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205 lines
10 KiB
XML
205 lines
10 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd-stub" conditional='ENABLE_EFI'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-stub</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-stub</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-stub</refname>
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<refname>linuxx64.efi.stub</refname>
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<refname>linuxia32.efi.stub</refname>
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<refname>linuxaa64.efi.stub</refname>
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<refpurpose>A simple UEFI kernel boot stub</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub</filename></para>
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<para><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub</filename></para>
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<para><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-stub</command> (stored in per-architecture files
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<filename>linuxx64.efi.stub</filename>, <filename>linuxia32.efi.stub</filename>,
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<filename>linuxaa64.efi.stub</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
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attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
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environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
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kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
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system into the Linux world.</para>
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<para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
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itself. This allows combining various resources inside a single PE binary image, which may then be signed
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via UEFI SecureBoot as a whole, covering all individual resources at once. Specifically it may
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include:</para>
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<itemizedlist>
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<listitem><para>The ELF Linux kernel images will be looked for in the <literal>.linux</literal> PE
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section of the executed image.</para></listitem>
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<listitem><para>The initial RAM disk (initrd) will be looked for in the <literal>.initrd</literal> PE
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section.</para></listitem>
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<listitem><para>The kernel command line to pass to the invoked kernel will be looked for in the
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<literal>.cmdline</literal> PE section.</para></listitem>
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<listitem><para>A boot splash (in Windows <filename>.BMP</filename> format) to show on screen before
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invoking the kernel will be looked for in the <literal>.splash</literal> PE section.</para></listitem>
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</itemizedlist>
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<para>If UEFI SecureBoot is enabled and the <literal>.cmdline</literal> section is present in the executed
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image, any attempts to override the kernel command line by passing one as invocation parameters to the
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EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
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SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
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is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR 8 (if a TPM is
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present).</para>
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</refsect1>
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<refsect1>
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<title>Companion Files</title>
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<para>The <command>systemd-stub</command> UEFI boot stub automatically collects two types of auxiliary
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companion files optionally placed in a drop-in directory next to the EFI binary and dynamically generates
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<command>cpio</command> initrd archives from them, and passes them to the kernel. Specifically:</para>
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<itemizedlist>
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<listitem><para>For a kernel binary called <filename><replaceable>foo</replaceable>.efi</filename> it
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will look for files with the <filename>.cred</filename> suffix in a directory named
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<filename><replaceable>foo</replaceable>.efi.extra.d/</filename>, next to it. A <command>cpio</command>
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archive is generated from all files found that way, placing them in the
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<filename>/.extra/credentials/</filename> directory of the initrd file hierarchy. The main initrd may
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then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
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authenticated credentials for use with <varname>LoadCredentialEncrypted=</varname> in the UEFI System
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Partition. See
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
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details on encrypted credentials. The generated <command>cpio</command> archive is measured into TPM
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PCR 4 (if a TPM is present)</para></listitem>
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<listitem><para>Similar, files <filename><replaceable>foo</replaceable>.efi.extra.d/*.raw</filename>
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are packed up as <command>cpio</command> archive and placed in the <filename>/.extra/sysext/</filename>
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directory in the initrd file hierarchy. This is supposed to be used to pass additional system extension
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images to the initrd. See
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<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
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details on system extension images. The generated <command>cpio</command> archive containing these
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system extension images is measured into TPM PCR 8 (if a TPM is present).</para></listitem>
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</itemizedlist>
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<para>Both mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
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images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
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further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
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by <command>systemd-creds encrypt -T</command> (see
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<citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
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details); in case of the system extension images by using signed Verity images.</para>
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</refsect1>
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<refsect1>
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<title>EFI Variables</title>
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<para>The following EFI variables are defined, set and read by <command>systemd-stub</command>, under the
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vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
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stub and the OS:</para>
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<variablelist class='efi-variables'>
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<varlistentry>
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<term><varname>LoaderDevicePartUUID</varname></term>
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<listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
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from. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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uses this information to automatically find the disk booted from, in order to discover various other
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partitions on the same disk automatically.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderFirmwareInfo</varname></term>
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<term><varname>LoaderFirmwareType</varname></term>
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<listitem><para>Brief firmware information. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
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data.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LoaderImageIdentifier</varname></term>
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<listitem><para>The path of EFI executable, relative to the EFI System Partition's root
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directory. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
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this data.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>StubInfo</varname></term>
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<listitem><para>Brief stub information. Use
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<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
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this data.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>Note that some of the variables above may also be set by the boot loader. The stub will only set
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them if they aren't set already. Some of these variables are defined by the <ulink
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url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
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</refsect1>
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<refsect1>
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<title>Assembling Kernel Images</title>
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<para>In order to assemble an UEFI PE kernel image from various components as described above, use an
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<citerefentry><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry> command line
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like this:</para>
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<programlisting>objcopy \
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--add-section .osrel=os-release --change-section-vma .osrel=0x20000 \
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--add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
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--add-section .splash=splash.bmp --change-section-vma .splash=0x40000 \
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--add-section .linux=vmlinux --change-section-vma .linux=0x2000000 \
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--add-section .initrd=initrd.cpio --change-section-vma .initrd=0x3000000 \
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/usr/lib/systemd/boot/efi/linuxx64.efi.stub \
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foo-unsigned.efi</programlisting>
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<para>This generates one PE executable file <filename>foo-unsigned.efi</filename> from the six individual
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files for OS release information, kernel command line, boot splash image, kernel image, main initrd and
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UEFI boot stub.</para>
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<para>To then sign the resulting image for UEFI SecureBoot use an
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<citerefentry><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry> command like
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the following:</para>
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<programlisting>sbsign \
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--key mykey.pem \
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--cert mykey.crt \
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--output foo.efi \
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foo-unsigned.efi</programlisting>
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<para>This expects a pair of X.509 private key and certificate as parameters and then signs the UEFI PE
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executable we generated above for UEFI SecureBoot and generates a signed UEFI PE executable as
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result.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-sysext</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<ulink url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>,
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<ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>,
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<citerefentry><refentrytitle>objcopy</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sbsign</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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