install: Add discoverable-partitions config option

Right now we default to DPS for composefs + systemd-boot.

In Fedora 43+, GRUB has the `bli` module and supports this,
so it *can* be used there.

Make this configurable (mainly intended for base image builders)
so that those with new enough GRUB can flip it on by default.

We had a hacky thing here that removed the auto-injected `root=`
arg if we detected composefs + sdboot; that can now instead
flip on this flag, and then we ensure we don't inject it
at all.

Assisted-by: OpenCode (Claude Opus 4)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

contrib/packaging/install-rpm-and-setup

@@ -21,5 +21,16 @@ env DRACUT_NO_XATTR=1 dracut --add bootc -vf /usr/lib/modules/$kver/initramfs.im
 # tests to know we're doing upstream CI.
 touch /usr/lib/.bootc-dev-stamp
 
+# Fedora 43+ ships a GRUB with the BLI module, so enable DPS
+# auto-discovery for root.  This must run after our RPM is installed
+# since older bootc doesn't recognize the discoverable-partitions key.
+. /usr/lib/os-release
+if [ "${ID}" = "fedora" ] && [ "${VERSION_ID}" -ge 43 ] 2>/dev/null; then
+  cat > /usr/lib/bootc/install/20-discoverable-partitions.toml <<'EOF'
+[install]
+discoverable-partitions = true
+EOF
+fi
+
 # Workaround for https://github.com/bootc-dev/bootc/issues/1546
 rm -rf /root/buildinfo /var/roothome/buildinfo

crates/lib/src/bootc_composefs/boot.rs

@@ -67,7 +67,7 @@ use std::io::Write;
 use std::path::Path;
 
 use anyhow::{Context, Result, anyhow, bail};
-use bootc_kernel_cmdline::utf8::{Cmdline, Parameter, ParameterKey};
+use bootc_kernel_cmdline::utf8::{Cmdline, Parameter};
 use bootc_mount::tempmount::TempMount;
 use camino::{Utf8Path, Utf8PathBuf};
 use cap_std_ext::{
@@ -582,12 +582,6 @@ pub(crate) fn setup_composefs_bls_boot(
         }
     };
 
-    // Remove "root=" from kernel cmdline as systemd-auto-gpt-generator should use DPS
-    // UUID
-    if bootloader == Bootloader::Systemd {
-        cmdline_refs.remove(&ParameterKey::from("root"));
-    }
-
     let is_upgrade = matches!(setup_type, BootSetupType::Upgrade(..));
 
     let current_root = if is_upgrade {

crates/lib/src/install/baseline.rs

@@ -34,6 +34,31 @@ use bootc_kernel_cmdline::utf8::Cmdline;
 #[cfg(feature = "install-to-disk")]
 use bootc_mount::is_mounted_in_pid1_mountns;
 
+/// Check whether DPS auto-discovery is enabled.  When `true`,
+/// `root=UUID=` is omitted and `systemd-gpt-auto-generator` discovers
+/// the root partition via its DPS type GUID instead.
+///
+/// Defaults to `true` for systemd-boot (which always implements BLI).
+/// For GRUB the default is `false` because we cannot know at install
+/// time whether the GRUB build includes the `bli` module — the module
+/// is baked into the signed EFI binary with no external manifest.
+/// Distros shipping a BLI-capable GRUB should set
+/// `discoverable-partitions = true` in their install config.
+#[cfg(feature = "install-to-disk")]
+fn use_discoverable_partitions(state: &State) -> bool {
+    // Explicit config takes priority
+    if let Some(ref config) = state.install_config {
+        if let Some(v) = config.discoverable_partitions {
+            return v;
+        }
+    }
+    // systemd-boot always supports BLI
+    matches!(
+        state.config_opts.bootloader,
+        Some(crate::spec::Bootloader::Systemd)
+    )
+}
+
 // This ensures we end up under 512 to be small-sized.
 pub(crate) const BOOTPN_SIZE_MB: u32 = 510;
 pub(crate) const EFIPN_SIZE_MB: u32 = 512;
@@ -226,10 +251,15 @@ pub(crate) fn install_create_rootfs(
     };
     let serial = device.serial.as_deref().unwrap_or("<unknown>");
     let model = device.model.as_deref().unwrap_or("<unknown>");
+    let discoverable = use_discoverable_partitions(state);
     println!("Block setup: {block_setup}");
     println!("       Size: {}", device.size);
     println!("     Serial: {serial}");
     println!("      Model: {model}");
+    println!(
+        " Partitions: {}",
+        if discoverable { "Discoverable" } else { "UUID" }
+    );
 
