libstorage/src/gpt.rs

// Copyright 2023, The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use crate::{BlockIo, Disk, Result};
use core::{
    array::from_fn,
    cmp::min,
    convert::TryFrom,
    default::Default,
    fmt::{Debug, Formatter},
    mem::size_of,
    num::NonZeroU64,
    ops::{Deref, DerefMut},
    str::from_utf8,
};
use crc32fast::Hasher;
use gbl_async::block_on;
use liberror::{Error, GptError};
use safemath::SafeNum;
use zerocopy::{AsBytes, ByteSlice, FromBytes, FromZeroes, Ref};

/// Number of bytes in GUID.
pub const GPT_GUID_LEN: usize = 16;
/// The maximum number of UTF-16 characters in a GPT partition name, including termination.
pub const GPT_NAME_LEN_U16: usize = 36;
const GPT_NAME_LEN_U8: usize = 2 * GPT_GUID_LEN;

/// The top-level GPT header.
#[repr(C, packed)]
#[derive(Debug, Default, Copy, Clone, AsBytes, FromBytes, FromZeroes, PartialEq, Eq)]
pub struct GptHeader {
    /// Magic bytes; must be [GPT_MAGIC].
    pub magic: u64,
    /// Header version.
    pub revision: u32,
    /// Header size in bytes.
    pub size: u32,
    /// CRC of the first `size` bytes, calculated with this field zeroed.
    pub crc32: u32,
    /// Reserved; must be set to 0.
    pub reserved0: u32,
    /// The on-disk block location of this header.
    pub current: u64,
    /// The on-disk block location of the other header.
    pub backup: u64,
    /// First usable block for partition contents.
    pub first: u64,
    /// Last usable block for partition contents (inclusive).
    pub last: u64,
    /// Disk GUID.
    pub guid: [u8; GPT_GUID_LEN],
    /// Starting block for the partition entries array.
    pub entries: u64,
    /// Number of partition entries.
    pub entries_count: u32,
    /// The size of each partition entry in bytes.
    pub entries_size: u32,
    /// CRC of the partition entries array.
    pub entries_crc: u32,
}

impl GptHeader {
    /// Casts a bytes slice into a mutable GptHeader structure.
    pub fn from_bytes_mut(bytes: &mut [u8]) -> &mut GptHeader {
        Ref::<_, GptHeader>::new_from_prefix(bytes).unwrap().0.into_mut()
    }

    /// Computes the actual crc32 value.
    fn calculate_header_crc(&self) -> u32 {
        let mut hasher = Hasher::new();
        hasher.update(&self.as_bytes()[..GPT_CRC32_OFFSET]);
        hasher.update(&[0u8; size_of::<u32>()]);
        hasher.update(&self.as_bytes()[GPT_CRC32_OFFSET + size_of::<u32>()..]);
        hasher.finalize()
    }

    /// Update the header crc32 value.
    pub fn update_crc(&mut self) {
        self.crc32 = self.calculate_header_crc();
    }

    /// Updates entries and header crc according to the given entries buffer.
    fn update_entries_crc(&mut self, entries: &[u8]) {
        let size = SafeNum::from(self.entries_count) * self.entries_size;
        self.entries_crc = crc32(&entries[..size.try_into().unwrap()]);
        self.update_crc();
    }
}

/// Computes the number of blocks for the 128 partition entries reserved space in GPT.
fn gpt_entries_blk(block_size: u64) -> Result<u64> {
    let size = u64::try_from(GPT_MAX_NUM_ENTRIES_SIZE).unwrap();
    match size % block_size {
        0 => Ok(size / block_size),
        _ => Err(Error::InvalidInput),
    }
}

/// Checks a header against a block device.
///
/// # Args
///
/// * `io`: An implementation of [BlockIo],
/// * `header`: The GPT header to verify.
/// * `is_primary`: If the header is a primary header.
fn check_header(io: &mut impl BlockIo, header: &GptHeader, is_primary: bool) -> Result<()> {
    let num_blks = SafeNum::from(io.info().num_blocks);
    let blk_sz = io.info().block_size;

    // GPT spec requires that at least 128 entries worth of space be reserved.
    let min_reserved_entries_blk = gpt_entries_blk(blk_sz)?;
    // Minimum space needed: 2 * (header + entries) + MBR.
    let min_disk_blks: u64 = ((min_reserved_entries_blk + 1) * 2 + 1).try_into().unwrap();
    if min_disk_blks > u64::try_from(num_blks).unwrap() {
        return Err(Error::GptError(GptError::DiskTooSmall));
    }

    if header.magic != GPT_MAGIC {
        return Err(Error::GptError(GptError::IncorrectMagic(header.magic)));
    }

    if header.calculate_header_crc() != header.crc32 {
        return Err(Error::GptError(GptError::IncorrectHeaderCrc));
    }

    if header.size != size_of::<GptHeader>().try_into().unwrap() {
        return Err(Error::GptError(GptError::UnexpectedHeaderSize {
            actual: header.size,
            expect: size_of::<GptHeader>(),
        }));
    }

    if header.entries_size != size_of::<GptEntry>().try_into().unwrap() {
        return Err(Error::GptError(GptError::UnexpectedEntrySize {
            actual: header.entries_size,
            expect: size_of::<GptEntry>(),
        }));
    }

    // Checks first/last usable block.
    //
    // Assuming maximum range where partition entries are adjacent to GPT headers.
    //
    // Should leave a minimum space for MBR + primary header + primary entries before.
    let min_first: u64 = (min_reserved_entries_blk + 2).try_into().unwrap();
    // Should leave a minimum space for secondary header + secondary entries space after.
    let max_last: u64 = (num_blks - 1 - min_reserved_entries_blk - 1).try_into().unwrap();
    if header.first > header.last + 1 || header.first < min_first || header.last > max_last {
        return Err(Error::GptError(GptError::InvalidFirstLastUsableBlock {
            first: header.first,
            last: header.last,
            range: (min_first, max_last),
        }));
    }

    // Checks entries starting block.
    if is_primary {
        // For primary header, entries must be before first usable block and can hold up to
        // `GPT_MAX_NUM_ENTRIES` entries
        let right: u64 =
            (SafeNum::from(header.first) - min_reserved_entries_blk).try_into().unwrap();
        if !(header.entries >= 2 && header.entries <= right) {
            return Err(Error::GptError(GptError::InvalidPrimaryEntriesStart {
                value: header.entries,
                expect_range: (2, right),
            }));
        }
    } else {
        // For secondary header, entries must be after last usable block and can hold up to
        // `GPT_MAX_NUM_ENTRIES` entries.
        if !(header.entries > header.last && header.entries <= max_last + 1) {
            return Err(Error::GptError(GptError::InvalidSecondaryEntriesStart {
                value: header.entries,
                expect_range: (header.last + 1, max_last + 1),
            }));
        }
    }

    if header.entries_count > GPT_MAX_NUM_ENTRIES.try_into().unwrap() {
        return Err(Error::GptError(GptError::NumberOfEntriesOverflow {
            entries: header.entries_count,
            max_allowed: GPT_MAX_NUM_ENTRIES,
        }));
    }

    Ok(())
}

/// Verifies the given entries against a verifed GPT header.
///
/// # Args
///
/// * `header`: The verified GPT header corresponding to the entries.
/// * `entries`: The buffer containing the entries.
fn check_entries(header: &GptHeader, entries: &[u8]) -> Result<()> {
    // Checks entries CRC.
    assert!(header.entries_count <= GPT_MAX_NUM_ENTRIES.try_into().unwrap());
    let entries_size: usize =
        (SafeNum::from(header.entries_count) * GPT_ENTRY_SIZE).try_into().unwrap();
    let entries = entries.get(..entries_size).ok_or(Error::GptError(GptError::EntriesTruncated))?;
    if header.entries_crc != crc32(entries) {
        return Err(Error::GptError(GptError::IncorrectEntriesCrc));
    }

    // Checks each entry.
    let entries = Ref::<_, [GptEntry]>::new_slice(entries)
        .ok_or(Error::GptError(GptError::EntriesTruncated))?
        .into_slice();
    let entries = &entries[..header.entries_count.try_into().unwrap()];
    for (idx, ele) in entries.iter().take_while(|v| !v.is_null()).enumerate() {
        // Error information uses 1-base partition index.
        let idx = idx.checked_add(1).unwrap();
        let (first, last) = (ele.first, ele.last);
        if first > last + 1 || last > header.last || first < header.first {
            return Err(Error::GptError(GptError::InvalidPartitionRange {
                idx,
                part_range: (first, last),
                usable_range: (header.first, header.last),
            }));
        } else if ele.part_type == [0u8; GPT_GUID_LEN] {
            return Err(Error::GptError(GptError::ZeroPartitionTypeGUID { idx }));
        } else if ele.guid == [0u8; GPT_GUID_LEN] {
            return Err(Error::GptError(GptError::ZeroPartitionUniqueGUID { idx }));
        }
    }

