fix(tvix/tests/tvix-init): run as pid1, support init= r/6651

We were previously not running as PID1, but were executing things via
uinitcmd (by u-root init).
This had some problems - a real system pid1 really wants to be pid1.

u-root init itself seems to be too complicated to configure as a
chainloader, so instead, we just provide init/pid1 ourselves and do the
little bit of mount setup / stage 1 that's necessary.

`tvix.exec` is renamed to `tvix.run`, to further distinguish it from the
`exec` bash function.

Our init will now also power off the machine if the shell, or the custom
tvix.run command finished. In the case of switch_root (`init=` being
passed), we terminate before, so it's up to the real system anyways.

Change-Id: If00c0c11ca9393968e76ae0d131594b5c135fa48
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9454
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

3 files changed, 133 insertions, 53 deletions

diff --git a/tvix/tests/README.md b/tvix/tests/README.md
index b079a85dbddd..8cb4e9a6595c 100644
--- a/tvix/tests/README.md
+++ b/tvix/tests/README.md
@@ -61,22 +61,16 @@ Run the VM like this:
 CH_CMDLINE=tvix.shell mg run //tvix/tests:runVM --
 ```
 
-You'll get dropped into a shell, from which you can list the store contents:
-
-```
-[    0.282381] Run /init as init process
-2023/09/24 13:03:38 Welcome to u-root!
-                              _
-   _   _      _ __ ___   ___ | |_
-  | | | |____| '__/ _ \ / _ \| __|
-  | |_| |____| | | (_) | (_) | |_
-   \__,_|    |_|  \___/ \___/ \__|
-
-2023/09/24 13:03:38 Running tvix-init…
-2023/09/24 13:03:38 Creating /nix/store
-2023/09/24 13:03:38 Mounting…
-2023/09/24 13:03:38 Invoking shell
-…
+You'll get dropped into an interactive shell, from which you can do things with
+the store:
+
+```
+  ______      _         ____      _ __
+ /_  __/   __(_)  __   /  _/___  (_) /_
+  / / | | / / / |/_/   / // __ \/ / __/
+ / /  | |/ / />  <   _/ // / / / / /_
+/_/   |___/_/_/|_|  /___/_/ /_/_/\__/
+
 /# ls -la /nix/store/
 dr-xr-xr-x root 0 0   Jan  1 00:00 .
 dr-xr-xr-x root 0 989 Jan  1 00:00 aw2fw9ag10wr9pf0qk4nk5sxi0q0bn56-glibc-2.37-8
@@ -86,30 +80,63 @@ dr-xr-xr-x root 0 141 Jan  1 00:00 mdi7lvrn2mx7rfzv3fdq3v5yw8swiks6-hello-2.12.1
 dr-xr-xr-x root 0 5   Jan  1 00:00 s2gi8pfjszy6rq3ydx0z1vwbbskw994i-libunistring-1.1
 ```
 
-Once you're done, run `poweroff` to turn off the VM.
+Once you exit the shell, the VM will power off itself.
 
 #### Execute a specific binary
 Run the VM like this:
 
 ```
 hello_cmd=$(mg build //third_party/nixpkgs:hello)/bin/hello
-CH_CMDLINE=tvix.exec=$hello_cmd mg run //tvix/tests:runVM --
+CH_CMDLINE=tvix.run=$hello_cmd mg run //tvix/tests:runVM --
 ```
 
