1 files changed, 129 insertions, 0 deletions
diff --git a/utils/src/shim.rs b/utils/src/shim.rs
new file mode 100644
index 0000000..504335c
--- /dev/null
+++ b/utils/src/shim.rs
@@ -0,0 +1,129 @@
+use anyhow::Context;
+use nix::errno::Errno;
+use nix::mount::{mount, MsFlags};
+use nix::sys::wait::{waitid, Id, WaitPidFlag};
+use nix::unistd::Pid;
+use std::env;
+use std::fs::{create_dir_all, metadata, remove_dir_all, remove_file, OpenOptions};
+use std::os::unix::io::{FromRawFd, IntoRawFd};
+use std::os::unix::process::CommandExt;
+use std::path::Path;
+use std::process::{Command, Stdio};
+
+fn unscrew_dev_shm() -> anyhow::Result<()> {
+    log::trace!("Unscrewing /dev/shm...");
+
+    let dev_shm = Path::new("/dev/shm");
+
+    if dev_shm.is_symlink() {
+        remove_file(dev_shm).context("When removing /dev/shm symlink")?;
+    } else if dev_shm.is_dir() {
+        remove_dir_all(dev_shm).context("When removing old /dev/shm")?;
+    }
+
+    create_dir_all("/dev/shm").context("When creating new /dev/shm")?;
+    mount(
+        Some("/run/shm"),
+        "/dev/shm",
+        None::<&str>,
+        MsFlags::MS_MOVE,
+        None::<&str>,
+    )
+    .context("When relocating /dev/shm")?;
+    mount(
+        Some("/dev/shm"),
+        "/run/shm",
+        None::<&str>,
+        MsFlags::MS_BIND,
+        None::<&str>,
+    )
+    .context("When bind mounting /run/shm to /dev/shm")?;
+
+    Ok(())
+}
+
+fn real_main() -> anyhow::Result<()> {
+    if metadata("/dev/shm")
+        .context("When checking /dev/shm")?
+        .is_symlink()
+    {
+        unscrew_dev_shm()?;
+    } else {
+        log::trace!("/dev/shm is not a symlink, leaving as-is...");
+    };
+
+    log::trace!("Remounting / shared...");
+
+    mount(
+        None::<&str>,
+        "/",
+        None::<&str>,
+        MsFlags::MS_REC | MsFlags::MS_SHARED,
+        None::<&str>,
+    )
+    .context("When remounting /")?;
+
+    log::trace!("Remounting /nix/store read-only...");
+
+    mount(
+        Some("/nix/store"),
+        "/nix/store",
+        None::<&str>,
+        MsFlags::MS_BIND,
+        None::<&str>,
+    )
+    .context("When bind mounting /nix/store")?;
+
+    mount(
+        Some("/nix/store"),
+        "/nix/store",
+        None::<&str>,
+        MsFlags::MS_BIND | MsFlags::MS_REMOUNT | MsFlags::MS_RDONLY,
+        None::<&str>,
+    )
+    .context("When remounting /nix/store read-only")?;
+
+    log::trace!("Running activation script...");
+
+    let kmsg_fd = OpenOptions::new()
+        .write(true)
+        .open("/dev/kmsg")
+        .context("When opening /dev/kmsg")?
+        .into_raw_fd();
+
+    let child = Command::new("/nix/var/nix/profiles/system/activate")
+        .env("LANG", "C.UTF-8")
+        // SAFETY: we just opened this
+        .stdout(unsafe { Stdio::from_raw_fd(kmsg_fd) })
+        .stderr(unsafe { Stdio::from_raw_fd(kmsg_fd) })
+        .spawn()
+        .context("When activating")?;
+
+    let pid = Pid::from_raw(child.id() as i32);
+
+    // If the child catches SIGCHLD, `waitid` will wait for it to exit, then return ECHILD.
+    // Why? Because POSIX is terrible.
+    let result = waitid(Id::Pid(pid), WaitPidFlag::WEXITED);
+    match result {
+        Ok(_) | Err(Errno::ECHILD) => {}
+        Err(e) => return Err(e).context("When waiting"),
+    };
+
+    log::trace!("Spawning real systemd...");
+
+    // if things go right, we will never return from here
+    Err(
+        Command::new("/nix/var/nix/profiles/system/systemd/lib/systemd/systemd")
+            .arg0(env::args_os().next().expect("arg0 missing"))
+            .args(env::args_os().skip(1))
+            .exec()
+            .into(),
+    )
+}
+
+fn main() {
+    env::set_var("RUST_BACKTRACE", "1");
+    kernlog::init().expect("Failed to set up logger...");
+    let result = real_main();
+    log::error!("Error: {:?}", result);
+}