     let root_size = opts
         .root_size
@@ -415,7 +445,6 @@ pub(crate) fn install_create_rootfs(
         opts.wipe,
         mkfs_options.iter().copied(),
     )?;
-    let rootarg = format!("root=UUID={root_uuid}");
     let bootsrc = boot_uuid.as_ref().map(|uuid| format!("UUID={uuid}"));
     let bootarg = bootsrc.as_deref().map(|bootsrc| format!("boot={bootsrc}"));
     let boot = bootsrc.map(|bootsrc| MountSpec {
@@ -434,8 +463,14 @@ pub(crate) fn install_create_rootfs(
         }
     }
 
-    // Add root= and rw argument
-    kargs.extend(&Cmdline::from(format!("{rootarg} {RW_KARG}")));
+    // When discoverable-partitions is enabled, omit root= so that
+    // systemd-gpt-auto-generator discovers root by its DPS type GUID.
+    if discoverable {
+        kargs.extend(&Cmdline::from(RW_KARG));
+    } else {
+        let rootarg = format!("root=UUID={root_uuid}");
+        kargs.extend(&Cmdline::from(format!("{rootarg} {RW_KARG}")));
+    }
 
     // Add boot= argument if present
     if let Some(bootarg) = bootarg {

crates/lib/src/install/config.rs

@@ -119,6 +119,13 @@ pub(crate) struct InstallConfiguration {
     pub(crate) bootupd: Option<Bootupd>,
     /// Bootloader to use (grub, systemd, none)
     pub(crate) bootloader: Option<Bootloader>,
+    /// Use the Discoverable Partitions Specification for root partition
+    /// discovery.  When true, the `root=` kernel argument is omitted
+    /// and `systemd-gpt-auto-generator` discovers root via its DPS
+    /// type GUID.  Requires the bootloader to implement the Boot Loader
+    /// Interface (systemd-boot always does, GRUB needs the `bli` module).
+    /// Defaults to false for broad compatibility.
+    pub(crate) discoverable_partitions: Option<bool>,
 }
 
 fn merge_basic<T>(s: &mut Option<T>, o: Option<T>, _env: &EnvProperties) {
@@ -203,6 +210,11 @@ impl Mergeable for InstallConfiguration {
             merge_basic(&mut self.boot_mount_spec, other.boot_mount_spec, env);
             self.bootupd.merge(other.bootupd, env);
             merge_basic(&mut self.bootloader, other.bootloader, env);
+            merge_basic(
+                &mut self.discoverable_partitions,
+                other.discoverable_partitions,
+                env,
+            );
             if let Some(other_kargs) = other.kargs {
                 self.kargs
                     .get_or_insert_with(Default::default)
@@ -876,3 +888,30 @@ bootloader = "grub"
     install.merge(other, &env);
     assert_eq!(install.bootloader, Some(Bootloader::None));
 }
+
+#[test]
+fn test_parse_discoverable_partitions() {
+    let c: InstallConfigurationToplevel = toml::from_str(
+        r##"[install]
+discoverable-partitions = true
+"##,
+    )
+    .unwrap();
+    assert_eq!(c.install.unwrap().discoverable_partitions, Some(true));
+
+    let c: InstallConfigurationToplevel = toml::from_str(
+        r##"[install]
+discoverable-partitions = false
+"##,
+    )
+    .unwrap();
+    assert_eq!(c.install.unwrap().discoverable_partitions, Some(false));
+
+    let c: InstallConfigurationToplevel = toml::from_str(
+        r##"[install]
+root-fs-type = "xfs"
+"##,
+    )
+    .unwrap();
+    assert_eq!(c.install.unwrap().discoverable_partitions, None);
+}

