build: release-x86-64 — the flashable hybrid ISO (Etcher/dd + optical), built in-repo

README.md

@@ -54,6 +54,17 @@ the UEFI bootloader at `zig-out/EFI/BOOT/BOOTX64.efi`, the kernel at
 `zig-out/system/kernel`, init at `zig-out/system/services/init`, drivers under
 `zig-out/system/drivers/`, and the initial-ramdisk at `zig-out/boot/`.
 
+## Release media
+
+```sh
+zig build release-x86-64
+```
+
+Produces `zig-out/danos-x86-64.iso`, a hybrid ISO that boots flashed raw to a
+USB stick (balenaEtcher, dd) or burned to optical media — see
+[docs/release-iso.md](docs/release-iso.md). `zig build check-iso-image`
+validates it without booting.
+
 ## Run
 
 Boot it in QEMU with OVMF (opens a display window):

build.zig

@@ -684,6 +684,31 @@ pub fn build(b: *std.Build) void {
     const check_fat_step = b.step("check-fat-image", "Verify the FAT32 USB image is valid and bootable");
     check_fat_step.dependOn(&check_fat.step);
 
+    // --- release-x86-64: danos-x86-64.iso, the flashable release image ---
+    // Wrap the FAT32 boot volume in a hybrid ISO (the in-repo Python builder
+    // again, no xorriso/isohybrid): an ISO9660 whose El Torito EFI boot entry
+    // and MBR ESP partition entry both point at the embedded FAT image. One
+    // file then boots every way release media is consumed — flashed raw to a
+    // USB stick with Etcher or dd, or burned to optical media — while
+    // danos-usb.img stays the raw superfloppy QEMU and the test harness boot.
+    const mk_iso = b.addSystemCommand(&.{"python3"});
+    mk_iso.addFileArg(b.path("tools/make-iso-image.py"));
+    const iso_image = mk_iso.addOutputFileArg("danos-x86-64.iso");
+    mk_iso.addFileArg(fat_image);
+    const iso_install = b.addInstallFile(iso_image, "danos-x86-64.iso");
+    const release_step = b.step("release-x86-64", "Build the flashable x86-64 release ISO (zig-out/danos-x86-64.iso; flash with Etcher or dd)");
+    release_step.dependOn(&iso_install.step);
+
+    // `zig build check-iso-image` — the ISO builder's own --verify (mirroring
+    // check-fat-image): the MBR partition, the El Torito catalog, and the
+    // embedded FAT32 image must all agree.
+    const check_iso = b.addSystemCommand(&.{"python3"});
+    check_iso.addFileArg(b.path("tools/make-iso-image.py"));
+    check_iso.addArg("--verify");
+    check_iso.addFileArg(iso_image);
+    const check_iso_step = b.step("check-iso-image", "Verify the release ISO is a valid hybrid (MBR ESP partition + El Torito EFI entry)");
+    check_iso_step.dependOn(&check_iso.step);
+
     // --- run-x86-64: boot the x86-64 kernel in QEMU via UEFI/OVMF ---
     // Firmware lives in different places per OS/distro, so probe the known
     // layouts (Architecture, Debian/Ubuntu, Fedora, macOS Homebrew) and use the first

docs/README.md

@@ -117,6 +117,11 @@ Cutting across all of these:
   hardware needed to run danos: minimum specs (UEFI x86-64, ACPI, PCIe ECAM,
   xHCI, ~128 MiB RAM) grounded in what the boot path actually assumes, plus a
   plain-language guide matching Intel/AMD CPU generations by name.
+- **[release-iso.md](release-iso.md) — the release ISO.** The flashable boot
+  media: `zig build release-x86-64` wraps the FAT32 boot volume in a hybrid ISO
+  (MBR ESP partition + El Torito EFI entry, one embedded image) that Etcher/dd
+  flash to USB or a burner writes to disc — built by an in-repo pure-Python
+  tool, like the FAT image itself.
 - **[arch.md](arch.md) — the architecture split.** How CPU-specific code is kept
   behind a build-time `arch` module so the generic kernel never names x86_64,
   leaving room for other systems (e.g. an AArch64 Raspberry Pi) later.
@@ -261,5 +266,5 @@ exception in [coding-standards.md](coding-standards.md) applies to that seam.
 | danos-native runtime (`runtime`): syscall wrappers, heap, IPC, device access, the file API (`fs`) — the stable application ABI | `library/runtime/` |
 | System services (init, the VFS server + `protocol`, the device-manager) | `system/services/` |
 | Device drivers, one sub-project each (`pci-bus`, `ps2-bus`, `usb-xhci-bus` bus drivers) | `system/drivers/` |
-| Build + `run-x86-64` (QEMU/OVMF) | `build.zig` |
+| Build + `run-x86-64` (QEMU/OVMF) + `release-x86-64` (the flashable ISO) | `build.zig` |
 | QEMU integration test harness | `test/qemu_test.py` |

