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composefs: Fix uki boot with grub2 on fedora aarch64 #2527

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composefs: Fix uki boot with grub2 on fedora aarch64 #2527

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33 changes: 33 additions & 0 deletions crates/lib/src/bootc_composefs/boot.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -142,6 +142,18 @@ const AUTH_EXT: &str = "auth";
/// This is relative to the ESP
pub(crate) const BOOTC_UKI_DIR: &str = "EFI/Linux/bootc";

// These are the modules we insmod in out menu config files
const GRUB_MODULES_USED: [&str; 2] = ["fat.mod", "chain.mod"];

fn grub_efi_module_dir() -> Option<&'static str> {
match std::env::consts::ARCH {
"aarch64" => Some("arm64-efi"),
"x86_64" => Some("x86_64-efi"),
"riscv64" => Some("riscv64-efi"),
_ => None,
}
}

/// Directory (relative to the ESP) where systemd-stub looks for UKI addons that apply
/// to *every* UKI, as opposed to addons scoped to a single UKI (which live alongside
/// it under [`BOOTC_UKI_DIR`]). Unlike per-UKI addons, these aren't tied to a single
Expand Down Expand Up @@ -2158,6 +2170,27 @@ pub(crate) async fn setup_composefs_boot(
Some(bind_boot_path.as_path()),
)?;

if matches!(postfetch.detected_bootloader, Bootloader::Grub) {
// The menu config files insmod some modules, these may be either built
// into the GRUB EFI binary, or loaded from /boot/grub2. For example, the
// fedora aarch64 grub doesn't include chain.mod. We install all the modules
// we can find to avoid potential problems.
Comment on lines +2174 to +2177

@cgwalters cgwalters Oct 2, 2026

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This should be bootupd's job

@alexlarsson alexlarsson Oct 2, 2026

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How can bootupd know that we're using these particular grub modules though? Maybe it should copy all modules?

@alexlarsson alexlarsson Oct 2, 2026

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I guess it could copy all modules? Its not that big anyway.

@cgwalters cgwalters Oct 2, 2026

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Right now if a static grub config is used (which is the default for bootupd) that static config is embedded in that package. And that static config should certainly include or install modules it needs.

I really want to avoid having bootc know too many details of grub because it's a huge mess, that stuff should get squirreled away in bootupd so we can keep this project kind of detached from the reality of the so many years of technical debt.

One hope is that grub-cc fixes some of this.

But actually though,

fedora aarch64 grub doesn't include chain.mod.

Isn't that just something to fix there?

I mean let's at least weigh options like that

@alexlarsson alexlarsson Oct 2, 2026

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Honestly, i think for fedora we should probably fix it in the grub build. However, we might want to support non-fedora bootc images that aren't built that way? I'll have a look at the bootupd side.

@alexlarsson alexlarsson Oct 2, 2026

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I filed a fedora bug, lets see what they think: https://bugzilla.redhat.com/show_bug.cgi?id=2545182

@alexlarsson alexlarsson Oct 2, 2026

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Here is the bootupd version of this: coreos/bootupd#1165

if let Some(module_dir) = grub_efi_module_dir() {
for module in GRUB_MODULES_USED {
let source = format!("usr/lib/grub/{module_dir}/{module}");
if mounted_root.dir().try_exists(&source)? {
let target = format!("boot/grub2/{module_dir}");
root_setup.physical_root.create_dir_all(&target)?;
let target = root_setup.physical_root.open_dir(&target)?;
mounted_root
.dir()
.copy(&source, &target, module)
.with_context(|| format!("Installing GRUB module {module}"))?;
}
}
}
}

// FIXME: Remove this hack once we have support in bootupd
if matches!(postfetch.detected_bootloader, Bootloader::GrubCC) {
// bootupctl wrote this under the physical root's real /boot (via
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