[PATCH v1 10/15] add support for fully rootless builds

["'Felix Moessbauer' via isar-users" <isar-users@googlegroups.com>]

Currently isar requires passwordless sudo and an environment
where mounting file systems is possible. This has proven problematic
for security reasons, both when running in a privileged container or
locally.

To solve this, we implement fully rootless builds that rely on the
unshare syscall which allows us to avoid sudo and instead operate in
temporary kernel namespaces as a user that is just privileged within
that namespace. This comes with some challenges regarding the handling
of mounts (they are cleared when leaving the namespace), as well as
cross namespace deployments (the outer user might not be able to access
the inner data). For that, we rework the handling of mounts and artifact
passing to make it compatible with both chroot modes (schroot and
unshare).

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
---
 Kconfig                                       |  2 +-
 RECIPE-API-CHANGELOG.md                       | 21 +++++
 doc/user_manual.md                            |  2 +
 kas/isar.yaml                                 |  2 +-
 meta/classes-global/base.bbclass              | 82 +++++++++++++++++-
 meta/classes-recipe/deb-dl-dir.bbclass        |  9 +-
 meta/classes-recipe/dpkg-base.bbclass         | 22 ++++-
 meta/classes-recipe/dpkg.bbclass              | 15 +++-
 .../image-locales-extension.bbclass           |  9 +-
 .../image-tools-extension.bbclass             | 84 +++++++++++++++++++
 meta/classes-recipe/image.bbclass             |  7 +-
 .../imagetypes_container.bbclass              |  4 +-
 meta/classes-recipe/imagetypes_wic.bbclass    |  6 +-
 meta/classes-recipe/rootfs.bbclass            | 52 +++++++++---
 meta/classes-recipe/sbuild.bbclass            | 24 +++++-
 meta/classes-recipe/sdk.bbclass               | 10 ++-
 meta/conf/bitbake.conf                        |  7 +-
 .../isar-mmdebstrap/isar-mmdebstrap.inc       | 11 ++-
 .../sbuild-chroot/sbuild-chroot.inc           | 24 +++++-
 19 files changed, 353 insertions(+), 40 deletions(-)

diff --git a/Kconfig b/Kconfig
index 683c0da5..5ef2bfcb 100644
--- a/Kconfig
+++ b/Kconfig
@@ -14,7 +14,7 @@ config KAS_INCLUDE_MAIN
 
 config KAS_BUILD_SYSTEM
 	string
-	default "isar"
+	default "isar-rootless"
 
 source "kas/machine/Kconfig"
 source "kas/distro/Kconfig"
diff --git a/RECIPE-API-CHANGELOG.md b/RECIPE-API-CHANGELOG.md
index ad03ed68..7640d87d 100644
--- a/RECIPE-API-CHANGELOG.md
+++ b/RECIPE-API-CHANGELOG.md
@@ -978,3 +978,24 @@ specifies the rootfs path.
 Using these helpers instead of direct `sudo` invocations centralizes platform-specific
 privileged execution logic in `base.bbclass`. Direct use of `sudo` is discouraged
 in downstream layers.
+
+### Rootless isar execution
+
+Isar is able to run without the need for `sudo` in an environment that
+allows unprivileged users to unshare the kernels `user namespace`. Further,
+a sufficiently large set of sub ids needs to be configured in `/etc/subuid` / `etc/subgid`.
+This range should be `> 65536`, but smaller ranges might work as well, depending on the
+ids used in the rootfs.
+
+A simple check if rootless is supported can be done by running:
+
+```bash
+mmdebstrap --unshare-helper /bin/echo "rootless supported" || echo "rootless not supported"
+```
+
+To enable rootless builds, set the bitbake variable `ISAR_ROOTLESS = "1"`.
+This internally switches the chroot mode from `schroot` to `unshare`.
+
+When using kas, the `build_system` needs to be set to `isar-rootless`, but the final
+interfaces still need to be clarified. Further, kas patches are needed (for details,
+check the kas mailing list).
diff --git a/doc/user_manual.md b/doc/user_manual.md
index 7520854b..77a37e9b 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -74,6 +74,7 @@ Building `debian-trixie` requires host system >= bookworm.
