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volume-manager: GPT parsing — the partition GUID is the id, the name is the label (S1)

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Rung 1 of the identity ladder. A protective MBR (a type-0xEE entry) routes
probing to the GPT, authoritatively: gptFirstVolume verifies the LBA-1 header's
'EFI PART' signature and a header CRC-32 (inline reflected poly 0xEDB88320,
shared with the fixtures so parser and tests never drift onto a magic constant),
then walks the entry array — bounded by the declared gpt_entry_scan_maximum —
for the first entry with a non-zero type GUID and an overflow-safe in-device
range. That range check is the confinement-safety guard the driver's clamp
rests on, the invariant firstVolume already enforces for MBR, extended to
untrusted GPT metadata. The unique partition GUID becomes the identity key (the
id / mount-path handle); the 36-char partition name becomes the display label.
Three host tests (GUID-as-id; entry-past-device skipped and an all-out-of-range
table is null; a broken header/CRC is not a volume) — all three FAIL with the
GPT branch neutralized (3/7) and pass with it (7/7). Entry-array CRC deferred
(correctness-only; the range check carries the safety property).
This commit is contained in:
Daniel Samson
2026-08-09 23:15:23 +01:00
parent c81120ef0f
commit 020e31bc8f
1 changed files with 202 additions and 5 deletions
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system/services/volume-manager/partition.zig
+202 -5
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@@ -18,8 +18,12 @@ const std = @import("std");
/// logical sectors (4Kn media is a separate concern, noted in the plan).
pub const sector_bytes = 512;
/// The longest display label the parser records: a GPT partition name is 36
/// UTF-16 units; a FAT volume label is 11 bytes; 36 ASCII bytes covers both.
/// bound: bytes of a volume's display label the parser records (a GPT partition
/// name is 36 UTF-16 units; a FAT volume label is 11 bytes; 36 covers both)
/// decided-by: hardware
/// protects: the Identity.label buffer
/// at-limit: degrade - a longer name is truncated to this many ASCII bytes
/// observed-by: a volume whose displayed label is clipped
pub const label_maximum = 36;
/// Which rung of the identity ladder produced this identity. The rung tags the
@@ -99,14 +103,126 @@ fn hasBootSignature(block0: []const u8) bool {
return block0.len >= 512 and block0[510] == 0x55 and block0[511] == 0xAA;
}
/// GPT header signature at LBA 1.
const gpt_signature = "EFI PART";
/// bound: GPT partition entries scanned before the prober gives up
/// decided-by: ours
/// protects: the entry-array scan loop from an untrusted num_partition_entries
/// at-limit: degrade - stop scanning; a device whose usable entry sits past the
/// cap is treated as having no GPT volume (real tables carry <=128 entries)
/// observed-by: the gpt-entry-past-device host test
const gpt_entry_scan_maximum = 128;
/// Reflected CRC-32 (polynomial 0xEDB88320) — the ISO-HDLC variant GPT uses for
/// its header checksum. Inlined so the parser and the host fixtures compute it
/// the same way and never drift onto a magic constant.
fn crc32(bytes: []const u8) u32 {
var c: u32 = 0xFFFFFFFF;
for (bytes) |b| {
c ^= b;
var k: u8 = 0;
while (k < 8) : (k += 1) {
c = if (c & 1 != 0) (c >> 1) ^ 0xEDB88320 else c >> 1;
}
}
return c ^ 0xFFFFFFFF;
}
/// A GPT disk carries a protective MBR: a boot-signed block 0 with a partition
/// entry of type 0xEE. Its presence routes probing to the GPT (authoritative).
fn isProtectiveMbr(block0: []const u8) bool {
if (!hasBootSignature(block0)) return false;
var index: usize = 0;
while (index < 4) : (index += 1) {
if (block0[446 + index * 16 + 4] == 0xEE) return true;
}
return false;
}
/// Copy the GPT partition name (36 UTF-16LE units, the 72 bytes at entry+56)
/// into the identity's display label as ASCII, dropping non-ASCII units.