docs/src/man/bootc-install-config.5.md

@@ -33,6 +33,12 @@ The `install` section supports these subfields:
 - `boot-mount-spec`: A string specifying the /boot filesystem mount specification.
    If not provided and /boot is a separate mount, its UUID will be used.
    An empty string signals to omit boot mount kargs entirely.
+- `discoverable-partitions`: Boolean.  When `true`, root discovery uses the
+   Discoverable Partitions Specification via `systemd-gpt-auto-generator` and
+   the `root=` kernel argument is omitted.  This requires the bootloader to
+   implement the Boot Loader Interface (BLI); systemd-boot always does, GRUB
+   needs the `bli` module (available in newer builds).  Defaults to `true`
+   when using systemd-boot, `false` otherwise.
 
 # filesystem
 
@@ -78,6 +84,13 @@ boot-mount-spec = "UUID=abcd-1234"
 bls-append-except-default = 'grub_users=""'
 ```
 
+Enable DPS auto-discovery for root (requires a BLI-capable bootloader):
+
+```toml
+[install]
+discoverable-partitions = true
+```
+
 # SEE ALSO
 
 **bootc(1)**

docs/src/man/bootc-install-to-disk.8.md

@@ -41,15 +41,25 @@ use `install to-filesystem` if you need precise control over the partition layou
 
 ### Root filesystem discovery
 
-Note that by default when used with "type 1" bootloader setups (i.e. non-UKI)
-a kernel argument `root=UUID=<uuid of filesystem>` is injected by default.
-This provides compatibility with existing initramfs implementations.
-
-When used with the composefs backend and UKIs, it's recommended that
-a bootloader implementing the DPS specification is used and that the root
-partition is auto-discovered. In this configuration, `systemd-gpt-auto-generator`
-in the initramfs will automatically find and mount the root partition based on
-its DPS type GUID, without requiring an explicit `root=` kernel argument.
+The root partition can be discovered at boot time in two ways:
+
+- **UUID mode** (default): A kernel argument `root=UUID=<uuid>` is
+  injected, providing broad compatibility with all initramfs
+  implementations and bootloaders.
+
+- **DPS auto-discovery**: The `root=` kernel argument is omitted
+  entirely.  `systemd-gpt-auto-generator` in the initramfs discovers
+  the root partition by its DPS type GUID.  This enables transparent
+  block-layer changes (such as adding LUKS encryption) without
+  updating kernel arguments.  DPS auto-discovery requires the
+  bootloader to implement the Boot Loader Interface (BLI).
+  systemd-boot always supports this; GRUB supports it only with
+  newer builds that include the `bli` module.
+
+When using systemd-boot, DPS auto-discovery is enabled by default.
+For GRUB, container base images that ship a BLI-capable build should
+set `discoverable-partitions = true` in their install configuration
+(see **bootc-install-config**(5)).
 
 # OPTIONS
 

tmt/tests/booted/readonly/052-test-bli-detection.nu

@@ -0,0 +1,30 @@
+use std assert
+use tap.nu
+
+tap begin "DPS root discovery when partition-uuids is false"
+
+# Parse os-release
+let os = open /usr/lib/os-release
+    | lines
+    | filter {|l| $l != "" and not ($l | str starts-with "#") }
+    | parse "{key}={value}"
+    | reduce -f {} {|it, acc| $acc | upsert $it.key ($it.value | str trim -c '"') }
+
+let os_id = ($os.ID? | default "unknown")
+let version_id = ($os.VERSION_ID? | default "0" | into int)
+
+# We inject this in our builds, but hopefully C10S gets this too at some point
+if not ($os_id == "fedora" and $version_id >= 43) {
+    print $"# skip: only applies to Fedora 43+ \(found ($os_id) ($version_id)\)"
+    tap ok
+    exit 0
+}
+
+print $"Running on ($os_id) ($version_id), checking DPS root discovery"
+
+let cmdline = (open /proc/cmdline)
+let has_root_karg = ($cmdline | str contains "root=")
+
+assert (not $has_root_karg) "Fedora 43+ should use DPS auto-discovery (no root= in cmdline)"
+
+tap ok