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nix: Fix segfaults in nix build with ClangTool compatibility patches #12

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87 changes: 82 additions & 5 deletions documentation/nix/nix.md
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ The goals of using Nix in this repository are to:

> ⚠️ **Linux only:** This flake currently supports **Linux only** because [`libbpf`](https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/os-specific/linux/libbpf/default.nix) is Linux-specific.

Feedback and merge requests are welcome. If we're missing a tool, please open an issue or PR. See the `corePackages` section in `flake.nix`.
Feedback and merge requests are welcome. If we're missing a tool, please open an issue or PR. See `nix/packages.nix` for package definitions.

---

Expand Down Expand Up @@ -63,12 +63,76 @@ Feedback and merge requests are welcome. If we're missing a tool, please open an

### What This Repository Provides

This repository includes `flake.nix` and `flake.lock`:
- **`flake.nix`** defines the development environment (compilers, libraries, tools, helper functions)
- **`flake.lock`** pins exact versions so all developers use **identical** inputs
This repository includes `flake.nix`, `flake.lock`, and modular Nix files in `nix/`:

- **`flake.nix`** - Main entry point (~90 lines), imports modules from `nix/`
- **`flake.lock`** - Pins exact versions so all developers use **identical** inputs
- **`nix/packages.nix`** - Package definitions (nativeBuildInputs, buildInputs, devTools)
- **`nix/llvm.nix`** - LLVM/Clang configuration with wrapped llvm-config
- **`nix/env-vars.nix`** - Environment variable exports
- **`nix/devshell.nix`** - Development shell configuration
- **`nix/derivation.nix`** - Package derivation for `nix build`
- **`nix/patches/`** - Source code patches for Nix compatibility (see [Nix-Specific Patches](#nix-specific-patches))
- **`nix/shell-functions/`** - Modular shell functions (build, clean, configure, etc.)

Running `nix develop` spawns a shell with the correct toolchains, libraries, and environment variables configured for you.

### Current Toolchain Versions

The Nix development environment provides the following tool and library versions (as of February 2026):

| Package | Version | Purpose |
|---------|---------|---------|
| GCC | 15.2.0 | Primary C/C++ compiler for final libraries |
| LLVM/Clang | 21.1.8 | Host compiler for xdp2-compiler, AST parsing |
| Boost | 1.87.0 | C++ libraries (graph, wave, program_options) |
| libbpf | 1.5.0 | BPF library for eBPF/XDP programs |
| libelf | 0.192 | ELF file handling |
| libpcap | 1.10.5 | Packet capture library |
| Python | 3.13.3 | Scripting and packet generation (with scapy) |

### Nix-Specific Patches

The Nix build uses patches to handle differences between traditional Linux distributions and the Nix build environment. These patches are located in `nix/patches/` and are automatically applied during `nix build`.

**Why patches?** When using libclang's `ClangTool` API directly (as xdp2-compiler does), it bypasses the Nix clang wrapper that normally sets up include paths. Additionally, different clang versions handle certain C constructs differently. The patches address these Nix-specific issues without modifying the original source code.

#### Patch 1: System Include Paths (`01-nix-clang-system-includes.patch`)

**Problem:** ClangTool bypasses the Nix clang wrapper script which normally adds `-isystem` flags for system headers. Without these flags, header resolution fails and the AST contains error nodes.

**Solution:** Reads include paths from environment variables set by the Nix derivation and adds them as `-isystem` arguments to ClangTool. These environment variables are only set during `nix build`, so this is a no-op on Ubuntu/Fedora.

Environment variables used:
- `XDP2_C_INCLUDE_PATH`: Clang builtins (stddef.h, stdint.h, etc.)
- `XDP2_GLIBC_INCLUDE_PATH`: glibc headers (stdlib.h, stdio.h, etc.)
- `XDP2_LINUX_HEADERS_PATH`: Linux kernel headers (<linux/types.h>, etc.)

#### Patch 2: Tentative Definition Null Check (`02-tentative-definition-null-check.patch`)

**Problem:** C tentative definitions like `static const struct T name;` (created by `XDP2_DECL_PROTO_TABLE` macro) behave differently across clang versions:
- Ubuntu clang 18.1.3: `hasInit()` returns false, these are skipped
- Nix clang 18.1.8+: `hasInit()` returns true with void-type InitListExpr

When `getAs<RecordType>()` is called on void type, it returns nullptr, causing a segfault.

**Solution:** Adds a null check and skips tentative definitions gracefully. The actual definition is processed when encountered later in the AST.

For detailed investigation notes, see [phase6_segfault_defect.md](phase6_segfault_defect.md).

### BPF/XDP Development Tools

The development shell includes additional tools for BPF/XDP development and debugging:

| Tool | Purpose |
|------|---------|
| `bpftools` | BPF program inspection and manipulation |
| `bpftrace` | High-level tracing language for eBPF |
| `bcc` | BPF Compiler Collection with Python bindings |
| `perf` | Linux performance analysis tool |
| `pahole` | DWARF debugging info analyzer (useful for BTF) |
| `clang-tools` | clang-tidy, clang-format, and other code quality tools |

---

## Quick Start
Expand Down Expand Up @@ -161,13 +225,26 @@ After running `build-all` once, the necessary changes will have been applied to

### Debugging nix develop

The `flake.nix` and embedded bash code make use of the environment variable `XDP2_NIX_DEBUG`. This variable uses syslog levels between 0 (default) and 7.
The shell functions use the environment variable `XDP2_NIX_DEBUG` at runtime. This variable uses syslog-style levels between 0 (default) and 7.

Debug levels:
- **0** - No debug output (default)
- **3** - Basic debug info
- **5** - Show compiler selection and config.mk details
- **7** - Maximum verbosity (all debug info)

For maximum debugging:
```bash
XDP2_NIX_DEBUG=7 nix develop --verbose --print-build-logs
```

You can also set the debug level after entering the shell:
```bash
nix develop
export XDP2_NIX_DEBUG=5
build-all # Will show debug output
```

### Shellcheck

The `flake.nix` checks all the bash code within the flake to ensure there are no issues.
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6 changes: 3 additions & 3 deletions flake.lock
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