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# vmlinux-btf
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An OpenWrt package that builds a BTF-enabled `vmlinux` in a shadow kernel tree and installs it as a standalone file, enabling CO-RE (Compile Once – Run Everywhere) eBPF programs to run on OpenWrt without tainting or modifying the main kernel build.
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---
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## How It Works
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1. Downloads the upstream Linux kernel source that matches the current OpenWrt build environment.
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2. Extracts it into an isolated `shadow-kernel` directory inside the build tree.
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3. Applies the same generic and platform-specific patches that the main kernel receives, in the same order.
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4. Copies the main kernel's `.config` as a baseline, then enables the full set of BTF and BPF-related options on top of it.
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5. Compiles `vmlinux` with full DWARF debug info.
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6. Uses `pahole` to extract a detached BTF blob from the resulting `vmlinux`.
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7. Installs the blob to `/usr/lib/debug/boot/vmlinux` on the target device.
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This approach keeps the main kernel tree completely untouched — BTF information is generated entirely in the shadow build.
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---
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## Host Build Dependencies
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The following tools must be present on the **host machine** running the OpenWrt build system before compiling this package.
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### Required by the OpenWrt build system itself
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These are standard OpenWrt build prerequisites. If your environment already builds OpenWrt successfully, these should already be present.
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| Tool | Purpose |
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|------|---------|
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| `gcc` / `binutils` | Cross-compilation toolchain |
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| `make` | Build orchestration |
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| `xz-utils` | Decompression of kernel source tarball (`xzcat`) |
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| `tar` | Extraction of kernel source |
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| `bc` | Kernel version arithmetic during `prepare` |
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| `flex`, `bison` | Kernel parser generation |
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| `libssl-dev` | Kernel certificate handling |
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| `libelf-dev` | ELF binary manipulation, required by kernel build |
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| `python3` | Kernel build scripts |
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Install on Debian/Ubuntu:
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```bash
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sudo apt install build-essential libssl-dev libelf-dev \
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bc flex bison python3 xz-utils tar
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```
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Install on Fedora/RHEL:
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```bash
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sudo dnf install gcc binutils openssl-devel elfutils-libelf-devel \
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bc flex bison python3 xz tar
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```
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---
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### Required specifically by this package
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#### `dwarves` (provides `pahole`)
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`pahole` is used to extract the detached BTF blob from the compiled `vmlinux`. This is the only tool required by this package that is **not** a standard OpenWrt build dependency.
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Install on Debian/Ubuntu:
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```bash
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sudo apt install dwarves
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```
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Install on Fedora/RHEL:
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```bash
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sudo dnf install dwarves
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```
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> **Note:** A host `pahole` version of **1.16 or newer** is required for `--btf_encode_detached` support. The version bundled with the OpenWrt `dwarves/host` feed satisfies this requirement. If you install `pahole` manually from your system package manager, verify the version with `pahole --version`.
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---
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## Building
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Add this package to your OpenWrt build configuration:
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```bash
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# From the OpenWrt build root
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make menuconfig
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# Navigate to: Kernel → vmlinux-btf
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# Select with <M> or <*>
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make package/vmlinux-btf/compile V=s
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```
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Or build the IPK directly:
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```bash
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make package/vmlinux-btf/{clean,compile} V=s
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```
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---
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## Installed Files
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| Path | Description |
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|------|-------------|
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| `/usr/lib/debug/boot/vmlinux` | Detached BTF blob |
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| `/usr/lib/debug/boot/vmlinux-<kernel-version>` | Symlink created by `postinst` |
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The symlink is created at install time using the running kernel version (`uname -r`) and removed automatically on package removal.
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---
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## Notes
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- **Build time is significant.** This package compiles a complete kernel from source. Expect 20–60 minutes depending on your hardware and target architecture.
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- **Kernel version must match.** The shadow kernel is built from the same source version as the OpenWrt target kernel (`$(LINUX_VERSION)`). Installing this package on a device running a different kernel version will likely result in BTF type mismatches and broken CO-RE lookups.
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- **Architecture-specific JIT support** (`HAVE_EBPF_JIT`) is set automatically by the kernel build system based on the target architecture and does not need to be configured manually.
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---
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## License
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GPL-3.0-or-later — see [https://www.gnu.org/licenses/gpl-3.0.html](https://www.gnu.org/licenses/gpl-3.0.html)
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