Add kernel tests

This commit adds kernel tests running on x86_64 for 5.15, 6.0, 6.2,
and 6.6 using vmtest[0]. The main goal is to ensure that the BPF
programs load fine for various kernels.

The original plan was to have a nix derivation that would do everything:
1) download the kernels 2) build vmtest 3) build lightswitch 4) generate
the vmtest configuration, and finally 5) run vmtest with said configuration.
I attempted to do this in this branch [1] but I was taking too long and
decided for this approach that gets us 80% of the value for 20% of the work.

The downsides for this approach are:

1) As the vmtest config file is not generated on the fly, the kernel
   paths have to be hardcoded. This is not as bad as it might seem
because nix guarantees these paths to be the same accross machines, but
it's still a bit kludgy;
2) There is an expectation that lightswitch in dev mode has been built
   before runnign the vmtests;

How to run the kernel tests
===========================

```
nix develop
cargo build
vmtest
```

Notes
=====

We still require qemu-guest-agen to be running in the base OS (nix or
not) which is not ideal, but a proper fix might be quite involved.

Test Plan
=========

Forced a panic to make sure the kernel tests would fail.

- [0]: https://github.com/danobi/vmtest
- [1]: https://github.com/javierhonduco/lightswitch/commits/wip-vmtests

.github/workflows/vmtests.yml

@@ -0,0 +1,34 @@
+name: vmtests
+on:
+    pull_request:
+    push:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  vmtests:
+    runs-on: ubuntu-22.04
+    permissions:
+        id-token: write
+        contents: read
+
+    steps:
+    - uses: actions/checkout@main
+    - uses: DeterminateSystems/nix-installer-action@main
+    - uses: DeterminateSystems/magic-nix-cache-action@main
+
+    - name: Install system dependencies
+      run: |
+        export DEBIAN_FRONTEND=noninteractive
+        sudo apt update && sudo apt -y install --no-install-recommends qemu-system-x86 qemu-guest-agent
+
+    - name: Set up nix dev env
+      run: nix develop --command echo 0
+
+    - name: Build `lightswitch`
+      run: nix develop --ignore-environment --command bash -c 'cargo build'
+
+    - name: Run kernel tests
+      run: nix develop --command 'vmtest'

.gitignore

@@ -1,3 +1,4 @@
 /target
 flame.svg
 src/bpf/*_skel.rs
+.vmtest.log

flake.nix

@@ -53,6 +53,11 @@
               # snapshot testing plugin binary
               cargo-insta
               # ocamlPackages.magic-trace
+              (import ./vm.nix { inherit pkgs; }).vmtest
+              (import ./vm.nix { inherit pkgs; }).kernel_5_15
+              (import ./vm.nix { inherit pkgs; }).kernel_6_0
+              (import ./vm.nix { inherit pkgs; }).kernel_6_2
+              (import ./vm.nix { inherit pkgs; }).kernel_6_6
             ];
 
             LIBCLANG_PATH = lib.makeLibraryPath [ llvmPackages_16.libclang ];

vm.nix

@@ -0,0 +1,78 @@
+{ pkgs }:
+{
+  kernel_5_15 = pkgs.stdenv.mkDerivation {
+    name = "download-kernel-5.15";
+    src = pkgs.fetchurl {
+      url = "https://github.com/danobi/vmtest/releases/download/test_assets/bzImage-v5.15-fedora38";
+      hash = "sha256-nq8W72vuNKCgO1OS6aJtAfg7AjHavRZ7WAkP7X6V610=";
+    };
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p $out
+      cp -r $src $out/bzImage
+    '';
+  };
+
+  kernel_6_0 = pkgs.stdenv.mkDerivation {
+    name = "download-kernel-6.0";
+    src = pkgs.fetchurl {
+      url = "https://github.com/danobi/vmtest/releases/download/test_assets/bzImage-v6.0-fedora38";
+      hash = "sha256-ZBBQ0yVUn+Isd2b+a32oMEbNo8T1v46P3rEtZ+1j9Ic=";
+    };
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p $out
+      cp -r $src $out/bzImage
+    '';
+  };
+
+  kernel_6_2 = pkgs.stdenv.mkDerivation {
+    name = "download-kernel-6.2";
+    src = pkgs.fetchurl {
+      url = "https://github.com/danobi/vmtest/releases/download/test_assets/bzImage-v6.2-fedora38";
+      hash = "sha256-YO2HEIWTuEEJts9JrW3V7UVR7t4J3+8On+tjdELa2m8=";
+    };
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p $out
+      cp -r $src $out/bzImage
+    '';
+  };
+
+  kernel_6_6 = pkgs.stdenv.mkDerivation {
+    name = "download-kernel-6.6";
+    src = pkgs.fetchurl {
+      url = "https://github.com/danobi/vmtest/releases/download/test_assets/bzImage-v6.6-fedora38";
+      hash = "sha256-6Fu16SPBITP0sI3lapkckZna6GKBn2hID038itt82jA=";
+    };
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p $out
+      cp -r $src $out/bzImage
+    '';
+  };
+
+  vmtest = pkgs.rustPlatform.buildRustPackage {
+    name = "vmtest";
+    src = pkgs.fetchFromGitHub {
+      owner = "danobi";
+      repo = "vmtest";
+      rev = "51f11bf301fea054342996802a16ed21fb5054f4";
+      sha256 = "sha256-qtTq0dnDHi1ITfQzKrXz+1dRMymAFBivWpjXntD09+A=";
+    };
+    cargoHash = "sha256-SHjjCWz4FVVk1cczkMltRVEB3GK8jz2tVABNSlSZiUc=";
+    # nativeCheckInputs = [ pkgs.qemu ];
+
+    # There are some errors trying to access `/build/source/tests/*`.
+    doCheck = false;
+
+    meta = with pkgs.lib; {
+      description = "Helps run tests in virtual machines";
+      homepage = "https://github.com/danobi/vmtest/";
+      license = licenses.asl20;
+      mainProgram = "";
+      maintainers = with maintainers; [ ];
+      platforms = platforms.linux;
+    };
+  };
+}

vmtest.toml

@@ -0,0 +1,19 @@
+[[target]]
+name = "Fedora 5.15"
+kernel = "/nix/store/cg9wq48zz2dbvayh7sigr5smaf4dwcxp-download-kernel-5.15/bzImage"
+command = "/mnt/vmtest/target/debug/lightswitch --duration 0"
+
+[[target]]
+name = "Fedora 6.0"
+kernel = "/nix/store/p8xc9gqdkpfjpmcb9ja2gx6g6bd9cby7-download-kernel-6.0/bzImage"
+command = "/mnt/vmtest/target/debug/lightswitch --duration 0"
+
+[[target]]
+name = "Fedora 6.2"
+kernel = "/nix/store/79wzcqy12fncdf6pk1fj627ggh7v0nc7-download-kernel-6.2/bzImage"
+command = "/mnt/vmtest/target/debug/lightswitch --duration 0"
+
+[[target]]
+name = "Fedora 6.6"
+kernel = "/nix/store/77ixckavs2qidx1pglmlxsj6bfvjqijb-download-kernel-6.6/bzImage"
+command = "/mnt/vmtest/target/debug/lightswitch --duration 0"