docs/release-iso.md

@@ -0,0 +1,70 @@
+# The release ISO — flashable boot media
+
+`zig build release-x86-64` produces **`zig-out/danos-x86-64.iso`**, the file you
+hand to someone who wants to try danos on a real machine: point
+[balenaEtcher](https://etcher.balena.io) (or Raspberry Pi Imager, or plain `dd`)
+at it, flash a USB stick, and boot the stick. The same file also burns to
+optical media. `zig build check-iso-image` validates it without booting.
+
+```
+zig build release-x86-64
+# Etcher: select danos-x86-64.iso → select the stick → Flash
+# or: sudo dd if=zig-out/danos-x86-64.iso of=/dev/rdiskN bs=4m   (macOS; triple-check N)
+```
+
+## Why an ISO when danos-usb.img already boots
+
+`danos-usb.img` is a raw FAT32 **superfloppy** — a filesystem starting at
+sector 0, no partition table. UEFI firmware accepts that from a USB stick (it
+probes whole-disk FAT before giving up), which is why `dd`-ing the .img works
+and why QEMU and the test harness boot it directly. But it is a
+developer-shaped artifact: flashing apps expect an ISO, and a superfloppy
+can't be burned to a CD/DVD or carry a partition table for pickier firmware.
+
+The ISO wraps that same FAT image — bit-identical, built by the same
+`tools/make-fat-image.py` — in a container that boots everywhere release media
+gets consumed. One payload, two images: the .img stays the raw volume the QEMU
+harness mounts and boots, the .iso is what leaves the building.
+
+## How a hybrid ISO boots twice
+
+The trick (the same one Linux distribution ISOs use, usually via `xorriso
+-isohybrid…`) is that ISO9660 reserves its first 32 KiB as a **system area** it
+never touches — exactly where an MBR lives on a disk. So one file can carry two
+tables of contents, both pointing at the same embedded FAT image:
+
+* **Flashed to USB (Etcher, dd):** firmware sees a disk whose sector 0 is an
+  MBR with one partition of type `0xEF` (EFI System Partition) covering the
+  embedded FAT image. It mounts that ESP and runs `\EFI\BOOT\BOOTX64.efi` —
+  the standard removable-media path ([efi.md](efi.md)).
+* **Burned to optical media:** firmware reads the ISO9660 volume descriptors
+  at sector 16 and finds an **El Torito** boot record. Its catalog has one
+  entry, platform ID `0xEF` (EFI), whose start LBA is — again — the embedded
+  FAT image. The firmware exposes that image as a virtual disk and runs the
+  same `BOOTX64.efi` off it.
+
+Neither path involves the legacy BIOS boot-sector machinery: danos is
+UEFI-only ([system-requirements.md](system-requirements.md)), so the MBR holds
+no boot code, just the partition entry, and the El Torito entry is EFI-class,
+not floppy emulation.
+
+One El Torito wrinkle: the catalog's sector-count field is 16-bit (units of
+512 bytes), so it can name at most 32 MiB — less than the 64 MiB FAT image.
+That is fine in practice: firmware sizes the FAT filesystem from its own BPB,
+and the boot files sit in the first few MiB of the image (clusters are
+allocated from the front) either way. The USB path has no such cap.
+
+## The builder
+
+`tools/make-iso-image.py` follows the house rule of
+[make-fat-image.py](../tools/make-fat-image.py): pure Python 3 standard
+library, no external tools (no xorriso, mkisofs, or isohybrid), with a
+`--verify` mode the `check-iso-image` step runs — it checks that the MBR
+partition and the El Torito catalog agree on where the FAT image lives and
+that a FAT32 boot sector is actually there. Every timestamp field in the ISO
+is zeroed, so the build is reproducible byte-for-byte.
+
+The ISO9660 filesystem around the boot machinery is minimal but real: a root
+directory listing `BOOT.CAT` (the catalog) and `EFI.IMG` (the FAT image), so
+`file`, mount tools, and archive browsers can open the ISO and see what's in
+it.

