ukify: omit .osrel section when --os-release= is empty

The primary motivation for this is to allow users of ukify to build
UKI-like objects, without having them later be detected as a UKI by
tools like kernel-install and bootctl.

The common code used by these tools to determine if a PE binary is a UKI
checks that both .osrel and .linux sections are present. Hence, adding
a mechansim to skip .osrel provides a way to avoid being labeled a UKI.

(cherry picked from commit 75890d949f)

man/ukify.xml

@@ -365,7 +365,10 @@
           <listitem><para>The os-release description (the <literal>.osrel</literal> section). The argument
           may be a literal string, or <literal>@</literal> followed by a path name. If not specified, the
           <citerefentry><refentrytitle>os-release</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
-          will be picked up from the host system.</para>
+          will be picked up from the host system. If explicitly set to an empty string, the ".osrel" section
+          is omitted from the UKI (this is not recommended in most cases, and causes the resulting artifact
+          to not be recognized as a UKI by other tools like <command>kernel-install</command>
+          and <command>bootctl</command>).</para>
 
           <xi:include href="version-info.xml" xpointer="v253"/></listitem>
         </varlistentry>

src/ukify/test/test_ukify.py

@@ -641,7 +641,7 @@ def test_efi_signing_pesign(kernel_initrd, tmp_path):
 
     shutil.rmtree(tmp_path)
 
-def test_inspect(kernel_initrd, tmp_path, capsys):
+def test_inspect(kernel_initrd, tmp_path, capsys, osrel=True):
     if kernel_initrd is None:
         pytest.skip('linux+initrd not found')
     if not shutil.which('sbsign'):
@@ -653,7 +653,7 @@ def test_inspect(kernel_initrd, tmp_path, capsys):
 
     output = f'{tmp_path}/signed2.efi'
     uname_arg='1.2.3'
-    osrel_arg='Linux'
+    osrel_arg='Linux' if osrel else ''
     cmdline_arg='ARG1 ARG2 ARG3'
 
     args = [
@@ -680,8 +680,12 @@ def test_inspect(kernel_initrd, tmp_path, capsys):
 
     text = capsys.readouterr().out
 
-    expected_osrel = f'.osrel:\n  size: {len(osrel_arg)}'
-    assert expected_osrel in text
+    if osrel:
+        expected_osrel = f'.osrel:\n  size: {len(osrel_arg)}'
+        assert expected_osrel in text
+    else:
+        assert '.osrel:' not in text
+
     expected_cmdline = f'.cmdline:\n  size: {len(cmdline_arg)}'
     assert expected_cmdline in text
     expected_uname = f'.uname:\n  size: {len(uname_arg)}'
@@ -694,6 +698,9 @@ def test_inspect(kernel_initrd, tmp_path, capsys):
 
     shutil.rmtree(tmp_path)
 
+def test_inspect_no_osrel(kernel_initrd, tmp_path, capsys):
+    test_inspect(kernel_initrd, tmp_path, capsys, osrel=False)
+
 @pytest.mark.skipif(not slow_tests, reason='slow')
 def test_pcr_signing(kernel_initrd, tmp_path):
     if kernel_initrd is None:

src/ukify/ukify.py

@@ -1477,6 +1477,9 @@ def make_uki(opts: UkifyConfig) -> None:
         '.profile',
     }
 
+    if not opts.os_release:
+        to_import.remove('.osrel')
+
     for profile in opts.join_profiles:
         pe = pefile.PE(profile, fast_load=True)
         prev_len = len(uki.sections)
@@ -2412,7 +2415,12 @@ def finalize_options(opts: argparse.Namespace) -> None:
 
     opts.os_release = resolve_at_path(opts.os_release)
 
-    if not opts.os_release and opts.linux:
+    if opts.os_release == '':
+        # If --os-release= with an empty string was passed, treat that as
+        # explicitly disabling the .osrel section, and do not fallback to the
+        # system's os-release files.
+        pass
+    elif opts.os_release is None and opts.linux:
         p = Path('/etc/os-release')
         if not p.exists():
             p = Path('/usr/lib/os-release')