1 <?xml version='
1.0'
?> <!--*-nxml-*-->
2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
6 <refentry id=
"systemd-stub" conditional='ENABLE_BOOTLOADER'
7 xmlns:
xi=
"http://www.w3.org/2001/XInclude">
9 <title>systemd-stub
</title>
10 <productname>systemd
</productname>
14 <refentrytitle>systemd-stub
</refentrytitle>
15 <manvolnum>7</manvolnum>
19 <refname>systemd-stub
</refname>
20 <refname>sd-stub
</refname>
21 <refname>linuxx64.efi.stub
</refname>
22 <refname>linuxia32.efi.stub
</refname>
23 <refname>linuxaa64.efi.stub
</refname>
24 <refpurpose>A simple UEFI kernel boot stub
</refpurpose>
29 <member><filename>/usr/lib/systemd/boot/efi/linuxx64.efi.stub
</filename></member>
30 <member><filename>/usr/lib/systemd/boot/efi/linuxia32.efi.stub
</filename></member>
31 <member><filename>/usr/lib/systemd/boot/efi/linuxaa64.efi.stub
</filename></member>
32 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename></member>
33 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.cred
</filename></member>
34 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.raw
</filename></member>
35 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.sysext.raw
</filename></member>
36 <member><filename><replaceable>ESP
</replaceable>/.../
<replaceable>foo
</replaceable>.efi.extra.d/*.confext.raw
</filename></member>
37 <member><filename><replaceable>ESP
</replaceable>/loader/addons/*.addon.efi
</filename></member>
38 <member><filename><replaceable>ESP
</replaceable>/loader/credentials/*.cred
</filename></member>
43 <title>Description
</title>
45 <para><command>systemd-stub
</command> (stored in per-architecture files
46 <filename>linuxx64.efi.stub
</filename>,
<filename>linuxia32.efi.stub
</filename>,
47 <filename>linuxaa64.efi.stub
</filename> on disk) is a simple UEFI boot stub. An UEFI boot stub is
48 attached to a Linux kernel binary image, and is a piece of code that runs in the UEFI firmware
49 environment before transitioning into the Linux kernel environment. The UEFI boot stub ensures a Linux
50 kernel is executable as regular UEFI binary, and is able to do various preparations before switching the
51 system into the Linux world.
</para>
53 <para>The UEFI boot stub looks for various resources for the kernel invocation inside the UEFI PE binary
54 itself. This allows combining various resources inside a single PE binary image (usually called
"Unified
55 Kernel Image", or
"UKI" for short), which may then be signed via UEFI SecureBoot as a whole, covering all
56 individual resources at once. Specifically it may include:
</para>
59 <!-- Let's keep this in the canonical order we also measure the sections by, i.e. as in
60 src/fundamental/uki.h's UnifiedSection enum -->
62 <listitem><para>A
<literal>.linux
</literal> section with the ELF Linux kernel image.
</para></listitem>
64 <listitem><para>An
<literal>.osrel
</literal> section with OS release information, i.e. the contents of
65 the
<citerefentry><refentrytitle>os-release
</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
66 of the OS the kernel belongs to.
</para></listitem>
68 <listitem><para>A
<literal>.cmdline
</literal> section with the kernel command line to pass to the
69 invoked kernel.
</para></listitem>
71 <listitem><para>An
<literal>.initrd
</literal> section with the initrd.
</para></listitem>
73 <listitem><para>A
<literal>.splash
</literal> section with an image (in the Windows
74 <filename>.BMP
</filename> format) to show on screen before invoking the kernel.
</para></listitem>
76 <listitem><para>A
<literal>.dtb
</literal> section with a compiled binary DeviceTree.
</para></listitem>
78 <listitem><para>A
<literal>.uname
</literal> section with the kernel version information, i.e. the
79 output of
<command>uname -r
</command> for the kernel included in the
<literal>.linux
</literal>
80 section.
