feat(rsboot): initial commit — UEFI bootloader extracted from RNG

1 files changed, 82 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..a39c192
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,82 @@
+# AGENTS.md — rsboot (RaiserSoft Bootloader)
+
+## Identity
+
+rsboot is a custom **UEFI** bootloader (PE32+, x64) that loads a Linux kernel +
+initramfs from an ext2 partition using UEFI file system protocols, then
+launches the kernel via `LoadImage`/`StartImage`.  Single-stage design —
+no MBR, no real mode, no stage1/stage2 split.
+
+This is a standalone submodule of the [RNG](https://git.2137697.xyz/RaiserSoft/rng) project.
+
+## Code
+
+- **Language:** C (freestanding, `-m64`, `-ffreestanding`, `-fPIE`, `-mabi=ms`).
+- **Assembler:** none currently (no `.asm` files needed).
+- **Linker:** `ld` with `-Ttext=0x1000 -e efi_main`, producing an ELF64
+  intermediate (`main.elf`).
+- **PE conversion:** `objcopy -O pei-x86-64 --subsystem=10` converts the ELF
+  to a PE32+ image, then Python scripts fix up headers and pad for
+  firmware compatibility.
+- **Dependencies:** none.  No libc, no CRT — just raw UEFI protocol calls.
+- **Debug:** UEFI console output (`con_out->OutputString()` via serial +
+  framebuffer as configured by the firmware).
+- **UEFI types:** `uefi.h` provides minimal stand-alone type definitions,
+  GUIDs, and protocol structs (system table, boot services, file system
+  protocol, loaded image protocol, etc.).
+
+## Source Layout
+
+```
+main.c      — EFI entry point, loads kernel + initrd, starts image
+uefi.h      — UEFI types, GUIDs, protocol structs
+Makefile    — builds BOOTX64.EFI
+fix_pe_simple.py    — PE header fixup (ImageBase, DllChar, strip relocs)
+pad_for_optiplex.py — PE padding for Optiplex 7010 firmware
+```
+
+## Build
+
+```sh
+make          # produces ../build/bootloader/BOOTX64.EFI
+```
+
+The build pipeline:
+
+1. **main.o** — compiled with `-fPIE -fshort-wchar -mno-red-zone -mabi=ms`
+2. **main.elf** — linked as position-independent ELF64 starting at text 0x1000
+3. **BOOTX64.EFI** — objcopy to `pei-x86-64`, subsystem=10 (EFI application)
+4. **fix_pe_simple.py** — sets ImageBase, marks RELOCS_STRIPPED (PIE code
+   uses only PC-relative addressing, so no fixups needed), sets NX |
+   DYNAMIC_BASE | HIGH_ENTROPY_VA, clears .reloc data directory and
+   checksum
+5. **pad_for_optiplex.py** — if the PE is smaller than ~160KB, pad to match
+   the size systemd-boot uses (some AMI Aptio firmware versions on
+   Optiplex 7010 reject smaller PE files)
+
+## Kernel Loading Flow
+
+1. Get our own `LoadedImage` protocol via `handle_protocol`
+2. Get `SimpleFileSystem` from the device handle
+3. `OpenVolume` → root directory
+4. Open `\bzImage`, read into allocated pages
+5. Open `\initramfs.cpio.gz`, read into allocated pages
+6. `LoadImage` (from memory, not device path) → kernel handle
+7. Set kernel's `load_options` (command line) on its `LoadedImage` protocol
+8. `StartImage` → kernel takes over
+
+## Key Conventions
+
+- **GUIDs:** EFI_LOADED_IMAGE_GUID, EFI_SIMPLE_FILE_SYSTEM_GUID (defined
+  in `uefi.h` matching the UEFI 2.10 spec).
+- **Boot device:** The device we were loaded from (our `LoadedImage.device_handle`).
+- **File paths:** `\bzImage`, `\initramfs.cpio.gz` (backslash prefix for
+  UEFI device paths).
+- **Cmdline:** `console=ttyS0,115200 console=tty0 earlyprintk=serial,ttyS0,115200 loglevel=3 rng_tty=ttyS0 initrd=\initramfs.cpio.gz`
+- **Memory allocation:** UEFI boot services `AllocatePages` (type 6 =
+  EfiLoaderData) for kernel/initrd buffers; `AllocatePool` for cmdline copy.
+- **Compilation quirks:** `-mabi=ms` is required so UEFI calling convention
+  (Microsoft x64 ABI) is used; `-fshort-wchar` makes `wchar_t` 16-bit to
+  match UEFI's CHAR16; `-mno-red-zone` because the red-zone can be clobbered
+  by UEFI firmware handlers.
+- **Commits:** `feat(rsboot):` prefix.