diff --git a/Cargo.lock b/Cargo.lock
index ae2b741b9..8f5ac5e81 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -191,6 +191,12 @@ name = "bootc-initramfs-setup"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "clap",
+ "composefs",
+ "composefs-boot",
+ "rustix 1.0.8",
+ "serde",
+ "toml 0.9.5",
 ]
 
 [[package]]
diff --git a/crates/initramfs/Cargo.toml b/crates/initramfs/Cargo.toml
index 81bd62973..82950f93e 100644
--- a/crates/initramfs/Cargo.toml
+++ b/crates/initramfs/Cargo.toml
@@ -7,6 +7,17 @@ publish = false
 
 [dependencies]
 anyhow.workspace = true
+clap = { workspace = true, features = ["std", "help", "usage", "derive"] }
+rustix.workspace = true
+serde = { workspace = true, features = ["derive"] }
+composefs.workspace = true
+composefs-boot.workspace = true
+toml.workspace = true
 
 [lints]
 workspace = true
+
+[features]
+default = ['pre-6.15']
+rhel9 = ['composefs/rhel9']
+'pre-6.15' = ['composefs/pre-6.15']
diff --git a/crates/initramfs/bootc-root-setup.service b/crates/initramfs/bootc-root-setup.service
index aab7de785..23525c7bc 100644
--- a/crates/initramfs/bootc-root-setup.service
+++ b/crates/initramfs/bootc-root-setup.service
@@ -2,8 +2,7 @@
 Description=bootc setup root
 Documentation=man:bootc(1)
 DefaultDependencies=no
-# For now
-ConditionKernelCommandLine=ostree
+ConditionKernelCommandLine=composefs
 ConditionPathExists=/etc/initrd-release
 After=sysroot.mount
 After=ostree-prepare-root.service
diff --git a/crates/initramfs/src/main.rs b/crates/initramfs/src/main.rs
index 51b5f551d..088c57a5a 100644
--- a/crates/initramfs/src/main.rs
+++ b/crates/initramfs/src/main.rs
@@ -1,24 +1,15 @@
 //! Code for bootc that goes into the initramfs.
-//! At the current time, this is mostly just a no-op.
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+mod mount;
+
 use anyhow::Result;
 
-fn setup_root() -> Result<()> {
-    let _ = std::fs::metadata("/sysroot/usr")?;
-    println!("setup OK");
-    Ok(())
-}
+use clap::Parser;
+use mount::{gpt_workaround, setup_root, Args};
 