 Install the following packages:
 ```
 apt install \
+  acl \
   binfmt-support \
   bubblewrap \
   bzip2 \
@@ -88,6 +89,7 @@ apt install \
   qemu-user-static \
   reprepro \
   sudo \
+  uidmap \
   unzip \
   xz-utils \
   git-buildpackage \
diff --git a/kas/isar.yaml b/kas/isar.yaml
index 16ce8b42..3cfc4f96 100644
--- a/kas/isar.yaml
+++ b/kas/isar.yaml
@@ -4,7 +4,7 @@
 header:
   version: 14
 
-build_system: isar
+build_system: isar-rootless
 
 repos:
   isar:
diff --git a/meta/classes-global/base.bbclass b/meta/classes-global/base.bbclass
index 5f82b241..8f4b1189 100644
--- a/meta/classes-global/base.bbclass
+++ b/meta/classes-global/base.bbclass
@@ -380,7 +380,28 @@ def deb_list_beautify(d, varname):
 # shall be used outside of this class.
 
 def insert_isar_mounts(d, rootfs, mounts):
+    """
+    In unshare mode, all mounts must be created after unsharing the
+    mount namespace. As needs to happen within the unshared session,
+    we implement it as a code generator. Note, that the random and urandom
+    mounts are needed for DDI images.
+    """
     lines = []
+    to_touch = ['/dev/null', '/dev/random', '/dev/urandom']
+    to_mkdir = ['/dev/pts', '/dev/shm']
+    if d.getVar('ISAR_CHROOT_MODE') == 'unshare':
+        lines.append('touch ' + ' '.join(['{}/{}'.format(rootfs, f) for f in to_touch]))
+        lines.append('mkdir -p ' + ' '.join(['{}/{}'.format(rootfs, f) for f in to_mkdir]))
+        lines.append('mount -o bind,private,mode=666 /dev/null {}/dev/null'.format(rootfs))
+        lines.append('mount -t devpts -o noexec,nosuid,uid=5,mode=620,ptmxmode=666 none {}/dev/pts'.format(rootfs))
+        lines.append('( cd {}/dev; ln -sf pts/ptmx . )'.format(rootfs))
+        lines.append('mount -t tmpfs none {}/dev/shm'.format(rootfs))
+        lines.append('mount -o bind /dev/random {}/dev/random'.format(rootfs))
+        lines.append('mount -o bind /dev/urandom {}/dev/urandom'.format(rootfs))
+        lines.append('mount -t proc none {}/proc'.format(rootfs))
+        # we do not unshare the network namespace, so we cannot create a sysfs, hence bind-mount
+        lines.append('mount -o rbind /sys {}/sys'.format(rootfs))
+
     for m in mounts.split():
         host, inner = m.split(':') if ':' in m else (m, m)
         inner_full = os.path.join(rootfs, inner[1:])
@@ -389,7 +410,18 @@ def insert_isar_mounts(d, rootfs, mounts):
     return '\n'.join(lines)
 
 def insert_isar_umounts(d, rootfs, mounts):
+    """
+    In unshare mount we don't unmount the system mounts but just
+    remove the mountpoints.
+    """
     lines = []
+    to_unlink = ['/dev/null', '/dev/random', '/dev/urandom', '/dev/ptmx']
+    to_rmdir = ['/dev/pts', '/dev/shm']
+    if d.getVar('ISAR_CHROOT_MODE') == 'unshare':
+        lines.append('rm -f ' + ' '.join(['{}/{}'.format(rootfs, f) for f in to_unlink]))
+        for d in ['{}/{}'.format(rootfs, _d) for _d in to_rmdir]:
+            lines.append('[ -d {} ] && rmdir {}'.format(d, d))
+
     for m in mounts.split():
         host, inner = m.split(':') if ':' in m else (m, m)
         mp = '{}/{}'.format(rootfs, inner)
@@ -397,11 +429,52 @@ def insert_isar_umounts(d, rootfs, mounts):
         lines.append('[ -d {} ] && rmdir --ignore-fail-on-non-empty {}'.format(mp, mp))
     return '\n'.join(lines)
 
+def get_subid_range(idmap, d):
+    import getpass
+    with open(idmap, 'r') as f:
+        entries = f.readlines()
+    for e in entries:
+        user, base, cnt = e.split(':')
+        if user == os.getuid() or user == getpass.getuser():
+            return int(base), int(cnt)
+    bb.error("No sub-id range specified in %s" % idmap)
+
 def run_privileged_cmd(d):
-    cmd = 'sudo -E'
+    """
+    In unshare mode we need to map the rootfs uid/gid range into the
+    subuid/subgid range of the parent namespace. As we usually only
+    get 65534 ids, we cannot map the whole range, as two ids are already
+    used by the calling environment (root and builder user). Hence, map
+    as much as we can but also map the highest id (nobody / nogroup) as
+    these are used within the rootfs. It would be easier to use
+    mmdebstrap --unshare-helper as command (which is also internally used
+    by sbuild), but this only maps linear ranges, hence it cannot map the
+    nobody / nogroup on the default subid range. By that, we have to avoid
+    the nobody / nogroup when building packages in this case.