tools/make-iso-image.py

@@ -0,0 +1,261 @@
+#!/usr/bin/env python3
+"""Wrap the FAT32 boot volume in a hybrid ISO — the flashable danos release image.
+
+Mirrors tools/make-fat-image.py in spirit: pure Python 3 standard library, no
+external tools (no xorriso / mkisofs / isohybrid). The output is one file that
+boots both ways release media is consumed:
+
+  * Flashed raw to a USB stick (Etcher, dd): the ISO's system area carries an
+    MBR whose single partition (type 0xEF, "EFI System") points at the FAT32
+    image embedded in the ISO, so UEFI firmware finds the ESP and runs
+    \\EFI\\BOOT\\BOOTX64.efi off it.
+  * Burned to optical media: an El Torito boot catalog with an EFI platform
+    entry points at the same embedded FAT image.
+
+The ISO9660 filesystem itself is minimal but valid — a primary volume
+descriptor, the El Torito boot record, path tables, and a root directory that
+lists the boot catalog and the FAT image — so inspection tools can open it.
+
+    make-iso-image.py <out.iso> <esp.img>
+    make-iso-image.py --verify <out.iso>
+
+All timestamp fields are zero ("not specified") so the build is reproducible.
+"""
+
+import struct
+import sys
+
+ISO_SECTOR = 2048
+
+# Fixed layout, in ISO sectors (LBA). Sectors 0-15 are the system area (the
+# hybrid MBR lives in its first 512 bytes); volume descriptors start at 16.
+PVD_LBA = 16          # primary volume descriptor
+BOOT_RECORD_LBA = 17  # El Torito boot record volume descriptor
+TERMINATOR_LBA = 18   # volume descriptor set terminator
+PATH_TABLE_L_LBA = 19
+PATH_TABLE_M_LBA = 20
+ROOT_DIR_LBA = 21     # root directory (one sector holds our four records)
+CATALOG_LBA = 22      # El Torito boot catalog
+ESP_LBA = 23          # the embedded FAT32 image starts here
+
+MBR_PARTITION_TYPE_ESP = 0xEF
+
+
+def both16(value):
+    """ISO9660 both-byte-order encoding: little-endian then big-endian."""
+    return struct.pack("<H", value) + struct.pack(">H", value)
+
+
+def both32(value):
+    return struct.pack("<I", value) + struct.pack(">I", value)
+
+
+def directory_record(identifier, lba, size, flags):
+    length = 33 + len(identifier)
+    if length % 2:
+        length += 1  # records are padded to even length
+    record = bytearray(length)
+    record[0] = length
+    record[2:10] = both32(lba)
+    record[10:18] = both32(size)
+    # record[18:25] is the recording date; zero = unspecified (reproducible).
+    record[25] = flags  # 0x02 = directory
+    record[28:32] = both16(1)  # volume sequence number
+    record[32] = len(identifier)
+    record[33:33 + len(identifier)] = identifier
+    return bytes(record)
+
+
+def primary_volume_descriptor(total_sectors, path_table_size):
+    sector = bytearray(ISO_SECTOR)
+    sector[0] = 1  # type: primary
+    sector[1:6] = b"CD001"
+    sector[6] = 1  # version
+    sector[8:40] = b"DANOS".ljust(32)  # system identifier
+    sector[40:72] = b"DANOS".ljust(32)  # volume identifier
+    sector[80:88] = both32(total_sectors)
+    sector[120:124] = both16(1)  # volume set size
+    sector[124:128] = both16(1)  # volume sequence number
+    sector[128:132] = both16(ISO_SECTOR)  # logical block size
+    sector[132:140] = both32(path_table_size)
+    sector[140:144] = struct.pack("<I", PATH_TABLE_L_LBA)
+    sector[148:152] = struct.pack(">I", PATH_TABLE_M_LBA)
+    sector[156:190] = directory_record(b"\x00", ROOT_DIR_LBA, ISO_SECTOR, 0x02)
+    sector[190:318] = b" " * 128  # volume set identifier
+    sector[318:446] = b" " * 128  # publisher
+    sector[446:574] = b" " * 128  # data preparer
+    sector[574:702] = b"DANOS MAKE-ISO-IMAGE".ljust(128)  # application
+    sector[702:739] = b" " * 37  # copyright file
+    sector[739:776] = b" " * 37  # abstract file
