diff --git a/build-utils/lib/XCAT/BuildUtils.pm b/build-utils/lib/XCAT/BuildUtils.pm
index 9bf034c1e..6e610ee23 100644
--- a/build-utils/lib/XCAT/BuildUtils.pm
+++ b/build-utils/lib/XCAT/BuildUtils.pm
@@ -42,6 +42,7 @@ our @EXPORT_OK = qw(
     buildinfo_text
     targetarch_from_target
     openeuler_build_target openeuler_repo_subdir
+    mock_config_text
     genesis_chroot_name genesis_target_arch genesis_build_plan
     genesis_log_errors genesis_log_deny_rules deb_belongs_to_dist
     genesis_dists genesis_dist_reason
@@ -1115,3 +1116,47 @@ sub _pid_alive {
 }
 
 1;
+
+#-------------------------------------------------------------------------------
+
+=head3 mock_config_text
+
+    Descriptions: Render the per-package mock configuration from a target configuration.
+
+    Arguments:
+        $pkg    - the package being built, e.g. xCAT or perl-xCAT
+        $target - the mock target, e.g. openeuler-24.03-ppc64le
+        $base   - the text of /etc/mock/<target>.cfg
+        $epoch  - SOURCE_DATE_EPOCH, passed into the chroot
+
+    Returns: the text to write to /etc/mock/<pkg>-<target>.cfg
+
+=cut
+
+#-------------------------------------------------------------------------------
+sub mock_config_text {
+    my ($pkg, $target, $base, $epoch) = @_;
+    my $chroot = "$pkg-$target";
+    my $text = $base;
+    $text =~ s/config_opts\['root'\]\s+=.*/config_opts['root'] = \"$chroot\"/;
+    if ($pkg eq "perl-xCAT" && $target !~ /suse|sles|leap/i) {
+        # perl-generators exports perl(xCAT::...) provides on RHEL/Fedora; it does not
+        # exist on openSUSE/SLES (rpm there generates perl provides itself), so injecting
+        # it into a SUSE chroot aborts chroot setup. Suppress it for SUSE targets.
+        $text .= "config_opts['chroot_additional_packages'] = 'perl-generators'\n";
+    }
+    $text .= "config_opts['environment']['SOURCE_DATE_EPOCH'] = '$epoch'\n";
+    # Avoid systemd-nspawn: it INTERMITTENTLY fails chroot setup with
+    #   "Failed to determine whether the unified cgroups hierarchy is used: No medium found"
+    # (ENOMEDIUM), which drops that package from the (still-signed) core -> an incomplete build
+    # that only surfaces later as a confusing MN install failure. 'simple' isolation is a plain
+    # chroot -- reliable for these RPM builds -- and sidesteps the nspawn cgroup race entirely.
+    $text .= "config_opts['isolation'] = 'simple'\n";
+    # mock creates --resultdir and its logs as chrootuid, and buildrpms.pl passes a relative
+    # one under a tree it owns as root. This assignment replaces any uid the target declares,
+    # so the in-chroot build user becomes uid 0 as well.
+    $text .= "config_opts['chrootuid'] = 0\n";
+    # chrootgid is left unset on purpose: groupadd for the in-chroot group fails on gid 0.
+    return $text;
+}
+
diff --git a/buildrpms.pl b/buildrpms.pl
index c350d0351..1b2ca65e3 100755
--- a/buildrpms.pl
+++ b/buildrpms.pl
@@ -59,7 +59,8 @@ use XCAT::BuildUtils qw(git_revision source_date_epoch sh sh_or_die usage buildi
     sweep_build_sources_dirs stage_xcat_probe_sources stage_xcatsn_templates
                         stage_genesis_base_sources
                         write_script read_line targetarch_from_target
-                        openeuler_build_target openeuler_repo_subdir);
+                        openeuler_build_target openeuler_repo_subdir
+                        mock_config_text);
 use POSIX ();                   # EEXIST, for the directory build lock (see main())
 use Getopt::Long qw(GetOptions);
 use POSIX qw(strftime);
@@ -325,23 +326,8 @@ sub createmockconfig {
     my $chroot = "$pkg-$target$ext";
     my $cfgfile = "/etc/mock/$chroot.cfg";
     return if -f $cfgfile && ! $opts{force};
-    cp "/etc/mock/$target.cfg", $cfgfile;
-    my $contents = read_text($cfgfile);
-    $contents =~ s/config_opts\['root'\]\s+=.*/config_opts['root'] = \"$chroot\"/;
-    if ($pkg eq "perl-xCAT" && $target !~ /suse|sles|leap/i) {
-        # perl-generators exports perl(xCAT::...) provides on RHEL/Fedora; it does not
-        # exist on openSUSE/SLES (rpm there generates perl provides itself), so injecting
-        # it into a SUSE chroot aborts chroot setup. Suppress it for SUSE targets.