</para></listitem>
82 <listitem><para>An
<literal>.sbat
</literal> section with
83 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">SBAT
</ulink> revocation
84 metadata.
</para></listitem>
86 <listitem><para>A
<literal>.pcrsig
</literal> section with a set of cryptographic signatures for the
87 expected TPM2 PCR values after the kernel has been booted, in JSON format. This is useful for
88 implementing TPM2 policies that bind disk encryption and similar to kernels that are signed by a
89 specific key.
</para></listitem>
91 <listitem><para>A
<literal>.pcrpkey
</literal> section with a public key in the PEM format matching the
92 signature data in the the
<literal>.pcrsig
</literal> section.
</para></listitem>
95 <para>If UEFI SecureBoot is enabled and the
<literal>.cmdline
</literal> section is present in the executed
96 image, any attempts to override the kernel command line by passing one as invocation parameters to the
97 EFI binary are ignored. Thus, in order to allow overriding the kernel command line, either disable UEFI
98 SecureBoot, or don't include a kernel command line PE section in the kernel image file. If a command line
99 is accepted via EFI invocation parameters to the EFI binary it is measured into TPM PCR
12 (if a TPM is
102 <para>If a DeviceTree is embedded in the
<literal>.dtb
</literal> section, it replaces an existing
103 DeviceTree in the corresponding EFI configuration table. systemd-stub will ask the firmware via the
104 <literal>EFI_DT_FIXUP_PROTOCOL
</literal> for hardware specific fixups to the DeviceTree.
</para>
106 <para>The contents of eight of these nine sections are measured into TPM PCR
11. It is otherwise not used
107 and thus the result can be pre-calculated without too much effort. The
<literal>.pcrsig
</literal> section
108 is not included in this PCR measurement, since it is supposed to contain signatures for the output of the
109 measurement operation, and thus cannot also be input to it.
</para>
111 <para>When
<literal>.pcrsig
</literal> and/or
<literal>.pcrpkey
</literal> sections are present in a
112 unified kernel image their contents are passed to the booted kernel in an synthetic initrd cpio archive
113 that places them in the
<filename>/.extra/tpm2-pcr-signature.json
</filename> and
114 <filename>/.extra/tpm2-pcr-public-key.pem
</filename> files. Typically, a
115 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line then
116 ensures they are copied into
<filename>/run/systemd/tpm2-pcr-signature.json
</filename> and
117 <filename>/run/systemd/tpm2-pcr-public-key.pem
</filename> where they remain accessible even after the
118 system transitions out of the initrd environment into the host file system. Tools such
119 <citerefentry><refentrytitle>systemd-cryptsetup@.service
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
120 <citerefentry><refentrytitle>systemd-cryptenroll
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
121 and
<citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
122 will automatically use files present under these paths to unlock protected resources (encrypted storage
123 or credentials) or bind encryption to booted kernels.
</para>
125 <para>For further details about the UKI concept, see the
<ulink
126 url=
"https://uapi-group.org/specifications/specs/unified_kernel_image/">UKI specification
</ulink>.
</para>
130 <title>Companion Files
</title>
132 <para>The
<command>systemd-stub
</command> UEFI boot stub automatically collects three types of auxiliary
133 companion files optionally placed in drop-in directories on the same partition as the EFI binary,
134 dynamically generates
<command>cpio
</command> initrd archives from them, and passes them to the kernel.
138 <listitem><para>For a kernel binary called
<filename><replaceable>foo
</replaceable>.efi
</filename>, it
139 will look for files with the
<filename>.cred
</filename> suffix in a directory named
140 <filename><replaceable>foo
</replaceable>.efi.extra.d/
</filename> next to it. If the kernel binary
141 uses a counter for the purpose of
142 <ulink url=
"https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot Assessment
</ulink>, this
143 counter will be ignored. For example,
<filename><replaceable>foo
</replaceable>+
3-
0.efi
</filename>
144 will look in directory
<filename><replaceable>foo
</replaceable>.efi.extra.d/
</filename>.