    // Checks overlap between partition ranges.
    // Sorts an index array because we don't want to modify input.
    let mut sorted_indices: [u8; GPT_MAX_NUM_ENTRIES] = from_fn(|i| i.try_into().unwrap());
    sorted_indices.sort_unstable_by_key(|v| match entries.get(usize::try_from(*v).unwrap()) {
        Some(v) if !v.is_null() => v.first,
        _ => u64::MAX,
    });

    let actual = entries.iter().position(|v| v.is_null()).unwrap_or(entries.len());
    if actual > 1 {
        for i in 0..actual - 1 {
            let prev: usize = sorted_indices[i].try_into().unwrap();
            let next: usize = sorted_indices[i + 1].try_into().unwrap();
            if entries[prev].last >= entries[next].first {
                return Err(Error::GptError(GptError::PartitionRangeOverlap {
                    prev: (prev + 1, entries[prev].first, entries[prev].last),
                    next: (next + 1, entries[next].first, entries[next].last),
                }));
            }
        }
    }

    Ok(())
}

/// GptEntry is the partition entry data structure in the GPT.
#[repr(C, packed)]
#[derive(Debug, Copy, Clone, AsBytes, FromBytes, FromZeroes, PartialEq)]
pub struct GptEntry {
    /// Partition type GUID.
    pub part_type: [u8; GPT_GUID_LEN],
    /// Unique partition GUID.
    pub guid: [u8; GPT_GUID_LEN],
    /// First block.
    pub first: u64,
    /// Last block (inclusive).
    pub last: u64,
    /// Partition flags.
    pub flags: u64,
    /// Partition name in UTF-16.
    pub name: [u16; GPT_NAME_LEN_U16],
}

impl GptEntry {
    /// Return the partition entry size in blocks.
    pub fn blocks(&self) -> Result<u64> {
        // Must perform "+1" first before subtracting `self.first`. Otherwise if partition size is
        // zero, where `self.first > self.last`, arithmetic will overflow.
        u64::try_from(SafeNum::from(self.last) + 1 - self.first).map_err(Into::into)
    }

    /// Return whether this is a `NULL` entry. The first null entry marks the end of the partition
    /// entries.
    fn is_null(&self) -> bool {
        self.first == 0 && self.last == 0
    }

    /// Decode the partition name into a string. A length N utf16 string can be at most 2N utf8
    /// bytes. Therefore, a safe size of `buffer` is 2*GPT_NAME_LEN_U16 = 72.
    pub fn name_to_str<'a>(&self, buffer: &'a mut [u8]) -> Result<&'a str> {
        let mut index = 0;
        for c in char::decode_utf16(self.name) {
            match c.unwrap_or(char::REPLACEMENT_CHARACTER) {
                '\0' => break,
                c if c.len_utf8() <= buffer[index..].len() => {
                    index += c.encode_utf8(&mut buffer[index..]).len()
                }
                _ => return Err(Error::InvalidInput), // Not enough space in `buffer`.
            }
        }
        // SAFETY:
        // _unchecked should be OK here since we wrote each utf8 byte ourselves,
        // but it's just an optimization, checked version would be fine also.
        unsafe { Ok(core::str::from_utf8_unchecked(&buffer[..index])) }
    }

    /// Checks if the partition name is the same as the given.
    pub fn match_name(&self, part: &str) -> Result<bool> {
        Ok(self.name_to_str(&mut [0u8; GPT_NAME_LEN_U16 * 2][..])? == part)
    }
}

impl core::fmt::Display for GptEntry {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        // Format: partition name: "abc", [first, last]: [123, 456]
        let mut name_conversion_buffer = [0u8; GPT_NAME_LEN_U16 * 2];
        let name = self.name_to_str(&mut name_conversion_buffer).map_err(|_| core::fmt::Error)?;
        // Note: The bracket around `{ self.first }` is for forcing a copy of the field because
        // GptEntry is a packed structure.
        write!(f, "partition: \"{}\", first: {}, last: {}", name, { self.first }, { self.last })
    }
}

// core::mem::offset_of!(GptHeader, crc32) is unsatble feature and rejected by the compiler in our
// settings. We pre-compute the value here.
const GPT_CRC32_OFFSET: usize = 16;
const GPT_ENTRY_SIZE: usize = size_of::<GptEntry>();
const GPT_MAX_NUM_ENTRIES: usize = 128;
const GPT_MAX_NUM_ENTRIES_SIZE: usize = GPT_MAX_NUM_ENTRIES * GPT_ENTRY_SIZE;
/// GPT header magic bytes ("EFI PART" in ASCII).
pub const GPT_MAGIC: u64 = 0x5452415020494645;

enum HeaderType {
    Primary,
    Secondary,
}

/// `Partition` contains information about a GPT partition.
#[derive(Debug, Copy, Clone, PartialEq)]
pub struct Partition {
    entry: GptEntry,
    block_size: u64,
    decoded_name: Option<([u8; GPT_NAME_LEN_U8], usize)>,
}

impl Partition {
    /// Creates a new instance.
    fn new(entry: GptEntry, block_size: u64) -> Self {
        let mut buf = [0u8; GPT_NAME_LEN_U8];
        let decoded_name = match entry.name_to_str(&mut buf[..]).ok().map(|v| v.len()) {
            Some(len) => Some((buf, len)),
            _ => None,
        };
        Self { entry, block_size, decoded_name }
    }

    /// Gets the decoded partition name.
    pub fn name(&self) -> Option<&str> {
        // Correct by construction. `from_utf8` should not fail.
        self.decoded_name.as_ref().map(|(buf, sz)| from_utf8(&buf[..*sz]).unwrap())
    }

    /// Returns the partition size in bytes.
    pub fn size(&self) -> Result<u64> {
        u64::try_from(SafeNum::from(self.entry.blocks()?) * self.block_size).map_err(Error::from)
    }

    /// Returns the block size of this partition.
    pub fn block_size(&self) -> u64 {
        self.block_size
    }

    /// Returns the partition entry structure in the GPT header.
    pub fn gpt_entry(&self) -> &GptEntry {
        &self.entry
    }

    /// Returns the partition's absolute start/end offset in number of bytes.
    pub fn absolute_range(&self) -> Result<(u64, u64)> {
        let start = SafeNum::from(self.entry.first) * self.block_size;
        let end = (SafeNum::from(self.entry.last) + 1) * self.block_size;
        Ok((start.try_into()?, end.try_into()?))
    }

    /// Checks a given sub range and returns its absolute offset.
    pub fn check_range(&self, off: u64, size: u64) -> Result<u64> {
        let off = SafeNum::from(off);
        let end: u64 = (off + size).try_into()?;
        match end > self.size()? {
            true => Err(Error::BadIndex(end as usize)),
            _ => Ok((off + self.absolute_range()?.0).try_into()?),
        }
    }
}

/// `PartitionIterator` iterates all GPT partition entries.
pub struct PartitionIterator<'a> {
    entries: &'a [GptEntry],
    block_size: u64,
    idx: usize,
}

impl Iterator for PartitionIterator<'_> {
    type Item = Partition;

    fn next(&mut self) -> Option<Self::Item> {
        let res = self
            .entries
            .get(self.idx)
            .filter(|v| !v.is_null())
            .map(|v| Partition::new(*v, self.block_size))?;
        self.idx += 1;
        Some(res)
    }
}

/// Contains result of GPT syncing/restoration.
#[derive(Copy, Clone, PartialEq, Debug, Default)]
pub enum GptSyncResult {
    /// Both primary and secondary GPT are valid.
    #[default]
    BothValid,
    /// Primary GPT is invalid and restored.
    PrimaryRestored(Error),
    /// Secondary GPT is invalid and restored.
    SecondaryRestored(Error),
    /// Neither primary or secondary GPT is valid.
    NoValidGpt {
        /// Primary GPT verify error.
        primary: Error,
        /// Secondary GPT verify error.
        secondary: Error,
    },
}

impl GptSyncResult {
    /// Combined into a result
    pub fn res(&self) -> Result<()> {
        match self {
            Self::NoValidGpt { primary: e, .. } => Err(*e),
            _ => Ok(()),
        }
    }
}

impl core::fmt::Display for GptSyncResult {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::BothValid => write!(f, "Found valid GPT."),
            Self::PrimaryRestored(e) => write!(f, "Primary GPT restored due to {e:?}."),
            Self::SecondaryRestored(e) => write!(f, "Secondary GPT restored due to {e:?}."),
            Self::NoValidGpt { primary, secondary } => {
                write!(f, "No valid GPT. primary: {primary:?}, secondary: {secondary:?}.")
            }
        }
    }
}