 Observe it executing the file (and closure) from the tvix-store:
 
 ```
-2023/09/24 13:06:13 Welcome to u-root!
-                              _
-   _   _      _ __ ___   ___ | |_
-  | | | |____| '__/ _ \ / _ \| __|
-  | |_| |____| | | (_) | (_) | |_
-   \__,_|    |_|  \___/ \___/ \__|
+[    0.277486] Run /init as init process
+  ______      _         ____      _ __
+ /_  __/   __(_)  __   /  _/___  (_) /_
+  / / | | / / / |/_/   / // __ \/ / __/
+ / /  | |/ / />  <   _/ // / / / / /_
+/_/   |___/_/_/|_|  /___/_/ /_/_/\__/
 
-2023/09/24 13:06:13 Running tvix-init…
-2023/09/24 13:06:13 Creating /nix/store
-2023/09/24 13:06:13 Mounting…
-2023/09/24 13:06:13 Invoking /nix/store/mdi7lvrn2mx7rfzv3fdq3v5yw8swiks6-hello-2.12.1/bin/hello
-…
 Hello, world!
-```
\ No newline at end of file
+2023/09/24 21:10:19 Nothing left to be done, powering off.
+[    0.299122] ACPI: PM: Preparing to enter system sleep state S5
+[    0.299422] reboot: Power down
+```
+
+#### Execute a NixOS system closure
+It's also possible to invoke a system closure. To do this, tvix-init honors the
+init= cmdline option, and will switch_root to it.
+
+
+```
+CH_CMDLINE=init=/nix/store/…-nixos-system-…/init mg run //tvix/tests:runVM --
+```
+
+```
+  ______      _         ____      _ __
+ /_  __/   __(_)  __   /  _/___  (_) /_
+  / / | | / / / |/_/   / // __ \/ / __/
+ / /  | |/ / />  <   _/ // / / / / /_
+/_/   |___/_/_/|_|  /___/_/ /_/_/\__/
+
+2023/09/24 21:16:43 switch_root: moving mounts
+2023/09/24 21:16:43 switch_root: Skipping "/run" as the dir does not exist
+2023/09/24 21:16:43 switch_root: Changing directory
+2023/09/24 21:16:43 switch_root: Moving /
+2023/09/24 21:16:43 switch_root: Changing root!
+2023/09/24 21:16:43 switch_root: Deleting old /
+2023/09/24 21:16:43 switch_root: executing init
+
+<<< NixOS Stage 2 >>>
+
+[    0.322096] booting system configuration /nix/store/g657sdxinpqfcdv0162zmb8vv9b5c4c5-nixos-system-client-23.11.git.82102fc37da
+running activation script...
+setting up /etc...
+starting systemd...
+[    0.980740] systemd[1]: systemd 253.6 running in system mode (+PAM +AUDIT -SELINUX +APPARMOR +IMA +SMACK +SECCOMP +GCRYPT -GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP +LIBFDISK +PCRE2 -PWQUALITY +P11KIT -QRENCODE +TPM2 +BZIP2 +LZ4 +XZ +ZLIB +ZSTD +BPF_FRAMEWORK -XKBCOMMON +UTMP -SYSVINIT default-hierarchy=unified)
+```
+
+This effectively replaces the NixOS Stage 1 entirely.
\ No newline at end of file
diff --git a/tvix/tests/default.nix b/tvix/tests/default.nix
index 9e9a22a66221..875fe0b9f1bf 100644
--- a/tvix/tests/default.nix
+++ b/tvix/tests/default.nix
@@ -58,7 +58,7 @@ rec {
       export HOME=$(mktemp -d)
       export GOROOT="$(go env GOROOT)"
 
-      GO111MODULE=off GOPATH=/tmp/go GOPROXY=off ${uroot}/bin/u-root -files ./tvix-init -uinitcmd "/tvix-init" -o $out
+      GO111MODULE=off GOPATH=/tmp/go GOPROXY=off ${uroot}/bin/u-root -files ./tvix-init -initcmd "/tvix-init" -o $out
     '';
   };
 
diff --git a/tvix/tests/tvix-init.go b/tvix/tests/tvix-init.go
index a81001159ae2..97a24bab3547 100644
--- a/tvix/tests/tvix-init.go
+++ b/tvix/tests/tvix-init.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"log"
 	"os"
 	"os/exec"
@@ -37,11 +38,41 @@ func parseCmdline(cmdline string) map[string]string {
 	return out
 }
 
+// mounts the nix store from the virtiofs tag to the given destination,
+// creating the destination if it doesn't exist already.
+func mountTvixStore(dest string) error {
+	if err := os.MkdirAll(dest, os.ModePerm); err != nil {
+		return fmt.Errorf("unable to mkdir dest: %w", err)
+	}
+	if err := run("mount", "-t", "virtiofs", "tvix", dest, "-o", "ro"); err != nil {
+		return fmt.Errorf("unable to run mount: %w", err)
+	}
+
+	return nil
+}
+
 func main() {
