# GRUB Integration Plan — Red Bear OS

**Date:** 2026-04-17
**Status:** Phase 1 — recipe builds successfully, post-build script ready, awaiting QEMU validation
**Approach:** Option A — GRUB as boot manager, chainloading Redox bootloader

## Overview

Add GNU GRUB as an optional boot manager for Red Bear OS. GRUB presents a menu
at boot and chainloads the existing Redox bootloader, which then boots the
kernel normally. This gives users:

- Multi-boot capability alongside Linux, Windows, or other OSes
- Boot menu with timeout and manual selection
- Familiar GRUB rescue shell for debugging
- No changes to the Redox kernel, RedoxFS, or existing boot flow

## Architecture

```
UEFI firmware
  → EFI/BOOT/BOOTX64.EFI (GRUB standalone image)
    → grub.cfg: default entry chainloads Redox bootloader
      → EFI/REDBEAR/redbear.efi (Redox bootloader)
        → Reads RedoxFS partition
          → Loads kernel
            → Boots Red Bear OS
```

### ESP Layout (GRUB mode)

```
EFI/
├── BOOT/
│   ├── BOOTX64.EFI      ← GRUB (primary, loaded by UEFI firmware)
│   └── grub.cfg          ← GRUB configuration
└── REDBEAR/
    └── redbear.efi       ← Redox bootloader (chainload target)
```

### ESP Layout (default, no GRUB)

```
EFI/
└── BOOT/
    └── BOOTX64.EFI       ← Redox bootloader (unchanged)
```

## Why GRUB?

1. **GRUB does not support RedoxFS.** Writing a GRUB filesystem module for
   RedoxFS is high-risk, GPL-licensing-sensitive work. Chainloading avoids it.
2. **The Redox bootloader works.** It reads RedoxFS directly and boots the
   kernel. No need to replicate that logic in GRUB.
3. **GRUB is universally understood.** System administrators know GRUB. A
   `grub.cfg` is easier to customize than a custom bootloader.
4. **Multi-boot.** GRUB can boot Linux, Windows, and other OSes alongside
   Red Bear OS without any changes to those systems.

## GRUB Module Set

The standalone EFI image includes these modules:

| Module | Purpose |
|--------|---------|
| `part_gpt` | GPT partition table support |
| `part_msdos` | MBR partition table support |
| `fat` | FAT32 filesystem (ESP) |
| `ext2` | ext2/3/4 filesystem |
| `normal` | Normal mode (menu, scripting) |
| `configfile` | Load configuration files |
| `search` | Search for files/volumes |
| `search_fs_uuid` | Search by filesystem UUID |
| `search_label` | Search by volume label |
| `chain` | Chainload other bootloaders |
| `echo` | Print messages |
| `test` | Conditional expressions |
| `ls` | List files and devices |
| `cat` | Display file contents |
| `halt` | Shut down |
| `reboot` | Reboot |

No RedoxFS module is needed — GRUB chainloads the Redox bootloader instead.

## GRUB Configuration

The default `grub.cfg`:

```cfg
# Red Bear OS GRUB Configuration
set default=0
set timeout=5

menuentry "Red Bear OS" {
    chainloader /EFI/REDBEAR/redbear.efi
    boot
}

menuentry "Reboot" {
    reboot
}

menuentry "Shutdown" {
    halt
}
```

Users can customize `grub.cfg` to add entries for other operating systems,
change the timeout, or add additional Red Bear OS entries (e.g., recovery
mode with different kernel parameters, once supported).

## ESP Size Requirements

| Component | Measured Size |
|-----------|--------------|
| GRUB EFI binary (with modules) | 540 KiB |
| Redox bootloader | 100–200 KiB |
| grub.cfg | < 1 KiB |
| **Total** | **~1 MiB** |

The default ESP is 1 MiB (too small for GRUB). Configs using GRUB must set:

```toml
[general]
efi_partition_size = 16   # 16 MiB, enough for GRUB + Redox bootloader + margin
```

## Implementation — Phase 1: Post-Build Script

Phase 1 uses a post-build script to modify the ESP in an existing disk image.
This approach requires **no changes to the installer** and works immediately.

### Files

| File | Purpose |
|------|---------|
| `local/recipes/core/grub/recipe.toml` | Build GRUB from source, produce `grub.efi` |
| `local/recipes/core/grub/grub.cfg` | Default GRUB configuration |
| `local/scripts/install-grub.sh` | Post-build ESP modification script |
| `local/scripts/fat_tool.py` | Python FAT32 tool (no mtools dependency) |
| `recipes/core/grub → local/recipes/core/grub` | Symlink for recipe discovery |

### Workflow

```bash
# 1. Build GRUB recipe
make r.grub

# 2. Build Red Bear OS (with larger ESP)
make all CONFIG_NAME=redbear-full   # Must have efi_partition_size = 16

# 3. Install GRUB into disk image
./local/scripts/install-grub.sh build/x86_64/harddrive.img

# 4. Test
make qemu
```

### Requirements

- Python 3 (for `fat_tool.py` — no mtools dependency)
- GRUB build dependencies: `gcc`, `make`, `bison`, `flex`, `autoconf`, `automake`
- ESP must be ≥ 8 MiB (set `efi_partition_size = 16` in config)

## Implementation — Phase 2: Installer-Native Support (Future)

Phase 2 adds GRUB awareness directly to the Redox installer, eliminating the
post-build script step. This requires modifying the installer patch.