/// A packed wrapper of `Option<NonZeroU64>`
#[repr(C, packed)]
#[derive(Debug, Copy, Clone, AsBytes, FromBytes, FromZeroes)]
struct BlockSize(Option<NonZeroU64>);

/// Represents the structure of a load buffer for loading/verifying/syncing up to N GPT entries.
#[repr(C, packed)]
#[derive(Debug, Copy, Clone, AsBytes, FromBytes, FromZeroes)]
pub struct GptLoadBufferN<const N: usize> {
    // GPT doesn't care about block size. But it's easier to have it available for computing offset
    // and size in bytes for partitions. It's also used as a flag for indicating whether a valid
    // GPT is loaded.
    block_size: BlockSize,
    primary_header: GptHeader,
    secondary_header: GptHeader,
    primary_entries: [GptEntry; N],
    secondary_entries: [GptEntry; N],
}

impl<const N: usize> Deref for GptLoadBufferN<N> {
    type Target = [u8];

    fn deref(&self) -> &Self::Target {
        self.as_bytes()
    }
}

impl<const N: usize> DerefMut for GptLoadBufferN<N> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.as_bytes_mut()
    }
}

/// Contains references corresponding to different GPT load entities parsed from a load buffer.
///
/// The structure is simply for organizing together the individual references of fields in
/// `GptLoadBufferN` parsed from a raw buffer. Note that we can't parse a `Ref<B, GptLoadBufferN>`
/// directly from a buffer because the number of entries (length of [GptEntry]) in this case needs
/// to be computed at run time based on the buffer size.
struct LoadBufferRef<B: ByteSlice> {
    block_size: Ref<B, BlockSize>,
    primary_header: Ref<B, GptHeader>,
    secondary_header: Ref<B, GptHeader>,
    primary_entries: Ref<B, [GptEntry]>,
    secondary_entries: Ref<B, [GptEntry]>,
}

impl<B: ByteSlice> LoadBufferRef<B> {
    fn from(buffer: B) -> Self {
        let n = min(GPT_MAX_NUM_ENTRIES, max_supported_entries(&buffer[..]).unwrap());
        let (block_size, rest) = Ref::new_from_prefix(buffer).unwrap();
        let (primary_header, rest) = Ref::new_from_prefix(rest).unwrap();
        let (secondary_header, rest) = Ref::new_from_prefix(rest).unwrap();
        let (primary_entries, rest) = Ref::new_slice_from_prefix(rest, n).unwrap();
        let (secondary_entries, _) = Ref::new_slice_from_prefix(rest, n).unwrap();
        Self { block_size, primary_header, secondary_header, primary_entries, secondary_entries }
    }

    /// Unpacks into the secondary GPT header/entries
    fn secondary(self) -> (Ref<B, GptHeader>, Ref<B, [GptEntry]>) {
        (self.secondary_header, self.secondary_entries)
    }
}

/// The minimum buffer size needed for creating a [Gpt] that can load `entries` number of
/// partitions.
///
/// # Returns
///
/// * Returns Ok(size) on success.
/// * Returns Err(Error::InvalidInput) if max_entries is greater than 128.
pub fn gpt_buffer_size(entries: usize) -> Result<usize> {
    match entries > GPT_MAX_NUM_ENTRIES {
        true => Err(Error::InvalidInput),
        _ => Ok(size_of::<GptLoadBufferN<0>>() + entries * GPT_ENTRY_SIZE * 2),
    }
}

/// Computes the maximum number of entries that can be loaded if using the given buffer for [Gpt].
fn max_supported_entries(buf: &[u8]) -> Result<usize> {
    match buf.len() < size_of::<GptLoadBufferN<0>>() {
        true => Err(Error::BufferTooSmall(Some(size_of::<GptLoadBufferN<0>>()))),
        _ => Ok((buf.len() - size_of::<GptLoadBufferN<0>>()) / 2 / GPT_ENTRY_SIZE),
    }
}

/// [Gpt] manages a buffer for loading, verifying and syncing GPT.
pub struct Gpt<B> {
    buffer: B,
}

impl<B: DerefMut<Target = [u8]>> Gpt<B> {
    /// Create an uninitialized Gpt instance from a provided buffer.
    ///
    /// The created [Gpt] can then be used in `Disk::sync_gpt()` for loading, verifying and syncing
    /// GPT on disk.
    ///
    /// # Args:
    ///
    /// * `buffer`: A buffer to use for loading, verifying and syncing primary and secondary GPT.
    ///   The size of the buffer determines the maximum number of partition entries that can be
    ///   loaded. If actual number of partitions, specified by `entries_count` in the GPT header,
    ///   exceeds it, verification and sync will eventually fail with `Error::BufferTooSmall`.
    ///   `gpt_buffer_size(num_entries)` can be used to compute the required size of buffer for
    ///   loading a specific number of entries. Note that most tools and OS fix the `entries_count`
    ///   value to the max 128 regardless of the actual number of partition entries used. Thus
    ///   unless you have full control of GPT generation in your entire system where you can always
    ///   ensure a smaller bound on it, it is recommended to always provide enough buffer for
    ///   loading 128 entries.
    ///
    /// # Returns
    ///
    /// * Returns Ok(Self) on success.
    /// * Returns Err(Error::BufferTooSmall) if buffer is less than the minimum size.
    pub fn new(mut buffer: B) -> Result<Self> {
        max_supported_entries(&buffer[..])?;
        LoadBufferRef::from(&mut buffer[..]).block_size.0 = None;
        Ok(Self { buffer })
    }

    /// Returns the maximum allowed entries.
    pub fn max_entries(&self) -> usize {
        max_supported_entries(&self.buffer[..]).unwrap()
    }

    /// Creates an instance of `Gpt<&mut [u8]>` that borrows the internal GPT buffer.
    pub fn as_borrowed(&mut self) -> Gpt<&mut [u8]> {
        Gpt { buffer: &mut self.buffer[..] }
    }

    /// Returns an iterator to GPT partition entries.
    ///
    /// If the object does not contain a valid GPT, the method returns Error.
    pub fn partition_iter(&self) -> Result<PartitionIterator> {
        let block_size = self.check_valid()?;
        let entries = LoadBufferRef::from(&self.buffer[..]).primary_entries.into_slice();
        Ok(PartitionIterator { entries, idx: 0, block_size })
    }

    /// Checks if a read/write range into a GPT partition overflows and returns the range's absolute
    /// offset in number of bytes.
    pub fn check_range(&self, part_name: &str, offset: u64, size: usize) -> Result<u64> {
        self.find_partition(part_name)?.check_range(offset, u64::try_from(size)?)
    }

    /// Return the list of GPT entries.
    ///
    /// If there is not a valid GPT, the method returns Error.
    pub fn entries(&self) -> Result<&[GptEntry]> {
        self.check_valid()?;
        let entries = LoadBufferRef::from(&self.buffer[..]).primary_entries.into_slice();
        let n = entries.iter().position(|v| v.is_null()).unwrap_or(entries.len());
        Ok(&entries[..n])
    }

    /// Returns the total number of partitions.
    pub fn num_partitions(&self) -> Result<usize> {
        Ok(self.entries()?.len())
    }

    /// Gets the `idx`th partition.
    pub fn get_partition(&self, idx: usize) -> Result<Partition> {
        let block_size = self.check_valid()?;
        let entry = *self.entries()?.get(idx).ok_or(Error::BadIndex(idx))?;
        Ok(Partition::new(entry, block_size))
    }

    /// Returns the `Partition` for a partition.
    ///
    /// # Args
    ///
    /// * `part`: Name of the partition.
    pub fn find_partition(&self, part: &str) -> Result<Partition> {
        let block_size = self.check_valid()?;
        for entry in self.entries()? {
            let mut name_conversion_buffer = [0u8; GPT_NAME_LEN_U16 * 2];
            if entry.name_to_str(&mut name_conversion_buffer)? != part {
                continue;
            }
            return Ok(Partition::new(*entry, block_size));
        }
        Err(Error::NotFound)
    }

    /// Checks whether the Gpt has been initialized and returns the block size.
    fn check_valid(&self) -> Result<u64> {
        Ok(LoadBufferRef::from(&self.buffer[..]).block_size.0.ok_or(Error::InvalidState)?.get())
    }