+    """
+    if d.getVar('ISAR_CHROOT_MODE') == 'unshare':
+        nobody_id = 65534
+        uid_base, uid_cnt = get_subid_range('/etc/subuid', d)
+        nobody_subid = uid_base + uid_cnt - 1
+        gid_base, gid_cnt = get_subid_range('/etc/subgid', d)
+        nogroup_subid = gid_base + gid_cnt - 1
+        cmd = 'unshare --mount --pid --uts --ipc --user' \
+              ' --kill-child' \
+              ' --setuid 0 --setgid 0 --fork' \
+              f' --map-users  1:{uid_base+1}:{uid_cnt-2}' \
+              f' --map-groups 1:{gid_base+1}:{gid_cnt-2}'
+        if uid_cnt < nobody_id:
+            cmd += f' --map-users  {nobody_id}:{nobody_subid}:1'
+        if gid_cnt < nobody_id:
+            cmd += f' --map-groups {nobody_id}:{nogroup_subid}:1'
+        cmd += " --map-root-user"
+    else:
+        cmd = 'sudo -E'
     bb.debug(1, "privileged cmd: %s" % cmd)
     return cmd
 
+UNSHARE_SUBUID_BASE  := "${@get_subid_range('/etc/subuid', d)[0] if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else '0'}"
+# store in variable to only compute once and make available to fetcher
 RUN_PRIVILEGED_CMD := "${@run_privileged_cmd(d)}"
 
 run_privileged() {
@@ -415,5 +488,10 @@ run_privileged_heredoc() {
 run_in_chroot() {
     rootfs="$1"
     shift
-    ${RUN_PRIVILEGED_CMD} chroot "$rootfs" "$@"
+
+    rootfs=$rootfs run_privileged_heredoc <<'EORIC' "$@"
+        set -e
+        ${@insert_isar_mounts(d, '$rootfs', '')}
+        chroot "$rootfs" "$@"
+EORIC
 }
diff --git a/meta/classes-recipe/deb-dl-dir.bbclass b/meta/classes-recipe/deb-dl-dir.bbclass
index 05a16585..dc83edad 100644
--- a/meta/classes-recipe/deb-dl-dir.bbclass
+++ b/meta/classes-recipe/deb-dl-dir.bbclass
@@ -121,8 +121,13 @@ deb_dl_dir_import() {
 
     # let our unprivileged user place downloaded packages in /var/cache/apt/archives/
     run_privileged_heredoc << '    EOSUDO'
-        mkdir -p "${rootfs}"/var/cache/apt/archives/partial/
-        chown -R ${uid}:${gid} "${rootfs}"/var/cache/apt/archives/
+        if [ "${ISAR_CHROOT_MODE}" = "unshare" ]; then
+            mkdir -p "${rootfs}"/var/cache/apt/archives
+            chmod 777 "${rootfs}"/var/cache/apt/archives
+        else
+            mkdir -p "${rootfs}"/var/cache/apt/archives/partial/
+            chown -R ${uid}:${gid} "${rootfs}"/var/cache/apt/archives/
+        fi
     EOSUDO
 
     # nothing to copy if download directory does not exist just yet
diff --git a/meta/classes-recipe/dpkg-base.bbclass b/meta/classes-recipe/dpkg-base.bbclass
index b3f4caad..c34c96bf 100644
--- a/meta/classes-recipe/dpkg-base.bbclass
+++ b/meta/classes-recipe/dpkg-base.bbclass
@@ -168,12 +168,30 @@ dpkg_schroot_create_configs() {
 EOSUDO
 }
 
+dpkg_chroot_prepare() {
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        dpkg_schroot_create_configs
+    fi
+}
+
+dpkg_chroot_finalize() {
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        schroot_delete_configs
+    fi
+}
+
+dpkg_prepare_unshare_ccache() {
+    mkdir -p "${CCACHE_DIR}"
+    # sbuild id from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1110942
+    setfacl -m u:${UNSHARE_SUBUID_BASE}:rwX -m u:${@int(d.getVar('UNSHARE_SUBUID_BASE')) + 999}:rwx "${CCACHE_DIR}"
+}
+
 python do_dpkg_build() {
