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Chain-load EFI applications from Windows Boot Manager

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// repository documentation

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# efiloader

`efiloader` is a Windows Boot Application that is launched by Windows Boot
Manager and then restores the UEFI firmware execution context so it can call
normal UEFI services and start another EFI application.

## What It Does

Windows Boot Applications are entered after Boot Manager has switched into its
own application context. In that context, direct calls into firmware services
such as `ST`, `BS`, or console protocols may partially work or may hang.

`efiloader` reads the firmware descriptor from
`BOOT_APPLICATION_PARAMETER_BLOCK`, prepares a firmware-callable context,
verifies screen output through `ConOut->OutputString`, and then launches a
target EFI application.

On x64, the restored firmware context includes:

- `CR3`
- `GDTR`
- `IDTR`
- `LDTR`
- `CS`
- `DS`, `ES`, `FS`, `GS`, `SS`

On ARM64, the restored firmware context includes:

- `SCTLR_EL1`
- `VBAR_EL1`
- `TPIDR_EL1`
- `TTBR0_EL1`, `TTBR1_EL1`
- `TCR_EL1`
- `MAIR_EL1`
- DAIF IRQ state

## Embedded Payload

`efiloader` can embed one EFI application or driver as an `RCDATA` resource.
Use Resource Hacker or a similar PE resource editor to replace the placeholder
in an existing `efiloader.efi`:

1. Open `efiloader.efi` in the resource editor.
2. Locate the `RCDATA` resource with ID `101`.
3. Replace that resource with the desired EFI application or driver.
4. Save the modified image as a new `efiloader.efi`.

After restoring the firmware execution context, `efiloader` loads the embedded
image directly from memory and starts it **before** the target selected by BCD
`loadoptions`.

The original resource is only a text placeholder and is ignored at runtime.

## BCD Options

The Boot Application entry should point to:

```text
\efiloader.efi
```

The BCD entry GUID and the path to `efiloader.efi` can be customized. Integrity
checks must be disabled for the Boot Application entry:

```batch
set guid={14530529-1111-2222-abcd-12345678abcd}
bcdedit /create %guid% /d "EFI LOADER" /application bootapp
bcdedit /set %guid% device boot
bcdedit /set %guid% path \efiloader.efi
bcdedit /set %guid% nointegritychecks true
bcdedit /set {bootmgr} displayorder %guid% /addlast
```

If you are editing an offline BCD store, add the same `/store` argument to the
`bcdedit` commands as appropriate for your environment.

`efiloader` also supports overriding the launched EFI application with the BCD
`loadoptions` string element (`0x12000030`):

```batch
bcdedit %bcd% /set %guid% loadoptions \EFI\okr.efi
```

The option is copied before switching to firmware context. Leading/trailing
spaces, NUL characters, and one pair of surrounding quotes are ignored.

If `loadoptions` is absent or invalid, the fallback target is:

```text
\shell.efi
```

## ISO Boot

When booting from optical media, `efiloader` first asks firmware to load the
target from the El Torito UEFI image. If that fails, it reads the target
directly from the ISO9660 or UDF filesystem. The launched EFI
application therefore does not need to be inside the El Torito UEFI image.

Example layout:

```text
[ISO]\bootmgr.efi
[ISO]\efiloader.efi
[ISO]\boot\boot.sdi
[ISO]\EFI\microsoft\boot\bcd
[ISO]\shell.efi
[IMG]\EFI\BOOT\BOOTX64.EFI
```

UDF media using physical (type 1) partition maps is supported. UDF metadata
partitions are not currently resolved.

## Serial Debug

Serial debug output goes to COM1 and is disabled by default. Define
`ENABLE_SERIAL_DEBUG` when building x86/x64 images to enable diagnostics.

## Acknowledgements

This project references work from:

- https://github.com/imbushuo/boot-shim
- https://github.com/reactos/reactos
- https://github.com/Mattiwatti/EfiGuard
- https://git.savannah.gnu.org/cgit/grub.git

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