145 A
<command>cpio
</command>
146 archive is generated from all files found that way, placing them in the
147 <filename>/.extra/credentials/
</filename> directory of the initrd file hierarchy. The main initrd may
148 then access them in this directory. This is supposed to be used to store auxiliary, encrypted,
149 authenticated credentials for use with
<varname>LoadCredentialEncrypted=
</varname> in the UEFI System
151 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
153 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
155 details on encrypted credentials. The generated
<command>cpio
</command> archive is measured into TPM
156 PCR
12 (if a TPM is present).
</para></listitem>
158 <listitem><para>Similarly, files
159 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.sysext.raw
</filename> are packed up in a
160 <command>cpio
</command> archive and placed in the
<filename>/.extra/sysext/
</filename> directory in the
161 initrd file hierarchy. This is supposed to be used to pass additional system extension images to the
163 <citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
164 details on system extension images. The generated
<command>cpio
</command> archive containing these
165 system extension images is measured into TPM PCR
13 (if a TPM is present).
</para></listitem>
167 <!-- Note: the actual suffix we look for for sysexts is just *.raw (not *.sysext.raw), for
168 compatibility reasons with old versions. But we want people to name their system extensions
169 properly, hence we document the *.sysext.raw suffix only. -->
171 <listitem><para>Similarly, files
172 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.confext.raw
</filename> are packed up in a
173 <command>cpio
</command> archive and placed in the
<filename>/.extra/confext/
</filename> directory in
174 the initrd file hierarchy. This is supposed to be used to pass additional configuration extension
175 images to the initrd. See
176 <citerefentry><refentrytitle>systemd-confext
</refentrytitle><manvolnum>8</manvolnum></citerefentry> for
177 details on configuration extension images. The generated
<command>cpio
</command> archive containing
178 these system extension images is measured into TPM PCR
12 (if a TPM is present).
</para></listitem>
180 <listitem><para>Similarly, files
181 <filename><replaceable>foo
</replaceable>.efi.extra.d/*.addon.efi
</filename> are loaded and verified as
182 PE binaries, and a
<literal>.cmdline
</literal> section is parsed from them. Addons are supposed to be
183 used to pass additional kernel command line parameters or Devicetree blobs, regardless of the kernel
184 image being booted, for example to allow platform vendors to ship platform-specific
185 configuration.
</para>
187 <para>In case Secure Boot is enabled, these files will be validated using keys in UEFI DB, Shim's DB or
188 Shim's MOK, and will be rejected otherwise. Additionally, if the both the addon and the UKI contain a a
189 <literal>.uname
</literal> section, the addon will be rejected if they do not match exactly. It is
190 recommended to always add a
<literal>.sbat
</literal> section to all signed addons, so that they may be
191 revoked with a SBAT policy update, without requiring blocklisting via DBX/MOKX. The
192 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool will add
193 a SBAT policy by default if none is passed when building addons. For more information on SBAT see
194 <ulink url=
"https://github.com/rhboot/shim/blob/main/SBAT.md">Shim documentation
</ulink>.
</para>
196 <para>Addon files are sorted, loaded, and measured into TPM PCR
12 (if a TPM is present) and appended
197 to the kernel command line. UKI command line options are listed first, then options from addons in
198 <filename>/loader/addons/*.addon.efi
</filename>, and finally UKI-specific addons. Device tree blobs are
199 loaded and measured following the same algorithm. Addons are always loaded in the same order based on
200 the filename, so that, given the same set of addons, the same set of measurements can be expected in
201 PCR12. However, note that the filename is not protected by the PE signature, and as such an attacker
202 with write access to the ESP could potentially rename these files to change the order in which they are
203 loaded, in a way that could alter the functionality of the kernel, as some options might be
204 order-dependent. If you sign such addons, you should pay attention to the PCR12 values and make use of
205 an attestation service so that improper use of your signed addons can be detected and dealt with using
206 one of the aforementioned revocation mechanisms.
