am-partitions 0.3.0

//! MBR (Master Boot Record) partition table.
//!
//! Layout:
//!
//! ```text
//!   0..446    bootloader code
//!   446..510  4 partition entries × 16 bytes
//!   510..512  signature 0x55 0xAA
//! ```
//!
//! Each partition entry:
//!
//! ```text
//!   0       boot indicator
//!   1..4    starting CHS (legacy, ignored)
//!   4       partition type byte
//!   5..8    ending CHS (legacy, ignored)
//!   8..12   starting LBA  (u32 little-endian)
//!   12..16  number of sectors  (u32 little-endian)
//! ```
//!
//! Sector size is assumed to be 512 bytes. Real-world hardware with 4K
//! sectors stores LBAs in 512-byte units in the MBR for compatibility, so
//! this assumption holds.

use crate::error::{Error, Result};
use crate::probe::{Partition, PartitionKind};
use fs_core::BlockDevice;

const SECTOR_SIZE: u64 = 512;
/// MBR LBAs are 32-bit, so a single primary entry can describe at most
/// 2^32 - 1 sectors = 2 TiB - 512 B at 512-byte sectors. The writer rejects
/// anything that doesn't fit.
const MBR_LBA_MAX: u64 = 0xFFFF_FFFF;

/// One 0xEE entry that spans the whole disk = protective MBR (GPT lives here).
pub const TYPE_GPT_PROTECTIVE: u8 = 0xEE;

/// Common MBR partition-type byte values, exported so callers can match.
pub mod types {
    pub const EMPTY: u8 = 0x00;
    pub const FAT12: u8 = 0x01;
    pub const FAT16_SMALL: u8 = 0x04;
    pub const EXTENDED_CHS: u8 = 0x05;
    pub const FAT16: u8 = 0x06;
    pub const NTFS_OR_EXFAT: u8 = 0x07;
    pub const FAT32_CHS: u8 = 0x0B;
    pub const FAT32_LBA: u8 = 0x0C;
    pub const FAT16_LBA: u8 = 0x0E;
    pub const EXTENDED_LBA: u8 = 0x0F;
    pub const LINUX_SWAP: u8 = 0x82;
    pub const LINUX: u8 = 0x83;
    pub const LINUX_LVM: u8 = 0x8E;
    pub const HFS_PLUS: u8 = 0xAF;
    pub const SOLARIS: u8 = 0xBF;
    pub const FREEBSD: u8 = 0xA5;
    pub const OPENBSD: u8 = 0xA6;
    pub const NETBSD: u8 = 0xA9;
    pub const GPT_PROTECTIVE: u8 = 0xEE;
    pub const EFI_SYSTEM: u8 = 0xEF;
}

/// True when the MBR is "protective" — exactly one non-empty entry of type
/// 0xEE. Real GPT lives at LBA 1 in this case.
pub fn is_protective(lba0: &[u8; 512]) -> bool {
    let mut nonempty = 0;
    let mut all_protective = true;
    for i in 0..4 {
        let off = 446 + i * 16;
        let type_byte = lba0[off + 4];
        if type_byte != types::EMPTY {
            nonempty += 1;
            if type_byte != TYPE_GPT_PROTECTIVE {
                all_protective = false;
            }
        }
    }
    nonempty == 1 && all_protective
}

/// MBR active/boot flag mask. The "status byte" at offset +0 of each entry
/// holds this in its high bit; legacy BIOS firmware boots from the partition
/// where this bit is set.
pub const STATUS_ACTIVE: u8 = 0x80;

/// Parse the four primary entries. Empty entries are skipped. Extended-LBA
/// entries are reported as-is — chain walking is not yet implemented.
pub fn parse(lba0: &[u8; 512]) -> Result<Vec<Partition>> {
    let mut out = Vec::new();
    for i in 0..4 {
        let off = 446 + i * 16;
        let status = lba0[off];
        let type_byte = lba0[off + 4];
        if type_byte == types::EMPTY {
            continue;
        }
        let starting_lba =
            u32::from_le_bytes([lba0[off + 8], lba0[off + 9], lba0[off + 10], lba0[off + 11]]);
        let sectors = u32::from_le_bytes([
            lba0[off + 12],
            lba0[off + 13],
            lba0[off + 14],
            lba0[off + 15],
        ]);
        if sectors == 0 {
            continue;
        }
        let start = (starting_lba as u64) * SECTOR_SIZE;
        let length = (sectors as u64) * SECTOR_SIZE;
        let active = (status & STATUS_ACTIVE) != 0;
        out.push(Partition {
            start,
            length,
            kind: PartitionKind::Mbr { type_byte, active },
            label: None,
            uuid: None,
        });
    }
    Ok(out)
}