+    sector[776:813] = b" " * 37  # bibliographic file
+    unspecified_date = b"0" * 16 + b"\x00"
+    for offset in (813, 830, 847, 864):  # creation/modification/expiry/effective
+        sector[offset:offset + 17] = unspecified_date
+    sector[881] = 1  # file structure version
+    return bytes(sector)
+
+
+def boot_record_descriptor():
+    sector = bytearray(ISO_SECTOR)
+    sector[0] = 0  # type: boot record
+    sector[1:6] = b"CD001"
+    sector[6] = 1
+    sector[7:39] = b"EL TORITO SPECIFICATION".ljust(32, b"\x00")
+    sector[71:75] = struct.pack("<I", CATALOG_LBA)
+    return bytes(sector)
+
+
+def terminator_descriptor():
+    sector = bytearray(ISO_SECTOR)
+    sector[0] = 255
+    sector[1:6] = b"CD001"
+    sector[6] = 1
+    return bytes(sector)
+
+
+def path_table(byte_order):
+    # A single entry: the root directory.
+    return (struct.pack("BB", 1, 0)
+            + struct.pack(byte_order + "I", ROOT_DIR_LBA)
+            + struct.pack(byte_order + "H", 1)
+            + b"\x00\x00")  # identifier 0x00 + pad to even
+
+
+def root_directory(esp_size):
+    # Records must be sorted by identifier; BOOT.CAT < EFI.IMG holds.
+    entries = (directory_record(b"\x00", ROOT_DIR_LBA, ISO_SECTOR, 0x02)
+               + directory_record(b"\x01", ROOT_DIR_LBA, ISO_SECTOR, 0x02)
+               + directory_record(b"BOOT.CAT;1", CATALOG_LBA, ISO_SECTOR, 0)
+               + directory_record(b"EFI.IMG;1", ESP_LBA, esp_size, 0))
+    return entries + b"\x00" * (ISO_SECTOR - len(entries))
+
+
+def boot_catalog(esp_size):
+    # Validation entry: EFI platform (0xEF), checksummed so its 16-bit words sum
+    # to zero, closed by the 0x55AA key bytes.
+    validation = bytearray(32)
+    validation[0] = 0x01
+    validation[1] = 0xEF
+    validation[4:28] = b"danos".ljust(24, b"\x00")
+    validation[30] = 0x55
+    validation[31] = 0xAA
+    checksum = (-sum(struct.unpack("<16H", validation))) & 0xFFFF
+    validation[28:30] = struct.pack("<H", checksum)
+    # Initial/default entry: bootable, no emulation, image at ESP_LBA. The
+    # sector-count field is 16-bit (units of 512 bytes) so it can't span a large
+    # ESP; UEFI firmware sizes the FAT filesystem from its own BPB, and the
+    # image's boot files sit well inside the capped span regardless.
+    default = bytearray(32)
+    default[0] = 0x88  # bootable
+    default[1] = 0x00  # no emulation
+    sector_count = min(0xFFFF, esp_size // 512)
+    default[6:8] = struct.pack("<H", sector_count)
+    default[8:12] = struct.pack("<I", ESP_LBA)
+    catalog = bytes(validation) + bytes(default)
+    return catalog + b"\x00" * (ISO_SECTOR - len(catalog))
+
+
+def hybrid_mbr(esp_size):
+    """The system-area MBR that makes the ISO flashable: one ESP partition."""
+    mbr = bytearray(512)
+    mbr[440:444] = b"dano"  # disk signature (fixed: reproducible builds)
+    start_lba = ESP_LBA * (ISO_SECTOR // 512)
+    partition = struct.pack(
+        "<B3sB3sII",
+        0x80,                      # status: active (harmless; helps picky firmware)
+        b"\xFE\xFF\xFF",           # CHS start: maxed out, LBA is authoritative
+        MBR_PARTITION_TYPE_ESP,    # type: EFI System
+        b"\xFE\xFF\xFF",           # CHS end
+        start_lba,
+        esp_size // 512,
+    )
+    mbr[446:462] = partition
+    mbr[510] = 0x55
+    mbr[511] = 0xAA
+    return bytes(mbr)
+
+
+def build(out_path, esp_path):
+    with open(esp_path, "rb") as handle:
+        esp = handle.read()
+    if len(esp) % ISO_SECTOR:
+        esp += b"\x00" * (ISO_SECTOR - len(esp) % ISO_SECTOR)
+    esp_sectors = len(esp) // ISO_SECTOR
+    total_sectors = ESP_LBA + esp_sectors
+
+    table_l = path_table("<")
+    image = bytearray(total_sectors * ISO_SECTOR)
+    image[0:512] = hybrid_mbr(len(esp))