</para></listitem>
208 <listitem><para>Files
<filename>/loader/credentials/*.cred
</filename> are packed up in a
209 <command>cpio
</command> archive and placed in the
<filename>/.extra/global_credentials/
</filename>
210 directory of the initrd file hierarchy. This is supposed to be used to pass additional credentials to
211 the initrd, regardless of the kernel being booted. The generated
<command>cpio
</command> archive is
212 measured into TPM PCR
12 (if a TPM is present).
</para></listitem>
214 <listitem><para>Additionally, files
<filename>/loader/addons/*.addon.efi
</filename> are loaded and
215 verified as PE binaries, and
<literal>.cmdline
</literal> and/or
<literal>.dtb
</literal> sections are
216 parsed from them. This is supposed to be used to pass additional command line parameters or Devicetree
217 blobs to the kernel, regardless of the kernel being booted.
</para></listitem>
220 <para>These mechanisms may be used to parameterize and extend trusted (i.e. signed), immutable initrd
221 images in a reasonably safe way: all data they contain is measured into TPM PCRs. On access they should be
222 further validated: in case of the credentials case by encrypting/authenticating them via TPM, as exposed
223 by
<command>systemd-creds encrypt -T
</command> (see
224 <citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
225 details); in case of the system extension images by using signed Verity images.
</para>
229 <title>TPM PCR Notes
</title>
231 <para>Note that when a unified kernel using
<command>systemd-stub
</command> is invoked the firmware will
232 measure it as a whole to TPM PCR
4, covering all embedded resources, such as the stub code itself, the
233 core kernel, the embedded initrd and kernel command line (see above for a full list).
</para>
235 <para>Also note that the Linux kernel will measure all initrds it receives into TPM PCR
9. This means
236 every type of initrd will be measured two or three times: the initrd embedded in the kernel image will be
237 measured to PCR
4, PCR
9 and PCR
11; the initrd synthesized from credentials (and the one synthesized
238 from configuration extensions) will be measured to both PCR
9 and PCR
12; the initrd synthesized from
239 system extensions will be measured to both PCR
4 and PCR
9. Let's summarize the OS resources and the PCRs
240 they are measured to:
</para>
243 <title>OS Resource PCR Summary
</title>
245 <tgroup cols='
2' align='left' colsep='
1' rowsep='
1'
>
246 <colspec colname=
"pcr" />
247 <colspec colname=
"definition" />
251 <entry>OS Resource
</entry>
252 <entry>Measurement PCR
</entry>
258 <entry><command>systemd-stub
</command> code (the entry point of the unified PE binary)
</entry>
263 <entry>Core kernel code (embedded in unified PE binary)
</entry>
264 <entry>4 +
11</entry>
268 <entry>OS release information (embedded in the unified PE binary)
</entry>
269 <entry>4 +
11</entry>
273 <entry>Main initrd (embedded in unified PE binary)
</entry>
274 <entry>4 +
9 +
11</entry>
278 <entry>Default kernel command line (embedded in unified PE binary)
</entry>
279 <entry>4 +
11</entry>
283 <entry>Overridden kernel command line
</entry>
288 <entry>Boot splash (embedded in the unified PE binary)
</entry>
289 <entry>4 +
11</entry>
293 <entry>TPM2 PCR signature JSON (embedded in unified PE binary, synthesized into initrd)
</entry>
298 <entry>TPM2 PCR PEM public key (embedded in unified PE binary, synthesized into initrd)
</entry>
299 <entry>4 +
9 +
11</entry>
303 <entry>Credentials (synthesized initrd from companion files)
</entry>
304 <entry>9 +
12</entry>
308 <entry>System Extensions (synthesized initrd from companion files)
</entry>
309 <entry>9 +
13</entry>
313 <entry>Configuration Extensions (synthesized initrd from companion files)
</entry>
314 <entry>9 +
12</entry>
322 <title>EFI Variables
</title>
324 <para>The following EFI variables are defined, set and read by
<command>systemd-stub
</command>, under the
325 vendor UUID
<literal>4a67b082-
0a4c-
41cf-b6c7-
440b29bb8c4f
</literal>, for communication between the boot
326 stub and the OS:
</para>
328 <variablelist class='efi-variables'
>
330 <term><varname>LoaderDevicePartUUID
</varname></term>
332 <listitem><para>Contains the partition UUID of the EFI System Partition the EFI image was run
333 from.