-    bb.build.exec_func('dpkg_schroot_create_configs', d)
+    bb.build.exec_func('dpkg_chroot_prepare', d)
     try:
         bb.build.exec_func("dpkg_runbuild", d)
     finally:
-        bb.build.exec_func('schroot_delete_configs', d)
+        bb.build.exec_func('dpkg_chroot_finalize', d)
 }
 do_dpkg_build[network] = "${TASK_USE_NETWORK_AND_SUDO}"
 
diff --git a/meta/classes-recipe/dpkg.bbclass b/meta/classes-recipe/dpkg.bbclass
index 8d7ff092..09dfab7e 100644
--- a/meta/classes-recipe/dpkg.bbclass
+++ b/meta/classes-recipe/dpkg.bbclass
@@ -85,7 +85,10 @@ dpkg_runbuild() {
     ext_deb_dir="${ext_root}${deb_dir}"
 
     if [ ${USE_CCACHE} -eq 1 ]; then
-        schroot_configure_ccache
+        ${ISAR_CHROOT_MODE}_configure_ccache
+    fi
+    if [ "${ISAR_CHROOT_MODE}" = "unshare" ]; then
+        sbuild_add_unshare_mounts
     fi
 
     profiles="${@ isar_deb_build_profiles(d)}"
@@ -110,22 +113,26 @@ dpkg_runbuild() {
     DSC_FILE=$(find ${WORKDIR} -maxdepth 1 -name "${DEBIAN_SOURCE}_*.dsc" -print)
 
     sbuild -A -n -c ${SBUILD_CHROOT} \
+        --chroot-mode=${ISAR_CHROOT_MODE} \
         --host=${PACKAGE_ARCH} --build=${BUILD_ARCH} ${profiles} \
         --no-run-lintian --no-run-piuparts --no-run-autopkgtest --resolve-alternatives \
         --bd-uninstallable-explainer=apt \
         --no-apt-update --apt-distupgrade \
         --chroot-setup-commands="echo \"Package: *\nPin: release n=${DEBDISTRONAME}\nPin-Priority: 1000\" > /etc/apt/preferences.d/isar-apt" \
-        --chroot-setup-commands="echo \"APT::Get::allow-downgrades 1;\" > /etc/apt/apt.conf.d/50isar-apt" \
+        --chroot-setup-commands="echo \"APT::Get::allow-downgrades 1;${@'\nAPT::Sandbox::User root;' if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''}\" > /etc/apt/apt.conf.d/50isar-apt" \
         --chroot-setup-commands="rm -f /var/log/dpkg.log" \
         --chroot-setup-commands="mkdir -p ${deb_dir}" \
         --chroot-setup-commands="find ${ext_deb_dir} -maxdepth 1 -name '*.deb' -exec ln -t ${deb_dir}/ -sf {} +" \
         --chroot-setup-commands="apt-get update -o Dir::Etc::SourceList=\"sources.list.d/isar-apt.list\" -o Dir::Etc::SourceParts=\"-\" -o APT::Get::List-Cleanup=\"0\"" \
         --finished-build-commands="rm -f ${deb_dir}/sbuild-build-depends-*-dummy_*.deb" \
         --finished-build-commands="find ${deb_dir} -maxdepth 1 -type f -name '*.deb' -print -exec cp ${CP_FLAGS} -t ${ext_deb_dir}/ {} +" \
-        --finished-build-commands="cp /var/log/dpkg.log ${ext_root}/dpkg_partial.log" \
+        ${@ '--finished-build-commands="cp /var/log/dpkg.log $ext_root/dpkg_partial.log"' if d.getVar('ISAR_CHROOT_MODE') == 'schroot' else '' } \
         --build-path="" --build-dir=${WORKDIR} --dist="${DEBDISTRONAME}" ${DSC_FILE}
 
-    sbuild_dpkg_log_export "${WORKDIR}/rootfs/dpkg_partial.log"
+    # TODO: port to unshare backend
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        sbuild_dpkg_log_export "${WORKDIR}/rootfs/dpkg_partial.log"
+    fi
     deb_dl_dir_export "${WORKDIR}/rootfs" "${distro}"
 
     # Cleanup apt artifacts
diff --git a/meta/classes-recipe/image-locales-extension.bbclass b/meta/classes-recipe/image-locales-extension.bbclass
index c1e8c175..bdaa55fb 100644
--- a/meta/classes-recipe/image-locales-extension.bbclass
