danos/system/services/volume-manager/partition.zig

//! Partition-table parsing, the policy the storage architecture places above the
//! block driver and below the filesystem (docs/file-system-development/
//! storage-architecture.md): read block 0, decide what block sub-ranges are
//! volumes, and read each volume's content identity. The block DRIVER never does
//! this — it clamps ranges it is told about; this is what tells it the numbers.
//!
//! Today: MBR (the four-entry table at offset 446) plus the bare-FAT case (a boot
//! sector right at LBA 0). GPT is the next entry in the identity ladder and slots
//! in here without touching anything above or below.

const std = @import("std");

/// One volume the parser found on the device: the block sub-range it occupies
/// and a content identity stable for the volume's life (the mount map keys on
/// it; the boot volume is recorded by it). `identity` is derived from the medium,
/// never from a port — a moved drive keeps it.
pub const Volume = struct {
    base_lba: u64,
    block_count: u64,
    identity: u64,
};

/// The MBR disk signature (offset 440, 4 bytes LE) — a 32-bit id written at
/// partition time. Weak (dd-cloned disks share it) but on the medium, and the
/// simplest rung of the identity ladder; the fuller rungs (GPT partition GUID,
/// FAT volume serial) refine `identityOf` without changing the shape.
fn diskSignature(block0: []const u8) u32 {
    if (block0.len < 444) return 0;
    return std.mem.readInt(u32, block0[440..444], .little);
}

/// The identity of the volume at partition index `index`: the disk signature
/// paired with the index, so two partitions of one disk stay distinct. For a
/// bare FAT (no table) the index is 0.
fn identityOf(block0: []const u8, index: u8) u64 {
    return (@as(u64, diskSignature(block0)) << 8) | index;
}

/// Whether block 0 looks like a partition table (the 0x55AA boot signature). A
/// bare FAT also carries it, so the caller distinguishes by whether any partition
/// entry is non-empty.
fn hasBootSignature(block0: []const u8) bool {
    return block0.len >= 512 and block0[510] == 0x55 and block0[511] == 0xAA;
}

/// The first volume on a device whose block 0 is `block0` and whose whole-device
/// size is `device_blocks`, or null if none is found. An MBR with a non-empty
/// entry yields that partition's [start, size); otherwise a boot signature with
/// no partitions is treated as a bare FAT spanning the whole device.
pub fn firstVolume(block0: []const u8, device_blocks: u64) ?Volume {
    if (!hasBootSignature(block0)) return null;
    var index: u8 = 0;
    while (index < 4) : (index += 1) {
        const entry = block0[446 + @as(usize, index) * 16 ..][0..16];
        const kind = entry[4];
        const start = std.mem.readInt(u32, entry[8..12], .little);
        const size = std.mem.readInt(u32, entry[12..16], .little);
        if (kind == 0 or start == 0 or size == 0) continue;
        return .{ .base_lba = start, .block_count = size, .identity = identityOf(block0, index) };
    }
    // No partition entries: a bare FAT spanning the device.
    return .{ .base_lba = 0, .block_count = device_blocks, .identity = identityOf(block0, 0) };
}

test "an MBR with one partition yields its range and a distinct identity" {
    var block0 = [_]u8{0} ** 512;
    block0[510] = 0x55;
    block0[511] = 0xAA;
    std.mem.writeInt(u32, block0[440..444], 0xDEADBEEF, .little);
    // partition 0: type 0x0c (FAT32 LBA), start 2048, size 100000
    block0[446 + 4] = 0x0c;
    std.mem.writeInt(u32, block0[446 + 8 ..][0..4], 2048, .little);
    std.mem.writeInt(u32, block0[446 + 12 ..][0..4], 100000, .little);
    const v = firstVolume(&block0, 200000).?;
    try std.testing.expectEqual(@as(u64, 2048), v.base_lba);
    try std.testing.expectEqual(@as(u64, 100000), v.block_count);
    try std.testing.expectEqual((@as(u64, 0xDEADBEEF) << 8) | 0, v.identity);
}

test "a boot signature with no partitions is a bare FAT over the whole device" {
    var block0 = [_]u8{0} ** 512;
    block0[510] = 0x55;
    block0[511] = 0xAA;
    const v = firstVolume(&block0, 65536).?;
    try std.testing.expectEqual(@as(u64, 0), v.base_lba);
    try std.testing.expectEqual(@as(u64, 65536), v.block_count);
}

test "no boot signature is no volume" {
    const block0 = [_]u8{0} ** 512;
    try std.testing.expect(firstVolume(&block0, 65536) == null);
}