<citerefentry><refentrytitle>systemd-gpt-auto-generator
</refentrytitle><manvolnum>8</manvolnum></citerefentry>
334 uses this information to automatically find the disk booted from, in order to discover various other
335 partitions on the same disk automatically.
</para>
337 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
341 <term><varname>LoaderFirmwareInfo
</varname></term>
342 <term><varname>LoaderFirmwareType
</varname></term>
344 <listitem><para>Brief firmware information. Use
345 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
348 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
352 <term><varname>LoaderImageIdentifier
</varname></term>
354 <listitem><para>The path of EFI executable, relative to the EFI System Partition's root
356 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
359 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
363 <term><varname>StubInfo
</varname></term>
365 <listitem><para>Brief stub information. Use
366 <citerefentry><refentrytitle>bootctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view
369 <xi:include href=
"version-info.xml" xpointer=
"v250"/></listitem>
373 <term><varname>StubPcrKernelImage
</varname></term>
375 <listitem><para>The PCR register index the kernel image, initrd image, boot splash, devicetree
376 database, and the embedded command line are measured into, formatted as decimal ASCII string (e.g.
377 <literal>11</literal>). This variable is set if a measurement was successfully completed, and remains
378 unset otherwise.
</para>
380 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
384 <term><varname>StubPcrKernelParameters
</varname></term>
386 <listitem><para>The PCR register index the kernel command line and credentials are measured into,
387 formatted as decimal ASCII string (e.g.
<literal>12</literal>). This variable is set if a measurement
388 was successfully completed, and remains unset otherwise.
</para>
390 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
394 <term><varname>StubPcrInitRDSysExts
</varname></term>
396 <listitem><para>The PCR register index the system extensions for the initrd, which are picked up from
397 the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
398 <literal>13</literal>). This variable is set if a measurement was successfully completed, and remains
399 unset otherwise.
</para>
401 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
405 <term><varname>StubPcrInitRDConfExts
</varname></term>
407 <listitem><para>The PCR register index the configuration extensions for the initrd, which are picked
408 up from the file system the kernel image is located on. Formatted as decimal ASCII string (e.g.
409 <literal>12</literal>). This variable is set if a measurement was successfully completed, and remains
410 unset otherwise.
</para>
412 <xi:include href=
"version-info.xml" xpointer=
"v255"/></listitem>
416 <para>Note that some of the variables above may also be set by the boot loader. The stub will only set
417 them if they aren't set already. Some of these variables are defined by the
<ulink
418 url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink>.
</para>
422 <title>initrd Resources
</title>
424 <para>The following resources are passed as initrd cpio archives to the booted kernel, and thus make up
425 the initial file system hierarchy in the initrd execution environment:
</para>
429 <term><filename>/
</filename></term>
431 <listitem><para>The main initrd from the
<literal>.initrd
</literal> PE section of the unified kernel
434 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
438 <term><filename>/.extra/credentials/*.cred
</filename></term>
439 <listitem><para>Credential files (suffix
<literal>.cred
</literal>) that are placed next to the
440 unified kernel image (as described above) are copied into the
441 <filename>/.extra/credentials/
</filename> directory in the initrd execution
444 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
448 <term><filename>/.extra/global_credentials/*.cred
</filename></term>
449 <listitem><para>Similarly, credential files in the
<filename>/loader/credentials/
</filename>
450 directory in the file system the unified kernel image is placed in are copied into the
451 <filename>/.extra/global_credentials/
</filename> directory in the initrd execution
454 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
458 <term><filename>/.extra/sysext/*.sysext.raw
</filename></term>
459 <listitem><para>System extension image files (suffix
<literal>.sysext.raw
</literal>) that are placed
460 next to the unified kernel image (as described above) are copied into the
461 <filename>/.extra/sysext/
</filename> directory in the initrd execution environment.
