BPF Linker 🔗

bpf-linker aims to simplify building modern BPF programs while still supporting older, more restrictive kernels.

Overview

bpf-linker can be used to statically link multiple BPF object files together and optionally perform optimizations needed to target older kernels. It operates on LLVM bitcode, so the inputs must be bitcode files (.bc) or object files with embedded bitcode (.o), optionally stored inside ar archives (.a).

Installation

The linker requires LLVM 18. It can use the same LLVM used by the rust compiler, or it can use an external LLVM installation.

If your target is aarch64-unknown-linux-gnu (i.e. Linux on Apple Silicon) you will have to use the external LLVM method.

Using LLVM provided by rustc

All you need to do is run:

cargo install bpf-linker

Using external LLVM

On Debian based distributions you need to install the llvm-18-dev, libclang-18-dev and libpolly-18-dev packages. If your distro doesn't have them you can get them from the official LLVM repo at https://apt.llvm.org.

On rpm based distribution you need the llvm-devel and clang-devel packages. If your distro doesn't have them you can get them from Fedora Rawhide.

Once you have installed LLVM 18 you can install the linker running:

cargo install bpf-linker --no-default-features

If you don't have cargo you can get it from https://rustup.rs or from your distro's package manager.

Usage

Rust

Nightly

To compile your eBPF crate just run:

cargo +nightly build --target=bpfel-unknown-none -Z build-std=core --release

If you don't want to have to pass the target and build-std options every time, you can put them in .cargo/config.toml under the crate's root folder:

[build]
target = "bpfel-unknown-none"

[unstable]
build-std = ["core"]

(Experimental) BTF support

To emit BTF debug information, set the following rustflags:

-C debuginfo=1 -C link-arg=--btf

These flags will work only for the eBPF targets (bpfeb-unknown-none, bpfel-unknown-none). Make sure you are specifying them only for eBPF crates, not for the user-space ones!

When compiling an eBPF crate directly with cargo +nightly build, they can be defined through the RUSTFLAGS environment variable:

RUSTFLAGS="-C debuginfo=1 -C link-arg=--btf" cargo +nightly build --target=bpfel-unknown-none -Z build-std=core --release

To avoid specifying them manually, you can put them in .cargo/config.toml:

[build]
target = "bpfel-unknown-none"
rustflags = "-C debuginfo=1 -C link-arg=--btf"

[unstable]
build-std = ["core"]

After that, the BPF object file present in target/bpfel-unknown-none/release should contain a BTF section.

Clang

For a simple example of how to use the linker with clang see this gist. In the example lib.c is compiled as a static library which is then linked by program.c. The Makefile shows how to compile the C code and then link it.

Usage

bpf-linker

USAGE:
    bpf-linker [FLAGS] [OPTIONS] --output <output> [--] [inputs]...

FLAGS:
        --disable-expand-memcpy-in-order    Disable passing --bpf-expand-memcpy-in-order to LLVM
        --disable-memory-builtins           Disble exporting memcpy, memmove, memset, memcmp and bcmp. Exporting those
                                            is commonly needed when LLVM does not manage to expand memory intrinsics to
                                            a sequence of loads and stores
    -h, --help                              Prints help information
        --ignore-inline-never               Ignore `noinline`/`#[inline(never)]`. Useful when targeting kernels that
                                            don't support function calls
        --unroll-loops                      Try hard to unroll loops. Useful when targeting kernels that don't support
                                            loops
    -V, --version                           Prints version information

OPTIONS:
        --cpu <cpu>                  Target BPF processor. Can be one of `generic`, `probe`, `v1`, `v2`, `v3` [default:
                                     generic]
        --cpu-features <features>    Enable or disable CPU features. The available features are: alu32, dummy, dwarfris.
                                     Use +feature to enable a feature, or -feature to disable it.  For example --cpu-
                                     features=+alu32,-dwarfris [default: ]
        --dump-module <path>         Dump the final IR module to the given `path` before generating the code
        --emit <emit>                Output type. Can be one of `llvm-bc`, `asm`, `llvm-ir`, `obj` [default: obj]
        --export <symbols>...        Comma separated list of symbols to export. See also `--export-symbols`
        --export-symbols <path>      Export the symbols specified in the file `path`. The symbols must be separated by
                                     new lines
    -L <libs>...                     Add a directory to the library search path
        --llvm-args <args>...        Extra command line arguments to pass to LLVM
        --log-file <path>            Output logs to the given `path`
        --log-level <level>          Set the log level. Can be one of `off`, `info`, `warn`, `debug`, `trace`
    -O <optimize>...                 Optimization level. 0-3, s, or z [default: 2]
    -o, --output <output>            Write output to <output>
        --target <target>            LLVM target triple. When not provided, the target is inferred from the inputs

ARGS:
    <inputs>...    Input files. Can be object files or static libraries