/// Write a fresh MBR sector with up to four primary entries. The bootloader
/// region (offset 0..446) is zeroed — there is no provision for preserving
/// existing boot code. A future `with_boot_code` variant can carry caller-
/// supplied boot code through. TODO: add that variant when a caller wants
/// legacy BIOS boot support.
///
/// Validation:
/// - At most four partitions (extended chains aren't written here).
/// - Every partition must carry a [`PartitionKind::Mbr`] type byte.
/// - Every partition must be sector-aligned, non-empty, and fit inside the
///   32-bit LBA range MBR uses.
/// - Partitions must not overlap.
pub fn write_mbr(dev: &dyn BlockDevice, partitions: &[Partition]) -> Result<()> {
    if !dev.is_writable() {
        return Err(Error::Block(fs_core::Error::ReadOnly));
    }
    if partitions.len() > 4 {
        return Err(Error::Invalid("MBR supports at most 4 primary partitions"));
    }
    let total_bytes = dev.size_bytes();
    if total_bytes < SECTOR_SIZE {
        return Err(Error::DeviceTooSmall);
    }

    // Overlap check on a sorted-by-start view.
    let mut sorted: Vec<&Partition> = partitions.iter().collect();
    sorted.sort_by_key(|p| p.start);
    let mut prev_end_lba: Option<u64> = None;
    for p in &sorted {
        validate_mbr_partition(p, total_bytes)?;
        let start_lba = p.start / SECTOR_SIZE;
        let end_lba = (p.start + p.length) / SECTOR_SIZE - 1;
        if let Some(prev) = prev_end_lba {
            if start_lba <= prev {
                return Err(Error::Invalid("partitions overlap"));
            }
        }
        prev_end_lba = Some(end_lba);
    }

    let mut sector = [0u8; 512];
    for (i, p) in partitions.iter().enumerate() {
        let off = 446 + i * 16;
        let (type_byte, active) = match p.kind {
            PartitionKind::Mbr { type_byte, active } => (type_byte, active),
            _ => return Err(Error::Invalid("non-MBR partition kind in MBR write")),
        };
        let start_lba = (p.start / SECTOR_SIZE) as u32;
        let sectors = (p.length / SECTOR_SIZE) as u32;

        sector[off] = if active { STATUS_ACTIVE } else { 0x00 };
        // CHS first/last left as zeros — modern OSes ignore CHS once LBA is
        // present, and the legacy fields can't faithfully describe most
        // modern geometry anyway.
        sector[off + 4] = type_byte;
        sector[off + 8..off + 12].copy_from_slice(&start_lba.to_le_bytes());
        sector[off + 12..off + 16].copy_from_slice(&sectors.to_le_bytes());
    }
    sector[510] = 0x55;
    sector[511] = 0xAA;
    dev.write_at(0, &sector)?;
    Ok(())
}

fn validate_mbr_partition(p: &Partition, total_bytes: u64) -> Result<()> {
    if !matches!(p.kind, PartitionKind::Mbr { .. }) {
        return Err(Error::Invalid("non-MBR partition kind in MBR write"));
    }
    if p.length == 0 {
        return Err(Error::Invalid("partition has zero length"));
    }
    if !p.start.is_multiple_of(SECTOR_SIZE) || !p.length.is_multiple_of(SECTOR_SIZE) {
        return Err(Error::Invalid("partition not sector-aligned"));
    }
    let start_lba = p.start / SECTOR_SIZE;
    let sectors = p.length / SECTOR_SIZE;
    if start_lba > MBR_LBA_MAX || sectors > MBR_LBA_MAX {
        return Err(Error::Invalid("partition exceeds MBR 32-bit LBA range"));
    }
    // First sector reserved for the MBR itself.
    if start_lba < 1 {
        return Err(Error::Invalid("MBR partition cannot start at LBA 0"));
    }
    if (p.start + p.length) > total_bytes {
        return Err(Error::Invalid("partition extends past device end"));
    }
    Ok(())
}