+++ b/meta/classes-recipe/image-locales-extension.bbclass
@@ -29,8 +29,12 @@ ROOTFS_INSTALL_COMMAND_BEFORE_EXPORT += "image_install_localepurge_download"
 image_install_localepurge_download[weight] = "40"
 image_install_localepurge_download[network] = "${TASK_USE_NETWORK_AND_SUDO}"
 image_install_localepurge_download() {
-    run_in_chroot '${ROOTFSDIR}' \
+    run_privileged_heredoc <<'EOF'
+    set -e
+    ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), d.getVar('ROOTFS_MOUNTS') if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else '')}
+    chroot ${ROOTFSDIR} \
         /usr/bin/apt-get ${ROOTFS_APT_ARGS} -oDebug::NoLocking=1 --download-only localepurge
+EOF
 }
 
 ROOTFS_INSTALL_COMMAND += "image_install_localepurge_install"
@@ -62,6 +66,9 @@ __EOF__
     # Install configuration into image:
     run_privileged_heredoc <<'EOSUDO'
         set -e
+
+        ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), '')}
+
         localepurge_state='i'
         if chroot '${ROOTFSDIR}' dpkg -s localepurge 2>/dev/null >&2
         then
diff --git a/meta/classes-recipe/image-tools-extension.bbclass b/meta/classes-recipe/image-tools-extension.bbclass
index e88557f6..3fff4972 100644
--- a/meta/classes-recipe/image-tools-extension.bbclass
+++ b/meta/classes-recipe/image-tools-extension.bbclass
@@ -16,7 +16,14 @@ do_image_tools[depends] += " \
 SCHROOT_MOUNTS = "${WORKDIR}:${PP_WORK} ${IMAGE_ROOTFS}:${PP_ROOTFS} ${DEPLOY_DIR_IMAGE}:${PP_DEPLOY}"
 SCHROOT_MOUNTS += "${REPO_ISAR_DIR}/${DISTRO}:/isar-apt"
 
+# only used on unshare
+ROOTFS_IMAGETOOLS ?= "${WORKDIR}/rootfs-imgtools-${BB_CURRENTTASK}"
+
 imager_run() {
+    imager_run_${ISAR_CHROOT_MODE} "$@"
+}
+
+imager_run_schroot() {
     local_install="${@(d.getVar("INSTALL_%s" % d.getVar("BB_CURRENTTASK")) or '').strip()}"
     local_bom="${@(d.getVar("BOM_%s" % d.getVar("BB_CURRENTTASK")) or '').strip()}"
 
@@ -103,3 +110,80 @@ generate_imager_sbom() {
             --timestamp $TIMESTAMP ${SBOM_DEBSBOM_EXTRA_ARGS} \
     < ${WORKDIR}/imager.manifest
 }
+
+imager_run_unshare() {
+    exec 3<&0
+
+    # ignore everything before '--'. If the remaining list is empty,
+    # assume a here document is passed via stdin
+    while [ "$#" -gt 0 ]; do
+        case "$1" in
+        --) shift 1; break ;;
+        *) shift 1 ;;
+        esac
+    done
+
+    if [ "$#" -eq 0 ]; then
+        set -- "$@" '/bin/bash' '-s'
+    fi
+
+    local_install="${@(d.getVar("INSTALL_%s" % d.getVar("BB_CURRENTTASK")) or '').strip()}"
+
+    run_privileged_heredoc <<'EOF'
+    set -e
+    mkdir -p ${ROOTFS_IMAGETOOLS}
+    tar -xf "${SBUILD_CHROOT}" -C "${ROOTFS_IMAGETOOLS}"
+    mkdir -p ${ROOTFS_IMAGETOOLS}/isar-apt
+    cp -rL /etc/resolv.conf "${ROOTFS_IMAGETOOLS}/etc"
+EOF
+
+    # setting up error handler
+    imager_cleanup() {
+        run_privileged rm -rf ${ROOTFS_IMAGETOOLS}
+    }
+    trap 'exit 1' INT HUP QUIT TERM ALRM USR1
+    trap 'imager_cleanup' EXIT
+
+    if [ -n "${local_install}" ]; then
+        echo "Installing imager deps: ${local_install}"
+
+        distro="${BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
+        if [ ${ISAR_CROSS_COMPILE} -eq 1 ]; then
+            distro="${HOST_BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
+        fi
+