</para>
463 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
467 <term><filename>/.extra/confext/*.confext.raw
</filename></term>
468 <listitem><para>Configuration extension image files (suffix
<literal>.confext.raw
</literal>) that are
469 placed next to the unified kernel image (as described above) are copied into the
470 <filename>/.extra/confext/
</filename> directory in the initrd execution environment.
</para>
472 <xi:include href=
"version-info.xml" xpointer=
"v255"/></listitem>
476 <term><filename>/.extra/tpm2-pcr-signature.json
</filename></term>
477 <listitem><para>The TPM2 PCR signature JSON object included in the
<literal>.pcrsig
</literal> PE
478 section of the unified kernel image is copied into the
479 <filename>/.extra/tpm2-pcr-signature.json
</filename> file in the initrd execution environment.
</para>
481 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
485 <term><filename>/.extra/tpm2-pcr-pkey.pem
</filename></term>
486 <listitem><para>The PEM public key included in the
<literal>.pcrpkey
</literal> PE section of the
487 unified kernel image is copied into the
<filename>/.extra/tpm2-pcr-public-key.pem
</filename> file in
488 the initrd execution environment.
</para>
490 <xi:include href=
"version-info.xml" xpointer=
"v252"/></listitem>
494 <para>Note that all these files are located in the
<literal>tmpfs
</literal> file system the kernel sets
495 up for the initrd file hierarchy and are thus lost when the system transitions from the initrd execution
496 environment into the host file system. If these resources shall be kept around over this transition they
497 need to be copied to a place that survives the transition first, for example via a suitable
498 <citerefentry><refentrytitle>tmpfiles.d
</refentrytitle><manvolnum>5</manvolnum></citerefentry> line. By
499 default, this is done for the TPM2 PCR signature and public key files.
</para>
503 <title>SMBIOS Type
11 Strings
</title>
505 <para><command>systemd-stub
</command> can be configured using SMBIOS Type
11 strings. Applicable strings
506 consist of a name, followed by
<literal>=
</literal>, followed by the value.
507 <command>systemd-stub
</command> will search the table for a string with a specific name, and if found,
508 use its value. The following strings are read:
</para>
512 <term><varname>io.systemd.stub.kernel-cmdline-extra
</varname></term>
513 <listitem><para>If set, the value of this string is added to the list of kernel command line
514 arguments that are measured in PCR12 and passed to the kernel.
</para>
516 <xi:include href=
"version-info.xml" xpointer=
"v254"/></listitem>
522 <title>Assembling Kernel Images
</title>
524 <para>In order to assemble a bootable Unified Kernel Image from various components as described above, use
525 <citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
529 <title>See Also
</title>
530 <para><simplelist type=
"inline">
531 <member><citerefentry><refentrytitle>systemd-boot
</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
532 <member><citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
533 <member><citerefentry><refentrytitle>systemd-creds
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
534 <member><citerefentry><refentrytitle>systemd-sysext
</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
535 <member><ulink url=
"https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification
</ulink></member>
536 <member><ulink url=
"https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface
</ulink></member>
537 <member><citerefentry><refentrytitle>ukify
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
538 <member><citerefentry><refentrytitle>systemd-measure
</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
539 <member><ulink url=
"https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd
</ulink></member>