+    image[PVD_LBA * ISO_SECTOR:(PVD_LBA + 1) * ISO_SECTOR] = \
+        primary_volume_descriptor(total_sectors, len(table_l))
+    image[BOOT_RECORD_LBA * ISO_SECTOR:(BOOT_RECORD_LBA + 1) * ISO_SECTOR] = \
+        boot_record_descriptor()
+    image[TERMINATOR_LBA * ISO_SECTOR:(TERMINATOR_LBA + 1) * ISO_SECTOR] = \
+        terminator_descriptor()
+    image[PATH_TABLE_L_LBA * ISO_SECTOR:PATH_TABLE_L_LBA * ISO_SECTOR + len(table_l)] = table_l
+    table_m = path_table(">")
+    image[PATH_TABLE_M_LBA * ISO_SECTOR:PATH_TABLE_M_LBA * ISO_SECTOR + len(table_m)] = table_m
+    image[ROOT_DIR_LBA * ISO_SECTOR:(ROOT_DIR_LBA + 1) * ISO_SECTOR] = root_directory(len(esp))
+    image[CATALOG_LBA * ISO_SECTOR:(CATALOG_LBA + 1) * ISO_SECTOR] = boot_catalog(len(esp))
+    image[ESP_LBA * ISO_SECTOR:] = esp
+
+    with open(out_path, "wb") as handle:
+        handle.write(image)
+    print(f"make-iso-image: wrote {out_path} "
+          f"({total_sectors * ISO_SECTOR // (1024 * 1024)} MiB hybrid ISO, "
+          f"ESP at LBA {ESP_LBA}, {esp_sectors} sectors)")
+
+
+def verify(path):
+    with open(path, "rb") as handle:
+        data = handle.read()
+    # The hybrid MBR (the Etcher/dd boot path).
+    if data[510] != 0x55 or data[511] != 0xAA:
+        sys.exit("verify: missing MBR 0x55AA signature")
+    status, _, part_type, _, part_start, part_sectors = \
+        struct.unpack_from("<B3sB3sII", data, 446)
+    if part_type != MBR_PARTITION_TYPE_ESP:
+        sys.exit(f"verify: MBR partition type 0x{part_type:02X}, expected 0xEF (ESP)")
+    # The ISO9660 descriptors (the optical boot path).
+    if data[PVD_LBA * ISO_SECTOR + 1:PVD_LBA * ISO_SECTOR + 6] != b"CD001":
+        sys.exit("verify: no primary volume descriptor")
+    boot_record = data[BOOT_RECORD_LBA * ISO_SECTOR:(BOOT_RECORD_LBA + 1) * ISO_SECTOR]
+    if not boot_record.startswith(b"\x00CD001") or \
+            not boot_record[7:30].startswith(b"EL TORITO SPECIFICATION"):
+        sys.exit("verify: no El Torito boot record")
+    catalog_lba = struct.unpack_from("<I", boot_record, 71)[0]
+    catalog = data[catalog_lba * ISO_SECTOR:(catalog_lba + 1) * ISO_SECTOR]
+    if catalog[0] != 0x01 or catalog[1] != 0xEF or catalog[30:32] != b"\x55\xAA":
+        sys.exit("verify: boot catalog validation entry is not an EFI entry")
+    if sum(struct.unpack("<16H", catalog[0:32])) & 0xFFFF != 0:
+        sys.exit("verify: boot catalog validation checksum is wrong")
+    if catalog[32] != 0x88:
+        sys.exit("verify: default catalog entry is not bootable")
+    boot_lba = struct.unpack_from("<I", catalog, 40)[0]
+    # Both paths must agree on where the ESP lives, and it must be a FAT32 image.
+    if boot_lba * (ISO_SECTOR // 512) != part_start:
+        sys.exit(f"verify: catalog boot image (LBA {boot_lba}) and MBR partition "
+                 f"(sector {part_start}) disagree")
+    esp = data[boot_lba * ISO_SECTOR:]
+    if len(esp) < part_sectors * 512:
+        sys.exit("verify: MBR partition extends past the end of the file")
+    if esp[510] != 0x55 or esp[511] != 0xAA or esp[82:90] != b"FAT32   ":
+        sys.exit("verify: embedded image is not a FAT32 boot volume")
+    print(f"verify: {path} is a hybrid ISO — MBR ESP partition (sector {part_start}, "
+          f"{part_sectors} sectors, active={status == 0x80}) and El Torito EFI entry "
+          f"both point at the embedded FAT32 image")
+
+
+def main(argv):
+    if len(argv) == 3 and argv[1] == "--verify":
+        verify(argv[2])
+        return 0
+    if len(argv) != 3:
+        sys.exit("usage: make-iso-image.py <out.iso> <esp.img>\n"
+                 "       make-iso-image.py --verify <out.iso>")
+    build(argv[1], argv[2])
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))