+        E="${@ isar_export_proxies(d)}"
+        deb_dl_dir_import ${ROOTFS_IMAGETOOLS} ${distro}
+        ${SCRIPTSDIR}/lockrun.py -r -f "${REPO_ISAR_DIR}/isar.lock" -s <<'EOAPT'
+        local_install=$local_install ${@run_privileged_cmd(d)} /bin/bash -s <<'EOF'
+            set -e
+            ${@insert_isar_mounts(d, d.getVar('ROOTFS_IMAGETOOLS'), d.getVar('SCHROOT_MOUNTS'))}
+            chroot ${ROOTFS_IMAGETOOLS} apt-get update \
+                -o Dir::Etc::SourceList='sources.list.d/isar-apt.list' \
+                -o Dir::Etc::SourceParts='-' \
+                -o APT::Get::List-Cleanup='0'
+            chroot ${ROOTFS_IMAGETOOLS} apt-get -o Debug::pkgProblemResolver=yes --no-install-recommends -y \
+                --allow-unauthenticated --allow-downgrades --download-only install \
+                $local_install
+EOF
+EOAPT
+
+        deb_dl_dir_export ${ROOTFS_IMAGETOOLS} ${distro}
+        local_install=$local_install run_privileged_heredoc <<'EOF'
+            set -e
+            ${@insert_isar_mounts(d, d.getVar('ROOTFS_IMAGETOOLS'), d.getVar('SCHROOT_MOUNTS'))}
+            chroot ${ROOTFS_IMAGETOOLS} apt-get -o Debug::pkgProblemResolver=yes --no-install-recommends -y \
+                --allow-unauthenticated --allow-downgrades install \
+                $local_install
+EOF
+    fi
+
+    run_privileged_heredoc <<'EOF' "$@"
+        set -e
+        mkdir -p ${ROOTFS_IMAGETOOLS}/${SCRIPTSDIR}
+        ${@insert_isar_mounts(d, d.getVar('ROOTFS_IMAGETOOLS'), d.getVar('SCHROOT_MOUNTS'))}
+        chroot ${ROOTFS_IMAGETOOLS} "$@" <&3
+EOF
+
+    run_privileged rm -rf ${ROOTFS_IMAGETOOLS}
+}
diff --git a/meta/classes-recipe/image.bbclass b/meta/classes-recipe/image.bbclass
index 4a250964..48a96a9e 100644
--- a/meta/classes-recipe/image.bbclass
+++ b/meta/classes-recipe/image.bbclass
@@ -188,6 +188,7 @@ SUDO_CHROOT = "imager_run -d ${PP_ROOTFS} -u root --"
 python() {
     image_types = (d.getVar('IMAGE_FSTYPES') or '').split()
     conversions = set(d.getVar('IMAGE_CONVERSIONS').split())
+    chroot_mode = d.getVar('ISAR_CHROOT_MODE')
 
     basetypes = {}
     typedeps = {}
@@ -263,7 +264,8 @@ python() {
         if image_cmd:
             localdata.setVar('type', bt)
             cmds.append(localdata.expand(image_cmd))
-            cmds.append(localdata.expand('\tsudo chown $(id -u):$(id -g) ${IMAGE_FILE_HOST}'))
+            if chroot_mode == 'schroot':
+                cmds.append(localdata.expand('\tsudo chown $(id -u):$(id -g) ${IMAGE_FILE_HOST}'))
         else:
             bb.fatal("No IMAGE_CMD for %s" % bt)
         vardeps.add('IMAGE_CMD:' + bt_clean)
@@ -293,7 +295,8 @@ python() {
                     cmd = '\t' + localdata.getVar('CONVERSION_CMD:' + c)
                     if cmd not in cmds:
                         cmds.append(cmd)
-                        cmds.append(localdata.expand('\tsudo chown $(id -u):$(id -g) ${IMAGE_FILE_HOST}.%s' % c))
+                        if chroot_mode == 'schroot':
+                            cmds.append(localdata.expand('\tsudo chown $(id -u):$(id -g) ${IMAGE_FILE_HOST}.%s' % c))
                     vardeps.add('CONVERSION_CMD:' + c)
                     for dep in (localdata.getVar('CONVERSION_DEPS:' + c) or '').split():
                         conversion_install.add(dep)
