Show HN: DOOM in the kernel, or fibers in eBPF

This describes BPF Capsule, a compiler and runtime that reshapes large C/C++/no_std Rust programs so the unmodified Linux eBPF verifier and JIT will accept and run them. The project demonstrates the technique by running DOOM inside the kernel: initialization, game logic, and rendering execute entirely in-kernel with one BPF invocation per game tick. The work also runs Lua, QuickJS, SQLite, zlib, wasm3, llama2.c and CPython 3.14 under the same scheme and lets Lua/Python inspect live packets. It targets ordinary kernels (oldest profile Linux 5.15) on x86-64 and arm64 and is runnable with a Nix command plus a WAD file.

The technical challenge is the verifier’s strict model: only five argument registers, 512 bytes of stack, acyclic call graphs, limited function count, an instruction-budgeting verifier, and pointer provenance tracking that forbids storing and later dereferencing raw pointers. The solution pipeline progressed from hand-trimming to userland uBPF testing, then LLVM passes that launder pointers (double bookkeeping to restore provenance), force loops into a verifier-provable form, and eventually implement a software VM of regions, fibers, and a managed stack inside eBPF. The result shows complex application logic can run in-kernel without patches, but only after substantial automated transformations to preserve the verifier’s proof obligations and work around LLVM optimizer rewrites.