-	log.Println("Running tvix-init…")
+	fmt.Print(`
+  ______      _         ____      _ __
+ /_  __/   __(_)  __   /  _/___  (_) /_
+  / / | | / / / |/_/   / // __ \/ / __/
+ / /  | |/ / />  <   _/ // / / / / /_
+/_/   |___/_/_/|_|  /___/_/ /_/_/\__/
+
+`)
 
-	log.Println("Creating /nix/store")
-	os.MkdirAll("/nix/store", os.ModePerm)
+	// Set PATH to "/bbin", so we can find the u-root tools
+	os.Setenv("PATH", "/bbin")
+
+	if err := run("mount", "-t", "proc", "none", "/proc"); err != nil {
+		log.Printf("Failed to mount /proc: %v\n", err)
+	}
+	if err := run("mount", "-t", "sysfs", "none", "/sys"); err != nil {
+		log.Printf("Failed to mount /sys: %v\n", err)
+	}
+	if err := run("mount", "-t", "devtmpfs", "devtmpfs", "/dev"); err != nil {
+		log.Printf("Failed to mount /dev: %v\n", err)
+	}
 
 	cmdline, err := os.ReadFile("/proc/cmdline")
 	if err != nil {
@@ -49,37 +80,59 @@ func main() {
 	}
 	cmdlineFields := parseCmdline(string(cmdline))
 
-	log.Println("Mounting…")
-	if err := run("mount", "-t", "virtiofs", "tvix", "/nix/store", "-o", "ro"); err != nil {
-		log.Printf("Failed to run mount: %v\n", err)
-	}
-
-	// If tvix.find is set, invoke find /nix/store
 	if _, ok := cmdlineFields["tvix.find"]; ok {
-		log.Println("Listing…")
+		// If tvix.find is set, invoke find /nix/store
+		if err := mountTvixStore("/nix/store"); err != nil {
+			log.Printf("Failed to mount tvix store: %v\n", err)
+		}
+
 		if err := run("find", "/nix/store"); err != nil {
 			log.Printf("Failed to run find command: %s\n", err)
 		}
-	}
+	} else if _, ok := cmdlineFields["tvix.shell"]; ok {
+		// If tvix.shell is set, mount the nix store to /nix/store directly,
+		// then invoke the elvish shell
+		if err := mountTvixStore("/nix/store"); err != nil {
+			log.Printf("Failed to mount tvix store: %v\n", err)
+		}
 
-	// If tvix.shell is set, invoke the elvish shell
-	if v, ok := cmdlineFields["tvix.shell"]; ok {
-		log.Printf("Invoking shell%s\n…", v)
 		if err := run("elvish"); err != nil {
 			log.Printf("Failed to run shell: %s\n", err)
 		}
-	}
+	} else if v, ok := cmdlineFields["tvix.run"]; ok {
+		// If tvix.run is set, mount the nix store to /nix/store directly,
+		// then invoke the command.
+		if err := mountTvixStore("/nix/store"); err != nil {
+			log.Printf("Failed to mount tvix store: %v\n", err)
+		}
+
+		if err := run(v); err != nil {
+			log.Printf("Failed to run command: %s\n", err)
+		}
+	} else if v, ok := cmdlineFields["init"]; ok {
+		// If init is set, invoke the binary specified (with switch_root),
+		// and prepare /fs beforehand as well.
+		os.Mkdir("/fs", os.ModePerm)
+		if err := run("mount", "-t", "tmpfs", "none", "/fs"); err != nil {
+			log.Fatalf("Failed to mount /fs tmpfs: %s\n", err)
+		}
+
+		// Mount /fs/nix/store
+		if err := mountTvixStore("/fs/nix/store"); err != nil {
+			log.Fatalf("Failed to mount tvix store: %v\n", err)
+		}
 
-	// If tvix.exec is set, invoke the binary specified
-	if v, ok := cmdlineFields["tvix.exec"]; ok {
-		log.Printf("Invoking %s\n…", v)
-		if err := syscall.Exec(v, []string{v}, []string{}); err != nil {
-			log.Printf("Failed to exec: %s\n", err)
+		// Invoke switch_root, which will take care of moving /proc, /sys and /dev.
+		if err := syscall.Exec("/bbin/switch_root", []string{"switch_root", "/fs", v}, []string{}); err != nil {
+			log.Printf("Failed to switch root: %s\n", err)
 		}
+	} else {
+		log.Printf("No command detected, not knowing what to do!")
 	}
 
-	log.Println("Powering off")
+	// This is only reached in the non switch_root case.
+	log.Printf("Nothing left to be done, powering off.")
 	if err := run("poweroff"); err != nil {
-		log.Printf("Failed to run command: %v\n", err)
+		log.Printf("Failed to run poweroff command: %v\n", err)
 	}
 }