 fn main() -> Result<()> {
-    let v = std::env::args().collect::<Vec<_>>();
-    let args = match v.as_slice() {
-        [] => anyhow::bail!("Missing argument".to_string()),
-        [_, rest @ ..] => rest,
-    };
-    match args {
-        [] => anyhow::bail!("Missing argument".to_string()),
-        [s] if s == "setup-root" => setup_root(),
-        [o, ..] => anyhow::bail!(format!("Unknown command {o}")),
-    }
+    let args = Args::parse();
+    gpt_workaround()?;
+    setup_root(args)
 }
diff --git a/crates/initramfs/src/mount.rs b/crates/initramfs/src/mount.rs
new file mode 100644
index 000000000..e1e2516ee
--- /dev/null
+++ b/crates/initramfs/src/mount.rs
@@ -0,0 +1,294 @@
+//! Mount helpers for bootc-initramfs
+
+use std::{
+    ffi::OsString,
+    fmt::Debug,
+    io::ErrorKind,
+    os::fd::{AsFd, OwnedFd},
+    path::{Path, PathBuf},
+};
+
+use anyhow::{Context, Result};
+use clap::Parser;
+use rustix::{
+    fs::{major, minor, mkdirat, openat, stat, symlink, Mode, OFlags, CWD},
+    io::Errno,
+    mount::{
+        fsconfig_create, fsconfig_set_string, fsmount, open_tree, unmount, FsMountFlags,
+        MountAttrFlags, OpenTreeFlags, UnmountFlags,
+    },
+};
+use serde::Deserialize;
+
+use composefs::{
+    fsverity::{FsVerityHashValue, Sha256HashValue},
+    mount::{mount_at, FsHandle},
+    mountcompat::{overlayfs_set_fd, overlayfs_set_lower_and_data_fds, prepare_mount},
+    repository::Repository,
+};
+use composefs_boot::cmdline::get_cmdline_composefs;
+
+// Config file
+#[derive(Clone, Copy, Debug, Deserialize)]
+#[serde(rename_all = "lowercase")]
+enum MountType {
+    None,
+    Bind,
+    Overlay,
+    Transient,
+}
+
+#[derive(Debug, Default, Deserialize)]
+struct RootConfig {
+    #[serde(default)]
+    transient: bool,
+}
+
+#[derive(Debug, Default, Deserialize)]
+struct MountConfig {
+    mount: Option<MountType>,
+    #[serde(default)]
+    transient: bool,
+}
+
+#[derive(Deserialize, Default)]
+struct Config {
+    #[serde(default)]
+    etc: MountConfig,
+    #[serde(default)]
+    var: MountConfig,
+    #[serde(default)]
+    root: RootConfig,
+}
+
+/// Command-line arguments
+#[derive(Parser, Debug)]
+#[command(version)]
+pub struct Args {
+    #[arg(help = "Execute this command (for testing)")]
+    /// Execute this command (for testing)
+    pub cmd: Vec<OsString>,
+
+    #[arg(
+        long,
+        default_value = "/sysroot",
+        help = "sysroot directory in initramfs"
+    )]
+    /// sysroot directory in initramfs
+    pub sysroot: PathBuf,
+
+    #[arg(
+        long,
+        default_value = "/usr/lib/composefs/setup-root-conf.toml",
+        help = "Config path (for testing)"
+    )]
+    /// Config path (for testing)
+    pub config: PathBuf,
+
+    // we want to test in a userns, but can't mount erofs there
+    #[arg(long, help = "Bind mount root-fs from (for testing)")]
+    /// Bind mount root-fs from (for testing)
+    pub root_fs: Option<PathBuf>,
+
+    #[arg(long, help = "Kernel commandline args (for testing)")]
+    /// Kernel commandline args (for testing)
+    pub cmdline: Option<String>,
+
+    #[arg(long, help = "Mountpoint (don't replace sysroot, for testing)")]
+    /// Mountpoint (don't replace sysroot, for testing)
+    pub target: Option<PathBuf>,
+}
+
+// Helpers
+fn open_dir(dirfd: impl AsFd, name: impl AsRef<Path> + Debug) -> rustix::io::Result<OwnedFd> {
+    openat(
+        dirfd,
+        name.as_ref(),
+        OFlags::PATH | OFlags::DIRECTORY | OFlags::CLOEXEC,
+        Mode::empty(),
+    )
+    .inspect_err(|_| {
+        eprintln!("Failed to open dir {name:?}");
+    })
+}
+
+fn ensure_dir(dirfd: impl AsFd, name: &str) -> rustix::io::Result<OwnedFd> {
+    match mkdirat(dirfd.as_fd(), name, 0o700.into()) {
+        Ok(()) | Err(Errno::EXIST) => {}
+        Err(err) => Err(err)?,
+    }
+    open_dir(dirfd, name)
+}
+
+fn bind_mount(fd: impl AsFd, path: &str) -> rustix::io::Result<OwnedFd> {
+    open_tree(
+        fd.as_fd(),
+        path,
+        OpenTreeFlags::OPEN_TREE_CLONE
+            | OpenTreeFlags::OPEN_TREE_CLOEXEC
+            | OpenTreeFlags::AT_EMPTY_PATH,
+    )
+    .inspect_err(|_| {
+        eprintln!("Open tree failed for {path}");
+    })
+}
+
+fn mount_tmpfs() -> Result<OwnedFd> {
+    let tmpfs = FsHandle::open("tmpfs")?;
+    fsconfig_create(tmpfs.as_fd())?;
+    Ok(fsmount(
+        tmpfs.as_fd(),
+        FsMountFlags::FSMOUNT_CLOEXEC,
+        MountAttrFlags::empty(),
+    )?)
+}
+
+fn overlay_state(base: impl AsFd, state: impl AsFd, source: &str) -> Result<()> {
+    let upper = ensure_dir(state.as_fd(), "upper")?;
+    let work = ensure_dir(state.as_fd(), "work")?;
+
+    let overlayfs = FsHandle::open("overlay")?;
+    fsconfig_set_string(overlayfs.as_fd(), "source", source)?;
+    overlayfs_set_fd(overlayfs.as_fd(), "workdir", work.as_fd())?;