fn setLabelFromUtf16(id: *Identity, name_bytes: []const u8) void {
var out: usize = 0;
var i: usize = 0;
while (i + 1 < name_bytes.len and out < label_maximum) : (i += 2) {
const unit = std.mem.readInt(u16, name_bytes[i..][0..2], .little);
if (unit == 0) break;
if (unit < 0x80) {
id.label[out] = @intCast(unit);
out += 1;
}
}
id.label_len = @intCast(out);
}
/// The first GPT volume, or null if LBA 1 is not a valid GPT header or no entry
/// validates. The header CRC-32 and the per-entry overflow-safe range check are
/// the confinement-safety guards the driver's clamp rests on — the invariant
/// firstVolume documents for MBR, extended to untrusted GPT metadata. The
/// entry-array CRC is deferred (correctness-only; the range check carries safety).
fn gptFirstVolume(reader: SectorReader, device_blocks: u64) ?Volume {
var header: [sector_bytes]u8 = undefined;
if (!reader.read(1, &header)) return null;
if (!std.mem.eql(u8, header[0..8], gpt_signature)) return null;
const header_size = std.mem.readInt(u32, header[12..16], .little);
if (header_size < 92 or header_size > sector_bytes) return null;
const stored_crc = std.mem.readInt(u32, header[16..20], .little);
var check: [sector_bytes]u8 = undefined;
@memcpy(check[0..header_size], header[0..header_size]);
@memset(check[16..20], 0);
if (crc32(check[0..header_size]) != stored_crc) return null;
const entry_lba = std.mem.readInt(u64, header[72..80], .little);
const num_entries = std.mem.readInt(u32, header[80..84], .little);
const entry_size = std.mem.readInt(u32, header[84..88], .little);
if (entry_size != 128 and entry_size != 256 and entry_size != 512) return null;
if (entry_lba == 0 or entry_lba >= device_blocks) return null;
const scan = @min(num_entries, gpt_entry_scan_maximum);
var sector_buf: [sector_bytes]u8 = undefined;
var loaded: u64 = std.math.maxInt(u64);
var i: u32 = 0;
while (i < scan) : (i += 1) {
const abs = @as(u64, i) * entry_size;
const lba = entry_lba + abs / sector_bytes;
const off = @as(usize, @intCast(abs % sector_bytes));
if (lba != loaded) {
if (!reader.read(lba, &sector_buf)) return null;
loaded = lba;
}
const entry = sector_buf[off..][0..128]; // the fields we read live in the first 128 bytes
var type_nonzero = false;
for (entry[0..16]) |b| {
if (b != 0) {
type_nonzero = true;
break;
}
}
if (!type_nonzero) continue;
const start = std.mem.readInt(u64, entry[32..40], .little);
const end = std.mem.readInt(u64, entry[40..48], .little); // inclusive last LBA
// Untrusted range from removable media: overflow-safe validation. Reject a
// partition that starts at 0, is reversed, or ends outside the device; only
// then is start + count <= device_blocks guaranteed for the driver's clamp.
if (start == 0 or end < start or end >= device_blocks) continue;
var id = Identity{ .rung = .gpt_guid, .key = std.mem.readInt(u128, entry[16..32], .little) };
setLabelFromUtf16(&id, entry[56..128]);
return .{ .base_lba = start, .block_count = end - start + 1, .identity = id };
}
return null;
}
/// The first volume on the device `reader` addresses, 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.
/// `device_blocks`, or null if none is found. A GPT disk (protective MBR) is
/// handled by GPT, authoritatively — its null is final. Otherwise 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 device.
pub fn firstVolume(reader: SectorReader, device_blocks: u64) ?Volume {
var block0: [sector_bytes]u8 = undefined;
if (!reader.read(0, &block0)) return null;
if (!hasBootSignature(&block0)) return null;
if (isProtectiveMbr(&block0)) return gptFirstVolume(reader, device_blocks);
var index: u8 = 0;
while (index < 4) : (index += 1) {
const entry = block0[446 + @as(usize, index) * 16 ..][0..16];
@@ -194,3 +310,84 @@ test "a partition that runs past the device is skipped, not trusted" {
try std.testing.expectEqual(@as(u64, 2048), v.base_lba); // the fitting one, not the overflowing one
try std.testing.expectEqual(@as(u64, 1000), v.block_count);
}
/// A single 128-byte GPT partition entry for the tests.