### Changes Required

1. **`GeneralConfig`**: Add `bootloader: Option<String>` field (`"redox"` default, `"grub"` for GRUB)
2. **`DiskOption`**: Add `bootloader_grub: Option<&[u8]>`, `grub_config: Option<&[u8]>` fields
3. **`fetch_bootloaders`**: When `bootloader = "grub"`, also install the `grub` package
4. **`with_whole_disk` / `with_whole_disk_ext4`**: When GRUB data present:
   - Create `EFI/REDBEAR/` directory
   - Write GRUB as `EFI/BOOT/BOOTX64.EFI`
   - Write Redox bootloader as `EFI/REDBEAR/redbear.efi`
   - Write `grub.cfg` to `EFI/BOOT/grub.cfg`
   - Auto-set ESP size to 16 MiB if not configured
5. **CLI**: Add `--bootloader grub` flag to installer binary

### Config Usage (Phase 2)

```toml
# config/redbear-full-grub.toml
include = ["redbear-full.toml"]

[general]
bootloader = "grub"
efi_partition_size = 16
```

## GRUB Recipe Design

The GRUB recipe uses `template = "custom"` because GRUB must be built for the
**host machine** (it's a build tool that produces EFI binaries), not for the
Redox target. The cookbook's `configure` template cross-compiles for Redox,
which is wrong for GRUB.

Key build steps:
1. Configure with `--target=x86_64 --with-platform=efi` (produces x86_64 EFI)
2. Disable unnecessary components (themes, mkfont, mount, device-mapper)
3. Run `grub-mkimage` to create standalone EFI binary with curated modules
4. Stage `grub.efi` and `grub.cfg` to `/usr/lib/boot/`

### Build Notes

The recipe uses `template = "custom"` because the cookbook's default `configure`
template sets `--host="${GNU_TARGET}"` for Redox cross-compilation, which is wrong
for GRUB (a host build tool producing EFI binaries).

Two issues required workarounds:

1. **Cross-compiler override.** The cookbook sets `CC`, `CXX`, `CFLAGS`, etc. to
   the Redox cross-toolchain. GRUB must be built with the host compiler. Fix:
   `unset CC CXX CPP LD AR NM RANLIB OBJCOPY STRIP PKG_CONFIG` and
   `unset CFLAGS CXXFLAGS CPPFLAGS LDFLAGS` at the top of the script.

2. **Missing `extra_deps.lst`.** GRUB 2.12 release tarballs omit
   `grub-core/extra_deps.lst` (normally generated by `autogen.sh` from git).
   Fix: `touch "${COOKBOOK_SOURCE}/grub-core/extra_deps.lst"` before configure.

3. **grub.cfg location.** The config file lives in the recipe directory
   (`${COOKBOOK_RECIPE}/grub.cfg`), not in the extracted source tarball
   (`${COOKBOOK_SOURCE}/`). The copy step uses `COOKBOOK_RECIPE`.

## Security Considerations

- GRUB configuration is on the ESP (FAT32), which is readable/writable by any OS
- Secure Boot: GRUB standalone images are not signed. Users needing Secure Boot
  must sign `BOOTX64.EFI` with their own key or use `shim`
- The chainload target (`EFI/REDBEAR/redbear.efi`) is also on the ESP
- No credentials or secrets are stored in the GRUB configuration

## Limitations

- GRUB cannot read RedoxFS (no module exists)
- Cannot pass kernel parameters directly (chainloading bypasses this)
- BIOS boot is not supported in Phase 1 (only UEFI)
- Requires `mtools` on the host for Phase 1 post-build script
- ESP must be manually sized to ≥ 8 MiB in config

## Testing

```bash
# Build GRUB recipe
make r.grub

# Verify GRUB binary was produced
find repo -name "grub.efi" -path "*/usr/lib/boot/*"

# Build full image with larger ESP
# (Add efi_partition_size = 16 to config first)
make all CONFIG_NAME=redbear-full

# Install GRUB
./local/scripts/install-grub.sh build/x86_64/harddrive.img

# Verify ESP contents
mdir -i build/x86_64/harddrive.img@1048576 -/ ::/

# Boot in QEMU
make qemu

# Expected: GRUB menu appears, "Red Bear OS" entry boots successfully
```

## References

- GNU GRUB Manual: https://www.gnu.org/software/grub/manual/grub/grub.html
- GRUB EFI standalone image: `grub-mkimage -O x86_64-efi ...`
- UEFI boot specification: `EFI/BOOT/BOOTX64.EFI` is the fallback boot path
- Redox bootloader source: `recipes/core/bootloader/source/`
- Installer GPT layout: `recipes/core/installer/source/src/installer.rs`