+    overlayfs_set_fd(overlayfs.as_fd(), "upperdir", upper.as_fd())?;
+    overlayfs_set_lower_and_data_fds(&overlayfs, base.as_fd(), None::<OwnedFd>)?;
+    fsconfig_create(overlayfs.as_fd())?;
+    let fs = fsmount(
+        overlayfs.as_fd(),
+        FsMountFlags::FSMOUNT_CLOEXEC,
+        MountAttrFlags::empty(),
+    )?;
+
+    Ok(mount_at(fs, base, ".")?)
+}
+
+fn overlay_transient(base: impl AsFd) -> Result<()> {
+    overlay_state(base, prepare_mount(mount_tmpfs()?)?, "transient")
+}
+
+fn open_root_fs(path: &Path) -> Result<OwnedFd> {
+    let rootfs = open_tree(
+        CWD,
+        path,
+        OpenTreeFlags::OPEN_TREE_CLONE | OpenTreeFlags::OPEN_TREE_CLOEXEC,
+    )?;
+
+    // https://github.com/bytecodealliance/rustix/issues/975
+    // mount_setattr(rootfs.as_fd()), ..., { ... MountAttrFlags::MOUNT_ATTR_RDONLY ... }, ...)?;
+
+    Ok(rootfs)
+}
+
+/// Prepares a floating mount for composefs and returns the fd
+///
+/// # Arguments
+/// * sysroot  - fd for /sysroot
+/// * name     - Name of the EROFS image to be mounted
+/// * insecure - Whether fsverity is optional or not
+pub fn mount_composefs_image(sysroot: &OwnedFd, name: &str, insecure: bool) -> Result<OwnedFd> {
+    let mut repo = Repository::<Sha256HashValue>::open_path(sysroot, "composefs")?;
+    repo.set_insecure(insecure);
+    repo.mount(name).context("Failed to mount composefs image")
+}
+
+fn mount_subdir(
+    new_root: impl AsFd,
+    state: impl AsFd,
+    subdir: &str,
+    config: MountConfig,
+    default: MountType,
+) -> Result<()> {
+    let mount_type = match config.mount {
+        Some(mt) => mt,
+        None => match config.transient {
+            true => MountType::Transient,
+            false => default,
+        },
+    };
+
+    match mount_type {
+        MountType::None => Ok(()),
+        MountType::Bind => Ok(mount_at(bind_mount(&state, subdir)?, &new_root, subdir)?),
+        MountType::Overlay => overlay_state(
+            open_dir(&new_root, subdir)?,
+            open_dir(&state, subdir)?,
+            "overlay",
+        ),
+        MountType::Transient => overlay_transient(open_dir(&new_root, subdir)?),
+    }
+}
+
+pub(crate) fn gpt_workaround() -> Result<()> {
+    // https://github.com/systemd/systemd/issues/35017
+    let rootdev = stat("/dev/gpt-auto-root");
+
+    let rootdev = match rootdev {
+        Ok(r) => r,
+        Err(e) if e.kind() == ErrorKind::NotFound => return Ok(()),
+        Err(e) => Err(e)?,
+    };
+
+    let target = format!(
+        "/dev/block/{}:{}",
+        major(rootdev.st_rdev),
+        minor(rootdev.st_rdev)
+    );
+    symlink(target, "/run/systemd/volatile-root")?;
+    Ok(())
+}
+
+/// Sets up /sysroot for switch-root
+pub fn setup_root(args: Args) -> Result<()> {
+    let config = match std::fs::read_to_string(args.config) {
+        Ok(text) => toml::from_str(&text)?,
+        Err(err) if err.kind() == ErrorKind::NotFound => Config::default(),
+        Err(err) => Err(err)?,
+    };
+
+    let sysroot = open_dir(CWD, &args.sysroot)
+        .with_context(|| format!("Failed to open sysroot {:?}", args.sysroot))?;
+
+    let cmdline = match &args.cmdline {
+        Some(cmdline) => cmdline,
+        // TODO: Deduplicate this with composefs branch karg parser
+        None => &std::fs::read_to_string("/proc/cmdline")?,
+    };
+    let (image, insecure) = get_cmdline_composefs::<Sha256HashValue>(cmdline)?;
+
+    let new_root = match args.root_fs {
+        Some(path) => open_root_fs(&path).context("Failed to clone specified root fs")?,
+        None => mount_composefs_image(&sysroot, &image.to_hex(), insecure)?,
+    };
+
+    // we need to clone this before the next step to make sure we get the old one
+    let sysroot_clone = bind_mount(&sysroot, "")?;
+
+    // Ideally we build the new root filesystem together before we mount it, but that only works on
+    // 6.15 and later.  Before 6.15 we can't mount into a floating tree, so mount it first.  This
+    // will leave an abandoned clone of the sysroot mounted under it, but that's OK for now.
+    if cfg!(feature = "pre-6.15") {
+        mount_at(&new_root, CWD, &args.sysroot)?;
+    }
+
+    if config.root.transient {
+        overlay_transient(&new_root)?;
+    }
+
+    match mount_at(&sysroot_clone, &new_root, "sysroot") {
+        Ok(()) | Err(Errno::NOENT) => {}
+        Err(err) => Err(err)?,
+    }
+
+    // etc + var
+    let state = open_dir(open_dir(&sysroot, "state/deploy")?, image.to_hex())?;
+    mount_subdir(&new_root, &state, "etc", config.etc, MountType::Overlay)?;
+    mount_subdir(&new_root, &state, "var", config.var, MountType::Bind)?;
+
+    if cfg!(not(feature = "pre-6.15")) {
+        // Replace the /sysroot with the new composed root filesystem
+        unmount(&args.sysroot, UnmountFlags::DETACH)?;
+        mount_at(&new_root, CWD, &args.sysroot)?;
+    }
+
+    Ok(())
+}