fn gptEntry(type_nonzero: bool, unique_guid: u128, start: u64, end: u64) [128]u8 {
var e = [_]u8{0} ** 128;
if (type_nonzero) e[0] = 0x01; // any non-zero byte makes the type GUID non-zero
std.mem.writeInt(u128, e[16..32], unique_guid, .little);
std.mem.writeInt(u64, e[32..40], start, .little);
std.mem.writeInt(u64, e[40..48], end, .little);
return e;
}
/// Lay out a 4-sector disk: protective MBR (LBA 0), GPT header with a correct
/// CRC (LBA 1), and the entry array (LBA 2).
fn buildGptDisk(disk: []u8, entries: []const [128]u8) void {
@memset(disk, 0);
disk[510] = 0x55;
disk[511] = 0xAA;
disk[446 + 4] = 0xEE; // protective entry type
std.mem.writeInt(u32, disk[446 + 8 ..][0..4], 1, .little);
std.mem.writeInt(u32, disk[446 + 12 ..][0..4], 0xFFFFFFFF, .little);
const h = disk[sector_bytes..][0..sector_bytes];
@memcpy(h[0..8], gpt_signature);
std.mem.writeInt(u32, h[12..16], 92, .little); // header_size
std.mem.writeInt(u64, h[72..80], 2, .little); // partition_entry_lba
std.mem.writeInt(u32, h[80..84], @intCast(entries.len), .little);
std.mem.writeInt(u32, h[84..88], 128, .little); // size_of_partition_entry
@memset(h[16..20], 0);
std.mem.writeInt(u32, h[16..20], crc32(h[0..92]), .little);
const ea = disk[2 * sector_bytes ..][0..sector_bytes];
var i: usize = 0;
while (i < entries.len and i < 4) : (i += 1) {
@memcpy(ea[i * 128 ..][0..128], &entries[i]);
}
}
test "a GPT disk yields the partition GUID as the identity id" {
var disk = [_]u8{0} ** (4 * sector_bytes);
const guid: u128 = 0x112233445566778899AABBCCDDEEFF00;
const entries = [_][128]u8{gptEntry(true, guid, 2048, 4095)};
buildGptDisk(&disk, &entries);
const rd = RamDisk{ .sectors = &disk };
const v = firstVolume(rd.reader(), 200000).?;
try std.testing.expectEqual(@as(u64, 2048), v.base_lba);
try std.testing.expectEqual(@as(u64, 2048), v.block_count); // 4095 - 2048 + 1
try std.testing.expectEqual(Rung.gpt_guid, v.identity.rung);
try std.testing.expectEqual(guid, v.identity.key);
}
test "a GPT entry past the device is skipped; an all-out-of-range table is no volume" {
var disk = [_]u8{0} ** (4 * sector_bytes);
const entries = [_][128]u8{
gptEntry(true, 0xAAA, 2048, 999999), // ends past a 200000-block device
gptEntry(true, 0xBBB, 4096, 8191), // fits
};
buildGptDisk(&disk, &entries);
const rd = RamDisk{ .sectors = &disk };
const v = firstVolume(rd.reader(), 200000).?;
try std.testing.expectEqual(@as(u64, 4096), v.base_lba); // the fitting one, not the overflowing one
try std.testing.expectEqual(@as(u128, 0xBBB), v.identity.key);
var solo_disk = [_]u8{0} ** (4 * sector_bytes);
const solo = [_][128]u8{gptEntry(true, 0xAAA, 2048, 999999)};
buildGptDisk(&solo_disk, &solo);
const rd2 = RamDisk{ .sectors = &solo_disk };
try std.testing.expect(firstVolume(rd2.reader(), 200000) == null);
}
test "a protective MBR with a broken GPT header is not a volume" {
var disk = [_]u8{0} ** (4 * sector_bytes);
const entries = [_][128]u8{gptEntry(true, 0xCCC, 2048, 4095)};
buildGptDisk(&disk, &entries);
disk[sector_bytes] = 'X'; // wreck the 'EFI PART' signature
const rd = RamDisk{ .sectors = &disk };
try std.testing.expect(firstVolume(rd.reader(), 200000) == null);
var bad_crc = [_]u8{0} ** (4 * sector_bytes);
buildGptDisk(&bad_crc, &entries);
bad_crc[sector_bytes + 16] ^= 0xFF; // corrupt a header-CRC byte
const rd2 = RamDisk{ .sectors = &bad_crc };
try std.testing.expect(firstVolume(rd2.reader(), 200000) == null);
}