    /// Helper function for loading and validating GPT header and entries.
    async fn load_and_validate_gpt(
        &mut self,
        disk: &mut Disk<impl BlockIo, impl DerefMut<Target = [u8]>>,
        hdr_type: HeaderType,
    ) -> Result<()> {
        let blk_sz = disk.io().info().block_size;
        let load = LoadBufferRef::from(&mut self.buffer[..]);
        let (header_start, mut header, mut entries) = match hdr_type {
            HeaderType::Primary => (blk_sz, load.primary_header, load.primary_entries),
            HeaderType::Secondary => (
                ((SafeNum::from(disk.io().info().num_blocks) - 1) * blk_sz).try_into()?,
                load.secondary_header,
                load.secondary_entries,
            ),
        };

        // Loads the header
        disk.read(header_start, header.as_bytes_mut()).await?;
        // Checks header.
        check_header(disk.io(), &header, matches!(hdr_type, HeaderType::Primary))?;
        // Loads the entries.
        let entries_size = SafeNum::from(header.entries_count) * GPT_ENTRY_SIZE;
        let entries_offset = SafeNum::from(header.entries) * blk_sz;
        let out = entries.as_bytes_mut().get_mut(..entries_size.try_into().unwrap()).ok_or(
            Error::BufferTooSmall(Some(
                gpt_buffer_size(header.entries_count.try_into().unwrap()).unwrap(),
            )),
        )?;
        disk.read(entries_offset.try_into().unwrap(), out).await?;
        // Checks entries.
        check_entries(&header, entries.as_bytes())
    }

    /// Loads and syncs GPT from a block device.
    ///
    /// * Returns Ok(sync_result) if disk IO is successful, where `sync_result` contains the GPT
    ///   verification and restoration result,
    /// * Returns Err() if disk IO encounters error.
    pub(crate) async fn load_and_sync(
        &mut self,
        disk: &mut Disk<impl BlockIo, impl DerefMut<Target = [u8]>>,
    ) -> Result<GptSyncResult> {
        let blk_sz = disk.io().info().block_size;
        let nonzero_blk_sz = NonZeroU64::new(blk_sz).ok_or(Error::InvalidInput)?;
        let total_blocks: SafeNum = disk.io().info().num_blocks.into();

        let primary_header_blk = 1;
        let primary_header_pos = blk_sz;
        let secondary_header_blk = total_blocks - 1;

        // Entries position for restoring.
        let primary_entries_blk = 2;
        let primary_entries_pos = SafeNum::from(primary_entries_blk) * blk_sz;
        let primary_res = self.load_and_validate_gpt(disk, HeaderType::Primary).await;
        let secondary_res = self.load_and_validate_gpt(disk, HeaderType::Secondary).await;

        let LoadBufferRef {
            mut block_size,
            mut primary_header,
            mut secondary_header,
            mut primary_entries,
            mut secondary_entries,
        } = LoadBufferRef::from(&mut self.buffer[..]);
        block_size.0 = None;
        let primary_entries = primary_entries.as_bytes_mut();
        let secondary_entries = secondary_entries.as_bytes_mut();
        let sync_res = match (primary_res, secondary_res) {
            (Err(primary), Err(secondary)) => GptSyncResult::NoValidGpt { primary, secondary },
            (Ok(()), Ok(())) if is_consistent(&primary_header, &secondary_header) => {
                GptSyncResult::BothValid
            }
            (Err(e), Ok(())) => {
                // Restores to primary
                primary_header.as_bytes_mut().clone_from_slice(secondary_header.as_bytes());
                primary_entries.clone_from_slice(&secondary_entries);
                primary_header.current = primary_header_blk;
                primary_header.backup = secondary_header_blk.try_into()?;
                primary_header.entries = primary_entries_blk;
                primary_header.update_crc();

                disk.write(primary_header_pos, primary_header.as_bytes_mut()).await?;
                disk.write(primary_entries_pos.try_into()?, primary_entries).await?;
                GptSyncResult::PrimaryRestored(e)
            }
            (Ok(()), v) => {
                // Restores to secondary
                let pos = secondary_header_blk * blk_sz;
                let secondary_entries_pos = pos - GPT_MAX_NUM_ENTRIES_SIZE;
                let secondary_entries_blk = secondary_entries_pos / blk_sz;

                secondary_header.as_bytes_mut().clone_from_slice(primary_header.as_bytes());
                secondary_entries.clone_from_slice(primary_entries);
                secondary_header.current = secondary_header_blk.try_into()?;
                secondary_header.backup = primary_header_blk;
                secondary_header.entries = secondary_entries_blk.try_into()?;
                secondary_header.update_crc();

                disk.write(pos.try_into()?, secondary_header.as_bytes_mut()).await?;
                disk.write(secondary_entries_pos.try_into()?, secondary_entries).await?;

                GptSyncResult::SecondaryRestored(match v {
                    Err(e) => e,
                    _ => Error::GptError(GptError::DifferentFromPrimary),
                })
            }
        };

        block_size.0 = Some(nonzero_blk_sz);
        Ok(sync_res)
    }
}

/// Checks whether primary and secondary header
fn is_consistent(primary: &GptHeader, secondary: &GptHeader) -> bool {
    let mut expected_secondary = *primary;
    expected_secondary.crc32 = secondary.crc32;
    expected_secondary.current = secondary.current;
    expected_secondary.backup = 1;
    expected_secondary.entries = secondary.entries;
    &expected_secondary == secondary
}

/// A [Gpt] that owns a `GptLoadBufferN<N>` and can load up to N partition entries.
///
/// Note: The size of this type increases with N and can be expensive to store on stack. It is
/// typically intended for resource abundant environment such as test.
pub type GptN<const N: usize> = Gpt<GptLoadBufferN<N>>;

/// Creates an instance of GptN.
pub fn new_gpt_n<const N: usize>() -> GptN<N> {
    Gpt::new(GptLoadBufferN::<N>::new_zeroed()).unwrap()
}

/// A [Gpt] that owns a `GptLoadBufferN<128>` and can load the maximum 128 partition entries.
///
/// Note: The size of this type is approximately 34K and can be expensive to store on stack. It
/// is typically intended for resource abundant environment such as test.
pub type GptMax = GptN<GPT_MAX_NUM_ENTRIES>;

/// Creates an instance of GptMax.
pub fn new_gpt_max() -> GptMax {
    new_gpt_n::<GPT_MAX_NUM_ENTRIES>()
}

/// Updates GPT on a block device.
///
/// # Args
///
/// * `io`: An implementation of [BlockIo]
/// * `scratch`: Scratch buffer for unaligned read write.
/// * `mbr_primary`: A buffer containing the MBR block, primary GPT header and entries.
/// * `resize`: If set to true, the method updates the last partition to cover the rest of the
///    storage.
/// * `gpt`: The output [Gpt] to update.
pub(crate) async fn update_gpt(
    disk: &mut Disk<impl BlockIo, impl DerefMut<Target = [u8]>>,
    mbr_primary: &mut [u8],
    resize: bool,
    gpt: &mut Gpt<impl DerefMut<Target = [u8]>>,
) -> Result<()> {
    let blk_sz: usize = disk.io().info().block_size.try_into()?;
    let (header, remain) = mbr_primary
        .get_mut(blk_sz..)
        .map(|v| v.split_at_mut_checked(blk_sz))
        .flatten()
        .ok_or(Error::BufferTooSmall(Some(blk_sz * 2)))?;
    let header = Ref::<_, GptHeader>::new_from_prefix(&mut header[..]).unwrap().0.into_mut();

