gpu

gpu — the one place a Lux chain asks for a primitive

A chain needs a hash and a signature check. This directory is where it asks for one, in all three languages, and where the choice between computing the answer and handing the question to an installed kernel library is made — once, in one place, with one rule.

The boundary

Kernels are private and no kernel source is here. They live in their own repository (lux-gpu/gpu, checked out on this machine at ~/work/luxcpp/gpu). What crosses the line into this public tree is a C ABI: four symbol names.

lux_gpu_create()             -> LuxGPU*
lux_gpu_destroy(LuxGPU*)
lux_gpu_backend_name(LuxGPU*) -> const char*
lux_gpu_keccak256_batch(LuxGPU*, const uint8_t* inputs, uint8_t* out,
                        const size_t* input_lens, size_t n) -> LuxError

Nothing is linked at build time. The library is dlopen'd, so a build machine that has never heard of it produces the same binary as one that has, and a binary that never finds it is a whole node.

The rule

The CPU backend is complete. Every primitive has a CPU body, always compiled, and it is the DEFINITION: the plugin does not get to have its own opinion. Two backends that disagree about a hash disagree about a block, so a plugin answer is either byte-identical to the CPU answer or a bug.

The plugin is a strict positive overlay. It may only ever be faster. When it is absent, when it declines a batch, or when it errors, the CPU answers — a device's bad day never propagates into consensus.

The knob

One environment variable, read once, at first use.

LUX_GPU_LIB names the library path when it is not on the loader's path.

LUX_GPU=verify is how "we believe they agree" becomes "we checked". It is what make gpu-differential sets.

What actually has two paths

No operation diverges between the CPU and the plugin — "What was measured" below says exactly what that claim rests on, and what it does not. A cross-LANGUAGE divergence is a different thing, and consolidating the primitives turned one up; it is the last section.

The three homes

gpu/gpu.go, plugin_cgo.go, plugin_nocgo.go   Go     package github.com/luxfi/node2/gpu
gpu/rust/                                    Rust   crate lux-gpu
gpu/cpp/                                     C++    CMake target lux_gpu

Each is the same shape: a cpu half that is always there, a plugin half behind dlopen, one LUX_GPU policy, and one Backend() that names which is answering. In Go the plugin half is //go:build cgo — a CGO_ENABLED=0 build has no way to open a shared library, so it never has one. That is not a missing feature; it is the build make luxd RUNTIME=go produces, and it is complete.

The C++ target also compiles the reused luxcpp/crypto bodies its CPU half delegates to. Those used to be compiled twice, once into xvm and once into platformvm, and the P-chain carried a third SHA-256 of its own in a header — three builds of one hash in one tree, and three places for the chains to drift.

Who goes through it

Every chain, for every primitive it is defined over.

chains/rust/xvm/src/hash.rs              names the seam's primitives, defines none
chains/rust/xvm/src/fx/secp256k1.rs      recover_public_key
chains/rust/platformvm/src/ids.rs        hash256
chains/rust/platformvm/src/sign.rs       address, recover
chains/cpp/xvm/src/id.cpp                sha256, pubkey_to_address
chains/cpp/xvm/src/root.cpp              keccak, the tags, the fold
chains/cpp/xvm/src/fx.cpp                recover_address
chains/cpp/platformvm/include/.../sha256.hpp   sha256
chains/cpp/platformvm/src/fx.cpp         address_of_compressed_key, recover_address

Signing is deliberately outside: a chain verifies signatures and never makes them, so k256's SigningKey stays in the chains' test and tooling halves. The seam exists to make VERIFICATION agree across backends.

What was measured

Run make gpu-differential. It builds all three seams and runs each one's tests twice — once with no library visible, once with LUX_GPU=verify against an installed one — and every input is asked of both backends and compared byte for byte.

On the machine this was written on:

• plugin:cpu, not plugin:cuda. lux_backend_available reports 1 for the plugin's own SIMD CPU backend and 0 for Metal, CUDA and Dawn. The only device backend built here, libluxgpu_backend_webgpu.so, does not load: it needs libwgpu_native.so, which is not on this machine. So the differential proves the dispatch and the ABI agree with the CPU definition, on every batch shape tried; it does not exercise a device kernel, because there is no device backend installed to exercise. Backend() reports the plugin's own name for exactly this reason — a host that says plugin:cpu is telling you it found the library and no device.
• Every seam's test carries a negative control on the verify comparison itself: it is handed a deliberately flipped byte and must refuse. A comparison nothing has ever seen fail is a comparison nobody has checked can fail.

A one-byte fork, closed by consolidating

Recovery ids 2 and 3. Go accepts all four (luxfi/crypto/secp256k1 checkSignature refuses only v >= 4) and recovers correctly for each; Rust's k256 does the same. Ids 2 and 3 mean R.x = r + n, and the first-party C++ curve refuses r >= n at parse time, so it has no path to them.

Before this seam the two C++ chains disagreed about that, with each other and with Go:

• chains/cpp/xvm accepted them, by masking v & 1. That turns v = 2 into v = 0 and recovers the key of a different id — so take any valid signature and move its recovery byte from 0 to 2, and a C++ node accepted a transaction Go and Rust both refuse. A fork anyone could build in one byte.
• chains/cpp/platformvm refused them, which matched Go for every such input.

Both refuse now, and that closes the exploitable direction. Measured on the KAT in gpu/cpp/test/gpu_test.cpp and gpu/rust/tests/differential.rs — the same signature asked of both seams:

The residue is one case: a deliberately built r where r + n is a valid x coordinate. There Go recovers a key and C++ still refuses — C++ refusing what Go accepts, never the reverse. It belongs to whoever owns the P/X differential, and closing it properly means teaching the C++ curve the r + n case.

Building the kernel library

make gpu builds lux-gpu/gpu's luxgpu_core_static target in place. It is not needed to build or run a node; it is needed to have something for LUX_GPU_LIB to point at.