efi: Skip adopting for systemd-boot and systemd-stub

If the system is booted with systemd-boot or systemd-stub then `bootctl`
will handle updating the boot components. Both of these components write
identifying information to EFI variables as NUL terminated UTF-16
strings. Check these variables to detect if either systemd-boot or
systemd-stub are in use and skip adoption if so.

Cargo.toml

@@ -34,6 +34,7 @@ openssl = "^0.10"
 serde = { version = "^1.0", features = ["derive"] }
 serde_json = "^1.0"
 tempfile = "^3.4"
+widestring = "1.0.2"
 
 [profile.release]
 # We assume we're being delivered via e.g. RPM which supports split debuginfo

src/efi.rs

@@ -12,6 +12,7 @@ use std::process::Command;
 
 use anyhow::{bail, Context, Result};
 use openat_ext::OpenatDirExt;
+use widestring::U16CString;
 
 use crate::component::*;
 use crate::filetree;
@@ -27,6 +28,10 @@ pub(crate) const ESP_MOUNTS: &[&str] = &["boot/efi", "efi"];
 pub(crate) const COREOS_ESP_PART_LABEL: &str = "EFI-SYSTEM";
 pub(crate) const ANACONDA_ESP_PART_LABEL: &str = "EFI\\x20System\\x20Partition";
 
+/// Systemd boot loader info EFI variable names
+const LOADER_INFO_VAR_STR: &str = "LoaderInfo-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f";
+const STUB_INFO_VAR_STR: &str = "StubInfo-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f";
+
 #[derive(Default)]
 pub(crate) struct Efi {
     mountpoint: RefCell<Option<PathBuf>>,
@@ -105,6 +110,70 @@ impl Efi {
     }
 }
 
+/// Convert a nul-terminated UTF-16 byte array to a String.
+fn string_from_utf16_bytes(slice: &[u8]) -> String {
+    // For some reason, systemd appends 3 nul bytes after the string.
+    // Drop the last byte if there's an odd number.
+    let size = slice.len() / 2;
+    let v: Vec<u16> = (0..size)
+        .map(|i| u16::from_ne_bytes([slice[2 * i], slice[2 * i + 1]]))
+        .collect();
+    U16CString::from_vec(v).unwrap().to_string_lossy()
+}
+
+/// Read a nul-terminated UTF-16 string from an EFI variable.
+fn read_efi_var_utf16_string(name: &str) -> Option<String> {
+    let efivars = Path::new("/sys/firmware/efi/efivars");
+    if !efivars.exists() {
+        log::warn!("No efivars mount at {:?}", efivars);
+        return None;
+    }
+    let path = efivars.join(name);
+    if !path.exists() {
+        log::trace!("No EFI variable {name}");
+        return None;
+    }
+    match std::fs::read(&path) {
+        Ok(buf) => {
+            // Skip the first 4 bytes, those are the EFI variable attributes.
+            if buf.len() < 4 {
+                log::warn!("Read less than 4 bytes from {:?}", path);
+                return None;
+            }
+            Some(string_from_utf16_bytes(&buf[4..]))
+        }
+        Err(reason) => {
+            log::warn!("Failed reading {:?}: {reason}", path);
+            None
+        }
+    }
+}
+
+/// Read the LoaderInfo EFI variable if it exists.
+fn get_loader_info() -> Option<String> {
+    read_efi_var_utf16_string(LOADER_INFO_VAR_STR)
+}
+
+/// Read the StubInfo EFI variable if it exists.
+fn get_stub_info() -> Option<String> {
+    read_efi_var_utf16_string(STUB_INFO_VAR_STR)
+}
+
+/// Whether to skip adoption if a systemd bootloader is found.
+fn skip_systemd_bootloaders() -> bool {
+    if let Some(loader_info) = get_loader_info() {
+        if loader_info.starts_with("systemd") {
+            log::trace!("Skipping adoption for {:?}", loader_info);
+            return true;
+        }
+    }
+    if let Some(stub_info) = get_stub_info() {
+        log::trace!("Skipping adoption for {:?}", stub_info);
+        return true;
+    }
+    false
+}
+
 impl Component for Efi {
     fn name(&self) -> &'static str {
         "EFI"
@@ -130,6 +199,11 @@ impl Component for Efi {
         } else {
             log::trace!("No CoreOS aleph detected");
         }
+        // Don't adopt if the system is booted with systemd-boot or
+        // systemd-stub since those will be managed with bootctl.
+        if skip_systemd_bootloaders() {
+            return Ok(None);
+        }
         let ostree_deploy_dir = Path::new("/ostree/deploy");
         if ostree_deploy_dir.exists() {
             let btime = ostree_deploy_dir.metadata()?.created()?;