    // Adjusts last usable block according to this device in case the GPT was generated for a
    // different disk size. If this results in some partition being out of range, it will be
    // caught during `check_header()`.
    let entries_blk = SafeNum::from(GPT_MAX_NUM_ENTRIES_SIZE) / blk_sz;
    // Reserves only secondary GPT header and entries.
    let num_blks = SafeNum::from(disk.io().info().num_blocks);
    header.last = (num_blks - entries_blk - 2).try_into().unwrap();
    header.backup = (num_blks - 1).try_into().unwrap();
    header.update_crc();

    check_header(disk.io(), &header, true)?;
    // Computes entries offset in bytes relative to `remain`
    let entries_off: usize = ((SafeNum::from(header.entries) - 2) * blk_sz).try_into().unwrap();
    let entries_size: usize =
        (SafeNum::from(header.entries_count) * header.entries_size).try_into().unwrap();
    let entries = remain
        .get_mut(entries_off..)
        .map(|v| v.get_mut(..entries_size))
        .flatten()
        .ok_or(Error::BufferTooSmall(Some(2 * blk_sz + entries_off + entries_size)))?;
    check_entries(&header, entries)?;

    if resize {
        // Updates the last entry to cover the rest of the storage.
        let gpt_entries =
            Ref::<_, [GptEntry]>::new_slice(&mut entries[..]).unwrap().into_mut_slice();
        gpt_entries.iter_mut().filter(|e| !e.is_null()).last().map(|v| v.last = header.last);
        header.update_entries_crc(entries);
        // Re-verifies everything.
        check_header(disk.io(), &header, true).unwrap();
        check_entries(&header, entries).unwrap();
    }

    disk.write(0, mbr_primary).await?;
    disk.sync_gpt(gpt).await?.res()
}

/// Erases GPT if there is one on the device.
pub(crate) async fn erase_gpt(
    disk: &mut Disk<impl BlockIo, impl DerefMut<Target = [u8]>>,
    gpt: &mut Gpt<impl DerefMut<Target = [u8]>>,
) -> Result<()> {
    match disk.sync_gpt(gpt).await?.res() {
        Err(_) => Ok(()), // No valid GPT. Nothing to erase.
        _ => {
            let blk_sz = disk.block_info().block_size;
            let mut load = LoadBufferRef::from(&mut gpt.buffer[..]);
            let entries_size = SafeNum::from(load.primary_header.entries_count) * GPT_ENTRY_SIZE;
            let scratch = load.primary_entries.as_bytes_mut();
            // Invalidate GPT first.
            load.block_size.0 = None;
            // Erases primary header/entries.
            let header = load.primary_header.current;
            let entries = load.primary_header.entries;
            disk.fill(header * blk_sz, blk_sz, 0, scratch).await?;
            disk.fill(entries * blk_sz, entries_size.try_into().unwrap(), 0, scratch).await?;
            // Erases secondary header/entries.
            let header = load.secondary_header.current;
            let entries = load.secondary_header.entries;
            disk.fill(header * blk_sz, blk_sz, 0, scratch).await?;
            disk.fill(entries * blk_sz, entries_size.try_into().unwrap(), 0, scratch).await?;
            Ok(())
        }
    }
}

/// Computes the minimum blocks needed for creating a GPT.
fn min_required_blocks(block_size: u64) -> Result<u64> {
    // MBR + primary/secondary GPT header block + primary/secondary entries blocks.
    Ok(1 + (1 + gpt_entries_blk(block_size)?) * 2)
}

/// `GptBuilder` provides API for modifying/creating GPT partition table on a disk.
pub struct GptBuilder<D, G> {
    disk: D,
    gpt: G,
}

impl<D: Debug, G: Debug> Debug for GptBuilder<D, G> {
    fn fmt(&self, f: &mut Formatter<'_>) -> core::result::Result<(), core::fmt::Error> {
        write!(f, "GptBuilder {{ disk: {:?}, gpt: {:?} }}", self.disk, self.gpt)
    }
}
// Generic parameters:
//
// * T: The type that implement BlockIo.
// * S: The type for the scratch buffer in `Self::disk`.
// * B: The type for the GPT buffer in `Self::gpt`.
// * D: The type for `Self::disk` which can dereference to a Disk<T, S>.
// * G: The type for `Self::gpt` which can dereference to a Gpt<B>.
impl<'a, T, S, B, D, G> GptBuilder<D, G>
where
    T: BlockIo,
    S: DerefMut<Target = [u8]>,
    B: DerefMut<Target = [u8]>,
    D: DerefMut<Target = Disk<T, S>>,
    G: DerefMut<Target = Gpt<B>>,
{
    /// Creates a new instance.
    ///
    /// The method always re-syncs the GPT. If `disk` does not contain a valid GPT, a new GPT is
    /// started from scratch.
    ///
    /// The partition entries will always be sorted when writing back to disk by `Self::persist()`.
    ///
    /// # Returns
    ///
    /// * Returns Ok((Self, true)) if an instance is created and the disk has a valid GPT.
    /// * Returns Ok((Self, false)) if an instance is created but disk does not have a valid GPT.
    /// * Returns Err() otherwise.
    pub fn new(mut disk: D, mut gpt: G) -> Result<(Self, bool)> {
        if disk.block_info().num_blocks < min_required_blocks(disk.block_info().block_size)? {
            return Err(Error::GptError(GptError::DiskTooSmall));
        }
        let has_valid_gpt = block_on(disk.sync_gpt(&mut gpt))?.res().is_ok();
        // Uses the buffer for secondary GPT header/entries as construction buffer, as it is not
        // used by Gpt once loaded and synced.
        let (mut header, mut entries) = LoadBufferRef::from(&mut gpt.buffer[..]).secondary();
        if !has_valid_gpt {
            header.as_bytes_mut().fill(0);
            entries.as_bytes_mut().fill(0);
            let entries_blk = gpt_entries_blk(disk.block_info().block_size).unwrap();
            // Initializes a secondary header.
            let num_blks = SafeNum::from(disk.block_info().num_blocks);
            header.magic = GPT_MAGIC;
            header.current = (num_blks - 1).try_into().unwrap();
            header.backup = 1;
            header.size = size_of::<GptHeader>().try_into().unwrap();
            header.first = 1 + 1 + entries_blk; // MBR + GPT header blocks + entries block
            header.last = (num_blks - 1 - entries_blk - 1).try_into().unwrap();
            header.entries = (num_blks - 1 - entries_blk).try_into().unwrap();
            header.entries_count = 0;
            header.entries_size = size_of::<GptEntry>().try_into().unwrap();
        }
        // Normalizes `entries_count` to actual valid entries. Some GPT disk fixes `entry_count` to
        // 128.
        header.entries_count =
            entries.iter().position(|v| v.is_null()).unwrap_or(entries.len()).try_into().unwrap();
        entries.sort_unstable_by_key(|v| match v.is_null() {
            true => u64::MAX,
            _ => v.first,
        });
        Ok((Self { disk, gpt }, has_valid_gpt))
    }

    /// Removes a partition.
    ///
    /// # Returns
    ///
    /// * Returns Ok(true) if found and removed.
    /// * Returns Ok(false) if not found.
    /// * Returns Err() otherwise.
    pub fn remove(&mut self, part: &str) -> Result<bool> {
        let (mut header, mut entries) = LoadBufferRef::from(&mut self.gpt.buffer[..]).secondary();
        let entries = &mut entries[..header.entries_count.try_into().unwrap()];
        match entries.iter().position(|v| v.match_name(part).unwrap_or(false)) {
            Some(n) => {
                // Shift the elements behind forward.
                entries[n..].rotate_left(1);
                // Zeroizes the last element.
                entries.last_mut().unwrap().as_bytes_mut().fill(0);
                header.entries_count -= 1;
                Ok(true)
            }
            _ => Ok(false),
        }
    }

    /// Inserts a new partition before a partition.
    ///
    /// # Args
    ///
    /// * `idx`: Index of the partition to insert before. If index is out of range of valid entries,
    ///   the partition will be inserted at the last.
    /// * `name`: Name of the partition.
    /// * `part_type`: Type GUID.
    /// * `unique_guid`: Unique GUID.
    /// * `flags`: Partition flag.
    /// * `size`: If Some(_), specifies the size in number of bytes for the partition. The method
    ///   will round it up to multiple of disk block size and check that there is enough space for
    ///   the partition. If None, the method will insert the partition and consumes all the
    ///   available space in between.
    fn insert_before(
        &mut self,
        idx: usize,
        name: &str,
        part_type: [u8; GPT_GUID_LEN],
        unique_guid: [u8; GPT_GUID_LEN],
        flags: u64,
        size: Option<u64>,
    ) -> Result<()> {
        let (mut header, mut entries) = LoadBufferRef::from(&mut self.gpt.buffer[..]).secondary();
        // Gets position to the first NULL entry.
        let n = entries.iter().position(|v| v.is_null()).ok_or(Error::OutOfResources)?;
        let entries = &mut entries[..n + 1];
        // Caps `idx` to no more than the first NULL entry.
        let idx = min(n, idx);
        // Comptues the ending block index (non-inclusive) of the previous partition entry.
        // Entries are guaranteed sorted in `Self::new()`.
        let prev_end = match idx {
            0 => header.first,
            _ => entries[idx - 1].last + 1,
        };
        // Comptues the starting block index (inclusive) of the next partition entry.
        let next_start = match idx == n {
            true => header.last + 1,
            _ => entries[idx].first,
        };
        // Computes the size in number of blocks
        let blk_sz = self.disk.block_info().block_size;
        let blocks: u64 = match size {
            Some(v) => (SafeNum::from(v).round_up(blk_sz) / blk_sz).try_into()?,
            _ => next_start - prev_end, // If not given, uses up all the gap space
        };
        // Checks if there is enough space.
        if next_start - prev_end < blocks {
            return Err(Error::OutOfResources);
        }
        // Inserts the new entry.
        entries[idx..].rotate_right(1);
        let entry = &mut entries[idx];
        assert!(entry.is_null());
        entry.part_type = part_type;
        entry.guid = unique_guid;
        entry.flags = flags;
        entry.first = prev_end;
        entry.last = prev_end + blocks - 1;
        for (idx, ele) in name.encode_utf16().enumerate() {
            match idx < GPT_NAME_LEN_U16 {
                true => entry.name[idx] = ele,
                _ => break,
            }
        }
        header.entries_count += 1;
        Ok(())
    }

