# Final Image format

The main binary is bastion.elf (in some cases it calls kernel.elf).
It has format of ELF64

## Why ELF64?
ELF is the basic executable format for unix-like, and this format is very easy for understandable.
Also elf is what Clang/GCC emit by default when targeting Linux or bare-metal. 
And GDB/LLDB has great understanding of ELF format.

## What format is Linux image
Linux has its own format for each architecture.
For example for x86:
bzImage layout:
┌──────────────────────┐
│ Real-mode boot header│  "Boot protocol" setup header
│ (struct boot_params) │  at offset 0x1F1
├──────────────────────┤
│ PE/COFF header       │  Grafted on if CONFIG_EFI_STUB=y
│ (optional)           │  so UEFI can load it directly
├──────────────────────┤
│ Compressed kernel    │  The actual vmlinux (ELF), compressed
│ (gzip/lz4/zstd)      │  with a small decompressor stub
├──────────────────────┤
│ Decompressor code    │  Runs in protected/long mode,
│                      │  decompresses → real vmlinux
└──────────────────────┘

for aarch64, kernel Image is a flat binary with 64-byte header defined by the ARM64 boot protocol (with efi_stub config it also can have PE header).