-        $contents .= "config_opts['chroot_additional_packages'] = 'perl-generators'\n";
-    }
-    $contents .= "config_opts['environment']['SOURCE_DATE_EPOCH'] = '$SOURCE_DATE_EPOCH'\n";
-    # Avoid systemd-nspawn: it INTERMITTENTLY fails chroot setup with
-    #   "Failed to determine whether the unified cgroups hierarchy is used: No medium found"
-    # (ENOMEDIUM), which drops that package from the (still-signed) core -> an incomplete build that
-    # only surfaces later as a confusing MN install failure. 'simple' isolation is a plain chroot --
-    # reliable for these RPM builds -- and sidesteps the nspawn cgroup race entirely.
-    $contents .= "config_opts['isolation'] = 'simple'\n";
-    write_text($cfgfile, $contents);
+    write_text($cfgfile,
+        mock_config_text($pkg, "$target$ext", read_text("/etc/mock/$target.cfg"), $SOURCE_DATE_EPOCH));
 }
 
 sub buildsources_genesis_base($) {
diff --git a/xCAT-test/unit/buildrpms_mock_resultdir_uid.t b/xCAT-test/unit/buildrpms_mock_resultdir_uid.t
new file mode 100644
index 000000000..4d0cfc982
--- /dev/null
+++ b/xCAT-test/unit/buildrpms_mock_resultdir_uid.t
@@ -0,0 +1,54 @@
+#!/usr/bin/env perl
+# mock creates --resultdir as chrootuid, and buildrpms.pl passes a relative one under a
+# root-owned tree. These assertions read the RENDERED configuration, not the builder.
+use strict;
+use warnings;
+use FindBin;
+use lib "$FindBin::Bin/../../build-utils/lib";
+use Test::More;
+use XCAT::BuildUtils qw(mock_config_text);
+
+my $ppc = <<'CFG';
+config_opts['root'] = 'openeuler-24.03-ppc64le'
+config_opts['target_arch'] = 'ppc64le'
+config_opts['chrootuid'] = 1000
+config_opts['chrootgid'] = 1000
+config_opts['use_bootstrap'] = False
+CFG
+
+my $x86 = <<'CFG';
+include('templates/openeuler-24.03.tpl')
+config_opts['root'] = 'openeuler-24.03sp4-x86_64'
+config_opts['target_arch'] = 'x86_64'
+CFG
+
+# mock takes the last assignment.
+sub effective {
+    my ($text, $opt) = @_;
+    my $v;
+    for my $line (split /\n/, $text) {
+        $v = $1 if $line =~ /\Aconfig_opts\['\Q$opt\E'\]\s*=\s*(.+?)\s*\z/;
+    }
+    return $v;
+}
+
+my $out = mock_config_text('xCAT', 'openeuler-24.03-ppc64le', $ppc, 1790988617);
+is(effective($out, 'chrootuid'), '0', 'the ppc64le core build writes its resultdir as root');
+is(effective($out, 'chrootgid'), '1000', 'the group the target declared is left alone');
+is(effective($out, 'root'), '"xCAT-openeuler-24.03-ppc64le"', 'the chroot is still named per package');
+
+my $plain = mock_config_text('xCAT', 'openeuler-24.03sp4-x86_64', $x86, 1790988617);
+is(effective($plain, 'chrootuid'), '0', 'a target that declares no uid is pinned to root too');
+is(effective($plain, 'chrootgid'), undef, 'and no group is invented for it');
+is(effective($plain, 'target_arch'), "'x86_64'", 'nothing else in the base configuration changes');
+
+like($out, qr/\Qconfig_opts['environment']['SOURCE_DATE_EPOCH'] = '1790988617'\E/,
+    'the reproducible build date is passed into the chroot');
+like($out, qr/\Qconfig_opts['isolation'] = 'simple'\E/, 'nspawn is avoided');
+like(mock_config_text('perl-xCAT', 'openeuler-24.03-ppc64le', $ppc, 1),
+    qr/\Qconfig_opts['chroot_additional_packages'] = 'perl-generators'\E/,
+    'perl-xCAT still gets perl-generators on an rpm-md target');
+unlike(mock_config_text('perl-xCAT', 'opensuse-leap-15.6-x86_64', $x86, 1),
+    qr/perl-generators/, 'and still does not on a SUSE target');
+
+done_testing();