    /// Adds a partition.
    ///
    /// # Args
    ///
    /// * `name`: Name of the partition.
    /// * `part_type`: Type GUID.
    /// * `unique_guid`: Unique GUID.
    /// * `flags`: Partition flag.
    /// * `size`: If Some(_), specifies the size in number of bytes for the partition. The method
    ///   will round it up to multiple of disk block size and search for the first large enough
    ///   space in the unused spae for putting the partition. If None, the method will add the
    ///   partition at the last and have it consume all remaining usable disk space.
    pub fn add(
        &mut self,
        name: &str,
        part_type: [u8; GPT_GUID_LEN],
        unique_guid: [u8; GPT_GUID_LEN],
        flags: u64,
        size: Option<u64>,
    ) -> Result<()> {
        let (header, _) = LoadBufferRef::from(&mut self.gpt.buffer[..]).secondary();
        let entry_count = usize::try_from(header.entries_count).unwrap();
        let search_start = size.is_some().then_some(0).unwrap_or(entry_count);
        for i in search_start..entry_count + 1 {
            if self.insert_before(i, name, part_type, unique_guid, flags, size).is_ok() {
                return Ok(());
            }
        }
        Err(Error::OutOfResources)
    }

    /// Persists the constructed GPT table to the disk and syncs. The builder is consumed.
    pub async fn persist(mut self) -> Result<()> {
        let (mut header, mut entries) = LoadBufferRef::from(&mut self.gpt.buffer[..]).secondary();
        header.update_entries_crc(entries.as_bytes());
        // Check validity. Should not fail if implementation is correct.
        check_header(self.disk.io(), &header, false).unwrap();
        check_entries(&header, entries.as_bytes()).unwrap();
        let blk_sz = self.disk.block_info().block_size;
        // Writes to secondary header/ entries
        self.disk.write(header.current * blk_sz, header.as_bytes_mut()).await?;
        self.disk.write(header.entries * blk_sz, entries.as_bytes_mut()).await?;
        // Clears primary header magic
        self.disk.write(blk_sz, &mut 0u64.to_be_bytes()).await?;
        // Re-syncs GPT
        self.disk.sync_gpt(&mut self.gpt).await?.res()
    }
}

/// Helper for calculcating the Crc32.
fn crc32(data: &[u8]) -> u32 {
    let mut hasher = Hasher::new();
    hasher.update(data);
    hasher.finalize()
}

#[cfg(test)]
pub(crate) mod test {
    use super::*;
    use crate::test::TestDisk;
    use gbl_async::block_on;

    /// A helper for creating a [TestDisk] from given data.
    fn test_disk(data: impl AsRef<[u8]>) -> TestDisk {
        // All tests cases use pre-generated GPT disk of 512 block size.
        TestDisk::new_ram_alloc(512, 512, data.as_ref().to_vec()).unwrap()
    }

    /// A helper for creating a [TestDisk] from given data and a [Gpt] for 128 entries.
    fn test_disk_and_gpt(data: impl AsRef<[u8]>) -> (TestDisk, GptMax) {
        (test_disk(data), new_gpt_max())
    }

    #[test]
    fn test_load_and_sync() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        block_on(dev.sync_gpt(&mut gpt)).unwrap();

        assert_eq!(gpt.partition_iter().unwrap().count(), 2);
        gpt.find_partition("boot_a").unwrap();
        gpt.find_partition("boot_b").unwrap();
        assert!(gpt.find_partition("boot_c").is_err());

        // Creating a new [Gpt] using the same buffer should reset the valid state.
        let gpt = Gpt::new(gpt.buffer).unwrap();
        assert!(gpt.partition_iter().is_err());
        assert!(gpt.find_partition("boot_a").is_err());
        assert!(gpt.find_partition("boot_b").is_err());
    }

    #[test]
    fn test_load_with_unaligned_buffer() {
        #[repr(align(8))]
        struct AlignedBuffer([u8; 34 * 1024]);
        let mut buffer = AlignedBuffer([0u8; 34 * 1024]);
        let buffer = &mut buffer.0[1..];
        assert_ne!(buffer.as_ptr() as usize % 2, 0);
        let mut disk = test_disk(include_bytes!("../test/gpt_test_1.bin"));
        let mut gpt = Gpt::new(buffer).unwrap();
        block_on(disk.sync_gpt(&mut gpt)).unwrap();
    }

    #[test]
    fn test_gpt_buffer_too_small() {
        assert!(Gpt::new(vec![0u8; size_of::<GptLoadBufferN<0>>() - 1]).is_err());
    }

    #[test]
    fn test_gpt_buffer_not_enough_for_all_entries() {
        let mut dev = test_disk(include_bytes!("../test/gpt_test_1.bin"));
        let mut gpt = new_gpt_n::<127>();
        assert_eq!(gpt.max_entries(), 127);
        // Actual entries_count is 128 in the GPT.
        assert!(block_on(dev.sync_gpt(&mut gpt)).unwrap().res().is_err());
    }

    #[test]
    fn test_good_gpt_no_repair_write() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        assert_eq!(block_on(dev.sync_gpt(&mut gpt)).unwrap(), GptSyncResult::BothValid);
    }

    /// A helper for testing restoration of invalid primary/secondary header modified by caller.
    fn test_gpt_sync_restore<'a>(
        modify_primary: impl FnOnce(&mut GptHeader, Ref<&mut [u8], [GptEntry]>),
        modify_secondary: impl FnOnce(&mut GptHeader, Ref<&mut [u8], [GptEntry]>),
        expect_primary_err: Error,
        expect_secondary_err: Error,
    ) {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");

        // Restores from secondary to primary.
        let mut disk = disk_orig.to_vec();
        let (header, entries) = (&mut disk[512..]).split_at_mut(512);
        let mut header = GptHeader::from_bytes_mut(header);
        modify_primary(&mut header, Ref::<_, [GptEntry]>::new_slice(entries).unwrap());
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        assert_ne!(dev.io().storage(), disk_orig);
        let sync_res = block_on(dev.sync_gpt(&mut gpt)).unwrap();
        assert_eq!(sync_res, GptSyncResult::PrimaryRestored(expect_primary_err));
        assert_eq!(dev.io().storage(), disk_orig);

        // Restores from primary to secondary.
        let mut disk = disk_orig.to_vec();
        let (entries, header) = (&mut disk[512..]).split_last_chunk_mut::<512>().unwrap();
        let (_, entries) = entries.split_last_chunk_mut::<{ 512 * 32 }>().unwrap();
        let mut header = GptHeader::from_bytes_mut(&mut header[..]);
        modify_secondary(&mut header, Ref::<_, [GptEntry]>::new_slice(&mut entries[..]).unwrap());
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        assert_ne!(dev.io().storage(), disk_orig);
        let sync_res = block_on(dev.sync_gpt(&mut gpt)).unwrap();
        assert_eq!(sync_res, GptSyncResult::SecondaryRestored(expect_secondary_err));
        assert_eq!(dev.io().storage(), disk_orig);
    }

    #[test]
    fn test_sync_gpt_incorrect_magic() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.magic = 0x123456;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::IncorrectMagic(0x123456));
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_incorrect_crc() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.crc32 = !hdr.crc32;
        }
        let err = Error::GptError(GptError::IncorrectHeaderCrc);
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_unexpected_header_size() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.size += 1;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::UnexpectedHeaderSize { actual: 93, expect: 92 });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_unexpected_entry_size() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.entries_size += 1;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::UnexpectedEntrySize { actual: 129, expect: 128 });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_first_usable_gt_last() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.first = hdr.last;
            hdr.last = hdr.first - 2;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::InvalidFirstLastUsableBlock {
            first: 94,
            last: 92,
            range: (34, 94),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_first_usable_out_of_range() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.first = 33;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::InvalidFirstLastUsableBlock {
            first: 33,
            last: 94,
            range: (34, 94),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_last_usable_out_of_range() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.last += 1;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::InvalidFirstLastUsableBlock {
            first: 34,
            last: 95,
            range: (34, 94),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_primary_entries_out_of_range() {
        test_gpt_sync_restore(
            |hdr, _| {
                hdr.entries = 1;
                hdr.update_crc();
            },
            |hdr, _| {
                hdr.entries = hdr.last;
                hdr.update_crc();
            },
            Error::GptError(GptError::InvalidPrimaryEntriesStart {
                value: 1,
                expect_range: (2, 2),
            }),
            Error::GptError(GptError::InvalidSecondaryEntriesStart {
                value: 94,
                expect_range: (95, 95),
            }),
        );
    }