diff --git a/meta/classes-recipe/imagetypes_container.bbclass b/meta/classes-recipe/imagetypes_container.bbclass
index b6cc4a14..09f13a17 100644
--- a/meta/classes-recipe/imagetypes_container.bbclass
+++ b/meta/classes-recipe/imagetypes_container.bbclass
@@ -68,7 +68,9 @@ do_containerize() {
     run_privileged rm -rf "${oci_img_dir}_unpacked"
 
     # no root needed anymore
-    run_privileged chown --recursive $(id -u):$(id -g) "${oci_img_dir}"
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        run_privileged chown --recursive $(id -u):$(id -g) "${oci_img_dir}"
+    fi
 }
 
 convert_container() {
diff --git a/meta/classes-recipe/imagetypes_wic.bbclass b/meta/classes-recipe/imagetypes_wic.bbclass
index 63974a3e..9f1e3de4 100644
--- a/meta/classes-recipe/imagetypes_wic.bbclass
+++ b/meta/classes-recipe/imagetypes_wic.bbclass
@@ -193,8 +193,10 @@ generate_wic_image() {
         fi
 EOIMAGER
 
-    run_privileged chown -R $(stat -c "%U" ${LAYERDIR_core}) ${LAYERDIR_core} ${LAYERDIR_isar} ${SCRIPTSDIR} || true
-    run_privileged chown -R $(id -u):$(id -g) "${DEPLOY_DIR_IMAGE}/${IMAGE_FULLNAME}.wic"*
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        run_privileged chown -R $(stat -c "%U" ${LAYERDIR_core}) ${LAYERDIR_core} ${LAYERDIR_isar} ${SCRIPTSDIR} || true
+        run_privileged chown -R $(id -u):$(id -g) "${DEPLOY_DIR_IMAGE}/${IMAGE_FULLNAME}.wic"*
+    fi
     rm -rf ${IMAGE_ROOTFS}/../pseudo
 
     cat ${DEPLOY_DIR_IMAGE}/${IMAGE_FULLNAME}.manifest \
diff --git a/meta/classes-recipe/rootfs.bbclass b/meta/classes-recipe/rootfs.bbclass
index 6823c610..9044bf93 100644
--- a/meta/classes-recipe/rootfs.bbclass
+++ b/meta/classes-recipe/rootfs.bbclass
@@ -145,7 +145,12 @@ rootfs_cmd() {
 }
 
 rootfs_do_mounts[weight] = "3"
-rootfs_do_mounts() {
+python rootfs_do_mounts() {
+    if d.getVar('ISAR_CHROOT_MODE') == 'schroot':
+        bb.build.exec_func('rootfs_do_mounts_priv', d)
+}
+
+rootfs_do_mounts_priv() {
     run_privileged_heredoc <<'EOSUDO'
         set -e
         mountpoint -q '${ROOTFSDIR}/dev' || \
@@ -168,7 +173,13 @@ rootfs_do_mounts() {
 EOSUDO
 }
 
-rootfs_do_umounts() {
+python rootfs_do_umounts() {
+    # unconditionally run the unmount code as this ignores missing
+    # mountpoints but also does the cleanup of the directories
+    bb.build.exec_func('rootfs_do_umounts_priv', d)
+}
+
+rootfs_do_umounts_priv() {
     run_privileged_heredoc <<'EOSUDO'
         set -e
 
@@ -215,7 +226,11 @@ ROOTFS_EXTRA_IMPORTED := "${@rootfs_extra_import(d)}"
 
 rootfs_prepare[weight] = "25"
 rootfs_prepare(){
-    run_privileged tar -xf "${BOOTSTRAP_SRC}" -C "${ROOTFSDIR}" --exclude="./dev/console"
+    rm -rf ${ROOTFSDIR}
+    run_privileged_heredoc << 'EOF'
+        mkdir -p ${ROOTFSDIR}
+        tar -xf "${BOOTSTRAP_SRC}" -C "${ROOTFSDIR}" --exclude="./dev/console"
+EOF
 
     # setup chroot
     run_privileged "${ROOTFSDIR}/chroot-setup.sh" "setup" "${ROOTFSDIR}"
@@ -285,10 +300,14 @@ rootfs_install_pkgs_update[weight] = "5"
 rootfs_install_pkgs_update[isar-apt-lock] = "acquire-before"
 rootfs_install_pkgs_update[network] = "${TASK_USE_NETWORK_AND_SUDO}"
 rootfs_install_pkgs_update() {
-    run_in_chroot '${ROOTFSDIR}' /usr/bin/apt-get update \
