Q<', $lba);
    }

  # Before we compute the checksum, we must zero-out the 4-byte
  # slot into which we'll store the result.
  substr ($$buf, 16, 4) = "\0" x 4;
  my $crc = crc32($$buf);
  substr ($$buf, 16, 4) = pack ('L<', $crc);
}

sub usage ($)
{
  my ($exit_code) = @_;
  my $STREAM = ($exit_code == 0 ? *STDOUT : *STDERR);
  if ($exit_code != 0)
    {
      print $STREAM "Try `$ME --help' for more information.\n";
    }
  else
    {
      print $STREAM <<EOF;
Usage: $ME [OPTIONS] FILE
Change the number of GPT partition array entries in FILE.

This option must be specified:

   --n-partition-array-entries=N  FIXME

The following are optional:

   --sector-size=N    assume sector size of N bytes (default: 512)
   --help             display this help and exit
   --version          output version information and exit

EXAMPLE:

  dd if=/dev/null of=F bs=1 seek=6MB
  parted -s F mklabel gpt
  $ME --n=9 F

EOF
    }
  exit $exit_code;
}

{
  my $n_partition_entries;

  use Getopt::Long;
  GetOptions
    (
     'n-partition-array-entries=i' => \$n_partition_entries,
     'sector-size=i' => \$ss,

     help => sub { usage 0 },
     version => sub { print "$ME version $VERSION\n"; exit },
    ) or usage 1;

  defined $n_partition_entries
    or (warn "$ME: --n-partition-array-entries=N not specified\n"), usage 1;

  defined $ss
    or $ss = 512;

  # Require sensible number:
  # It must either be <= 128, or else a multiple of 4 so that at 128 bytes each,
  # this array fully occupies a whole number of 512-byte sectors.
  1 <= $n_partition_entries
    && ($n_partition_entries <= 128
	|| $n_partition_entries % 4 == 0)
    or die "$ME: invalid number of partition entries: $n_partition_entries\n";

  @ARGV == 1
    or (warn "$ME: no file specified\n"), usage 1;

  my $in = $ARGV[0];
  local *F;
  open F, '<', $in
    or die "$ME: failed to open $in: $!\n";

  # Get length and perform some basic sanity checks.
  my $len = sysseek (F, 0, 2);
  defined $len
    or die "$ME: $in: failed to seek to EOF: $!\n";
  my $min_n_sectors = 34 + 33 + ($n_partition_entries * $pe_size + $ss - 1) / $ss;
  my $n_sectors = int (($len + $ss - 1) / $ss);
  $n_sectors < $min_n_sectors
    and die "$ME: $in: image file is too small to contain a GPT image\n";
  $len % $ss == 0
    or die "$ME: $in: size is not a multiple of $ss: $!\n";

  # Skip 1st sector.
  sysseek (F, $ss, 0)
    or die "$ME: $in: failed to seek to byte $ss: $!\n";

  # Read the primary GPT header.
  my $p;
  my $pri;
  ($p = sysread F, $pri, $gpt_header_len) && $p == $gpt_header_len
    or die "$ME: $in: failed to read the primary GPT header: $!\n";

  $backup_LBA = unpack ('Q<', substr ($pri, 32, 8));

  # Seek-to and read the backup GPT header.
  sysseek (F, $backup_LBA * $ss, 0)
    or die "$ME: $in: failed to seek to backup LBA $backup_LBA: $!\n";
  my $backup;
  ($p = sysread F, $backup, $gpt_header_len) && $p == $gpt_header_len
    or die "$ME: $in: read failed: $!\n";

  close F;

  check_GPT_header ('primary', $in, $pri);
  check_GPT_header ('backup',  $in, $backup);

  poke_n_pe (\$pri,    $n_partition_entries);
  poke_n_pe (\$backup, $n_partition_entries);

  # set both PE CRC and header CRCs:
  set_CRCs 'primary', \$pri,    $in, $n_partition_entries;
  set_CRCs 'backup',  \$backup, $in, $n_partition_entries;

  # Write both headers back to the file:
  open F, '+<', $in
    or die "$ME: failed to open $in: $!\n";

  sysseek (F, $ss, 0)
    or die "$ME: $in: failed to seek to byte $ss: $!\n";
  syswrite F, $pri
    or die "$ME: $in: failed to write primary header: $!\n";

  sysseek (F, $backup_LBA * $ss, 0)
    or die "$ME: $in: failed to seek to backup LBA $backup_LBA: $!\n";
  syswrite F, $backup
    or die "$ME: $in: failed to write backup header: $!\n";

  close F
    or die "$ME: failed to close $in: $!\n";
  }