    #[test]
    fn test_sync_gpt_incorrect_entry_crc() {
        fn modify(hdr: &mut GptHeader, _: Ref<&mut [u8], [GptEntry]>) {
            hdr.entries_crc = !hdr.entries_crc;
            hdr.update_crc();
        }
        let err = Error::GptError(GptError::IncorrectEntriesCrc);
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_partition_range_overflow() {
        fn modify(hdr: &mut GptHeader, mut entries: Ref<&mut [u8], [GptEntry]>) {
            entries[1].last = hdr.last + 1;
            hdr.update_entries_crc(entries.as_bytes());
        }
        let err = Error::GptError(GptError::InvalidPartitionRange {
            idx: 2,
            part_range: (50, 95),
            usable_range: (34, 94),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_invalid_partition_range() {
        fn modify(hdr: &mut GptHeader, mut entries: Ref<&mut [u8], [GptEntry]>) {
            entries[1].first = entries[1].last;
            entries[1].last = entries[1].first - 2;
            hdr.update_entries_crc(entries.as_bytes());
        }
        let err = Error::GptError(GptError::InvalidPartitionRange {
            idx: 2,
            part_range: (73, 71),
            usable_range: (34, 94),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_partition_overlap() {
        fn modify(hdr: &mut GptHeader, mut entries: Ref<&mut [u8], [GptEntry]>) {
            entries[0].last = entries[1].first;
            entries.swap(0, 1);
            hdr.update_entries_crc(entries.as_bytes());
        }
        let err = Error::GptError(GptError::PartitionRangeOverlap {
            prev: (2, 34, 50),
            next: (1, 50, 73),
        });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_zero_partition_type_guid() {
        fn modify(hdr: &mut GptHeader, mut entries: Ref<&mut [u8], [GptEntry]>) {
            entries[1].part_type = [0u8; GPT_GUID_LEN];
            hdr.update_entries_crc(entries.as_bytes());
        }
        let err = Error::GptError(GptError::ZeroPartitionTypeGUID { idx: 2 });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_sync_gpt_zero_partition_unique_guid() {
        fn modify(hdr: &mut GptHeader, mut entries: Ref<&mut [u8], [GptEntry]>) {
            entries[1].guid = [0u8; GPT_GUID_LEN];
            hdr.update_entries_crc(entries.as_bytes());
        }
        let err = Error::GptError(GptError::ZeroPartitionUniqueGUID { idx: 2 });
        test_gpt_sync_restore(modify, modify, err, err);
    }

    #[test]
    fn test_load_gpt_disk_primary_override_secondary() {
        let mut disk = include_bytes!("../test/gpt_test_1.bin").to_vec();
        // Modifies secondary header.
        let secondary_hdr = GptHeader::from_bytes_mut(disk.last_chunk_mut::<512>().unwrap());
        secondary_hdr.revision = !secondary_hdr.revision;
        secondary_hdr.update_crc();
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        assert_eq!(
            block_on(dev.sync_gpt(&mut gpt)).unwrap(),
            GptSyncResult::SecondaryRestored(Error::GptError(GptError::DifferentFromPrimary)),
        );
    }

    #[test]
    fn test_load_gpt_disk_too_small() {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");
        let mut disk = disk_orig.to_vec();
        // Resizes so that it's not enough to hold a full 128 maximum entries.
        // MBR + (header + entries) * 2 - 1
        disk.resize((1 + (32 + 1) * 2 - 1) * 512, 0);
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        let sync_res = block_on(dev.sync_gpt(&mut gpt)).unwrap();
        let err = Error::GptError(GptError::DiskTooSmall);
        assert_eq!(sync_res, GptSyncResult::NoValidGpt { primary: err, secondary: err });
    }

    #[test]
    fn test_uninitialized_gpt() {
        let disk = include_bytes!("../test/gpt_test_1.bin");
        // Load a good GPT first.
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        assert_eq!(block_on(dev.sync_gpt(&mut gpt)).unwrap(), GptSyncResult::BothValid);
        gpt.find_partition("boot_a").unwrap();
        // Corrupt GPT.
        block_on(dev.write(0, &mut vec![0u8; disk.len()])).unwrap();
        assert!(block_on(dev.sync_gpt(&mut gpt)).unwrap().res().is_err());
        assert!(gpt.find_partition("").is_err());
    }

    #[test]
    fn test_update_gpt() {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");
        let mut disk = disk_orig.to_vec();
        // Erases all GPT headers.
        disk[512..][..512].fill(0);
        disk.last_chunk_mut::<512>().unwrap().fill(0);

        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);

        assert_ne!(dev.io().storage(), disk_orig);
        let mut mbr_primary = disk_orig[..34 * 512].to_vec();
        block_on(dev.update_gpt(&mut mbr_primary, false, &mut gpt)).unwrap();
        assert_eq!(dev.io().storage(), disk_orig);
    }

    #[test]
    fn test_update_gpt_has_existing_valid_secondary() {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");
        let mut disk = disk_orig.to_vec();
        // Erases all GPT headers.
        disk[512..][..512].fill(0);
        // Leaves a valid but different secondary GPT.
        let secondary_hdr = GptHeader::from_bytes_mut(disk.last_chunk_mut::<512>().unwrap());
        secondary_hdr.revision = !secondary_hdr.revision;
        secondary_hdr.update_crc();

        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);

        assert_ne!(dev.io().storage(), disk_orig);
        let mut mbr_primary = disk_orig[..34 * 512].to_vec();
        block_on(dev.update_gpt(&mut mbr_primary, false, &mut gpt)).unwrap();
        assert_eq!(dev.io().storage(), disk_orig);
    }

    #[test]
    fn test_update_gpt_last_usable_adjusted() {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");
        let mut disk = disk_orig.to_vec();
        // Erases all GPT headers.
        disk[512..][..512].fill(0);
        disk.last_chunk_mut::<512>().unwrap().fill(0);
        // Doubles the disk size.
        disk.resize(disk_orig.len() * 2, 0);

        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);

        assert_ne!(dev.io().storage, disk_orig);
        let mut mbr_primary = disk_orig[..34 * 512].to_vec();
        block_on(dev.update_gpt(&mut mbr_primary, true, &mut gpt)).unwrap();
        let expected_last = (disk.len() - GPT_MAX_NUM_ENTRIES_SIZE - 512) / 512 - 1;

        let (primary, secondary) = dev.io().storage().split_last_chunk_mut::<512>().unwrap();
        let primary_hdr = GptHeader::from_bytes_mut(&mut primary[512..]);
        let secondary_hdr = GptHeader::from_bytes_mut(secondary);
        // Header's last usable block is updated.
        assert_eq!({ primary_hdr.last }, expected_last.try_into().unwrap());
        assert_eq!({ primary_hdr.backup }, (disk.len() / 512 - 1).try_into().unwrap());
        assert_eq!({ secondary_hdr.last }, expected_last.try_into().unwrap());
    }

    #[test]
    fn test_update_gpt_resize() {
        let disk_orig = include_bytes!("../test/gpt_test_1.bin");
        let mut disk = disk_orig.to_vec();
        // Erases all GPT headers.
        disk[512..][..512].fill(0);
        disk.last_chunk_mut::<512>().unwrap().fill(0);
        // Doubles the disk size.
        disk.resize(disk_orig.len() * 2, 0);

        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);

        assert_ne!(dev.io().storage, disk_orig);
        let mut mbr_primary = disk_orig[..34 * 512].to_vec();
        block_on(dev.update_gpt(&mut mbr_primary, true, &mut gpt)).unwrap();
        // Last entry is extended.
        let expected_last = (disk.len() - GPT_MAX_NUM_ENTRIES_SIZE - 512) / 512 - 1;
        assert_eq!({ gpt.entries().unwrap()[1].last }, expected_last.try_into().unwrap());
    }

    #[test]
    fn test_update_gpt_new_partition_out_of_range() {
        // `gpt_test_1.bin` has a 8k "boot_a" and a 12k "boot_b". Thus partitions space is 40
        // blocks (512 bytes block size) and in total the GPT disk needs (40 + 1 + (33) * 2) = 107
        // blocks.
        let (mut dev, mut gpt) = test_disk_and_gpt(&vec![0u8; 106 * 512]);
        let mut mbr_primary = include_bytes!("../test/gpt_test_1.bin")[..34 * 512].to_vec();
        assert!(block_on(dev.update_gpt(&mut mbr_primary, true, &mut gpt)).is_err());
    }

    #[test]
    fn test_update_gpt_buffer_truncated() {
        let mut disk = include_bytes!("../test/gpt_test_1.bin").to_vec();
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);

        // Less than 1 MBR block.
        assert_eq!(
            block_on(dev.update_gpt(&mut disk[..511], false, &mut gpt)),
            Err(Error::BufferTooSmall(Some(1024)))
        );

        // Less than MBR + GPT header.
        assert_eq!(
            block_on(dev.update_gpt(&mut disk[..1023], false, &mut gpt)),
            Err(Error::BufferTooSmall(Some(1024)))
        );

        // Less than MBR + GPT header + entries.
        assert_eq!(
            block_on(dev.update_gpt(&mut disk[..34 * 512 - 1], false, &mut gpt)),
            Err(Error::BufferTooSmall(Some(34 * 512)))
        );
    }

    #[test]
    fn test_update_gpt_check_header_fail() {
        let disk = include_bytes!("../test/gpt_test_1.bin");
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        let mut mbr_primary = disk[..34 * 512].to_vec();
        // Corrupts the first byte of the GPT header.
        mbr_primary[512] = !mbr_primary[512];
        assert_eq!(
            block_on(dev.update_gpt(&mut mbr_primary, false, &mut gpt)),
            Err(Error::GptError(GptError::IncorrectMagic(0x54524150204946BA)))
        );
    }

    #[test]
    fn test_update_gpt_check_entries_fail() {
        let disk = include_bytes!("../test/gpt_test_1.bin");
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        let mut mbr_primary = disk[..34 * 512].to_vec();
        // Corrupts the first byte of the entries.
        mbr_primary[1024] = !mbr_primary[1024];
        assert_eq!(
            block_on(dev.update_gpt(&mut mbr_primary, false, &mut gpt)),
            Err(Error::GptError(GptError::IncorrectEntriesCrc))
        );
    }

    #[test]
    fn test_erase_gpt_no_gpt() {
        let (mut dev, mut gpt) = test_disk_and_gpt(&[0u8; 1024 * 1024]);
        block_on(dev.erase_gpt(&mut gpt)).unwrap();
    }

    #[test]
    fn test_erase_gpt() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        block_on(dev.erase_gpt(&mut gpt)).unwrap();
        const GPT_SECTOR: usize = 33 * 512;
        assert_eq!(dev.io().storage[512..][..GPT_SECTOR], vec![0u8; GPT_SECTOR]);
        assert_eq!(*dev.io().storage.last_chunk::<GPT_SECTOR>().unwrap(), *vec![0u8; GPT_SECTOR]);
        assert!(matches!(
            block_on(dev.sync_gpt(&mut gpt)).unwrap(),
            GptSyncResult::NoValidGpt { .. }
        ));
    }

    #[test]
    fn test_zero_partition_size() {
        let disk = include_bytes!("../test/gpt_test_1.bin").to_vec();
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        let (mut builder, _) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        assert_eq!(builder.remove("boot_a"), Ok(true));
        assert_eq!(builder.remove("boot_b"), Ok(true));
        builder.add("boot_b", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(0)).unwrap();
        block_on(builder.persist()).unwrap();
        assert_eq!(gpt.partition_iter().unwrap().next().unwrap().size().unwrap(), 0);
    }

    #[test]
    fn test_sync_gpt_non_sorted_entries() {
        let mut disk = include_bytes!("../test/gpt_test_1.bin").to_vec();
        let (header, entries) = disk[512..].split_at_mut(512);
        let header = GptHeader::from_bytes_mut(header);
        let mut entries = Ref::<_, [GptEntry]>::new_slice(entries).unwrap();
        // Makes partition non-sorted.
        entries.swap(0, 1);
        header.update_entries_crc(entries.as_bytes());
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        block_on(dev.sync_gpt(&mut gpt)).unwrap().res().unwrap();
    }

    #[test]
    fn test_gpt_builder_initialize_gpt_if_no_valid_gpt() {
        let (mut dev, mut gpt) = test_disk_and_gpt(vec![0u8; 1024 * 1024]);
        let (builder, valid) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        assert!(!valid);
        block_on(builder.persist()).unwrap();
        // A new GPT is created.
        block_on(dev.sync_gpt(&mut gpt)).unwrap().res().unwrap();
        assert!(gpt.partition_iter().unwrap().next().is_none());
    }

    #[test]
    fn test_gpt_builder_remove_partition() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        let (mut builder, valid) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        assert!(valid);
        assert_eq!(builder.remove("boot_b"), Ok(true));
        assert_eq!(builder.remove("non-existent"), Ok(false));
        block_on(builder.persist()).unwrap();
        block_on(dev.sync_gpt(&mut gpt)).unwrap().res().unwrap();
        let part_iter = gpt.partition_iter().unwrap();
        assert_eq!(
            part_iter.map(|v| v.name().unwrap().into()).collect::<Vec<String>>(),
            ["boot_a"]
        );
    }

    #[test]
    fn test_gpt_builder_add_partition_find_first() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        let (mut builder, _) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        assert!(builder.remove("boot_a").unwrap());
        // Adds at the beginning.
        builder.add("new_0", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(1024)).unwrap();
        // Adds following "new_0"
        builder.add("new_1", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(1)).unwrap();
        block_on(builder.persist()).unwrap();
        block_on(dev.sync_gpt(&mut gpt)).unwrap().res().unwrap();
        assert_eq!(gpt.find_partition("new_0").unwrap().absolute_range().unwrap(), (17408, 18432));
        assert_eq!(gpt.find_partition("new_1").unwrap().absolute_range().unwrap(), (18432, 18944));
        assert_eq!(gpt.find_partition("boot_b").unwrap().absolute_range().unwrap(), (25600, 37888));
    }

    #[test]
    fn test_gpt_builder_non_sorted_add_partition() {
        let mut disk = include_bytes!("../test/gpt_test_1.bin").to_vec();
        let (mut dev, mut gpt) = test_disk_and_gpt(&disk);
        let (header, entries) = disk[512..].split_at_mut(512);
        let header = GptHeader::from_bytes_mut(header);
        let mut entries = Ref::<_, [GptEntry]>::new_slice(entries).unwrap();
        // Makes partition non-sorted.
        entries.swap(0, 1);
        header.update_entries_crc(entries.as_bytes());

        let (mut builder, _) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        // Adds following boot_b.
        builder.add("new", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(1024)).unwrap();
        block_on(builder.persist()).unwrap();
        assert_eq!(gpt.find_partition("boot_a").unwrap().absolute_range().unwrap(), (17408, 25600));
        assert_eq!(gpt.find_partition("boot_b").unwrap().absolute_range().unwrap(), (25600, 37888));
        assert_eq!(gpt.find_partition("new").unwrap().absolute_range().unwrap(), (37888, 38912));
    }

    #[test]
    fn test_gpt_builder_add_partition_append() {
        let (mut dev, mut gpt) = test_disk_and_gpt(include_bytes!("../test/gpt_test_1.bin"));
        let (mut builder, _) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        assert!(builder.remove("boot_b").unwrap());
        // Adds following "boot_a".
        builder.add("new_0", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(1024)).unwrap();
        // Consumes the rest of the space.
        builder.add("new_1", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, None).unwrap();
        block_on(builder.persist()).unwrap();
        block_on(dev.sync_gpt(&mut gpt)).unwrap().res().unwrap();
        assert_eq!(gpt.find_partition("boot_a").unwrap().absolute_range().unwrap(), (17408, 25600));
        assert_eq!(gpt.find_partition("new_0").unwrap().absolute_range().unwrap(), (25600, 26624));
        assert_eq!(gpt.find_partition("new_1").unwrap().absolute_range().unwrap(), (26624, 48640));
    }

    #[test]
    fn test_gpt_builder_not_enough_resource() {
        // Create a Gpt that can only load 1 entry.
        let mut gpt = new_gpt_n::<1>();
        let mut dev = test_disk(vec![0u8; 64 * 1024]);
        let (mut builder, _) = GptBuilder::new(&mut dev, &mut gpt).unwrap();
        builder.add("new_0", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, Some(1024)).unwrap();
        assert!(builder.add("new_1", [1u8; GPT_GUID_LEN], [1u8; GPT_GUID_LEN], 0, None).is_err());
    }
}