-        -o Dir::Etc::SourceList="sources.list.d/isar-apt.list" \
-        -o Dir::Etc::SourceParts="-" \
-        -o APT::Get::List-Cleanup="0"
+    run_privileged_heredoc <<'EOF'
+        set -e
+        ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), d.getVar('ROOTFS_MOUNTS')) if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''}
+        chroot '${ROOTFSDIR}' /usr/bin/apt-get update \
+            -o Dir::Etc::SourceList="sources.list.d/isar-apt.list" \
+            -o Dir::Etc::SourceParts="-" \
+            -o APT::Get::List-Cleanup="0"
+EOF
 }
 
 ROOTFS_INSTALL_COMMAND += "rootfs_install_resolvconf"
@@ -316,9 +335,12 @@ rootfs_install_pkgs_download[isar-apt-lock] = "release-after"
 rootfs_install_pkgs_download[network] = "${TASK_USE_NETWORK}"
 rootfs_install_pkgs_download() {
     # download packages using apt in a non-privileged namespace
-    rootfs_cmd --bind "${ROOTFSDIR}/var/cache/apt/archives" /var/cache/apt/archives \
-               ${ROOTFSDIR} \
-               -- /usr/bin/apt-get ${ROOTFS_APT_ARGS} -oDebug::NoLocking=1 --download-only ${ROOTFS_PACKAGES}
+    run_privileged_heredoc <<'EOF'
+    set -e
+    ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), d.getVar('ROOTFS_MOUNTS')) if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''}
+    chroot ${ROOTFSDIR} \
+        /usr/bin/apt-get ${ROOTFS_APT_ARGS} -oDebug::NoLocking=1 --download-only ${ROOTFS_PACKAGES}
+EOF
 }
 
 ROOTFS_INSTALL_COMMAND_BEFORE_EXPORT ??= ""
@@ -345,8 +367,12 @@ rootfs_install_pkgs_install[weight] = "8000"
 rootfs_install_pkgs_install[progress] = "custom:rootfs_progress.PkgsInstallProgressHandler"
 rootfs_install_pkgs_install[network] = "${TASK_USE_SUDO}"
 rootfs_install_pkgs_install() {
-    run_in_chroot "${ROOTFSDIR}" \
+    run_privileged_heredoc <<'EOF'
+    set -e
+    ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), d.getVar('ROOTFS_MOUNTS')) if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''}
+    chroot "${ROOTFSDIR}" \
         /usr/bin/apt-get ${ROOTFS_APT_ARGS} ${ROOTFS_PACKAGES}
+EOF
 }
 
 ROOTFS_INSTALL_COMMAND += "rootfs_restore_initrd_tooling"
@@ -652,8 +678,10 @@ rootfs_install_sstate_finalize() {
     # - after building the rootfs, the tar won't be there, but we also don't need to unpack
     # - after restoring from cache, there will be a tar which we unpack and then delete
     if [ -f rootfs.tar ]; then
+        run_privileged_heredoc <<'EOF'
         mkdir -p ${ROOTFSDIR}
-        run_privileged tar -C ${ROOTFSDIR} -xp ${SSTATE_TAR_ATTR_FLAGS} < rootfs.tar
+        tar -C ${ROOTFSDIR} -xp ${SSTATE_TAR_ATTR_FLAGS} -f rootfs.tar
+EOF
         rm rootfs.tar
     fi
 }
diff --git a/meta/classes-recipe/sbuild.bbclass b/meta/classes-recipe/sbuild.bbclass
index ea0c5841..1fa70bd8 100644
--- a/meta/classes-recipe/sbuild.bbclass
+++ b/meta/classes-recipe/sbuild.bbclass
@@ -7,7 +7,8 @@ SCHROOT_MOUNTS ?= ""
 
 inherit crossvars
 
-SBUILD_CHROOT ?= "${DEBDISTRONAME}-${SCHROOT_USER}-${ISAR_BUILD_UUID}-${@os.getpid()}"
+SBUILD_CHROOT:unshare ?= "${SCHROOT_DIR}.tar.zst"
+SBUILD_CHROOT:schroot ?= "${DEBDISTRONAME}-${SCHROOT_USER}-${ISAR_BUILD_UUID}-${@os.getpid()}"
 
 SBUILD_CONF_DIR ?= "${SCHROOT_CONF}/${SBUILD_CHROOT}"
 SCHROOT_CONF_FILE ?= "${SCHROOT_CONF}/chroot.d/${SBUILD_CHROOT}"
@@ -144,6 +145,13 @@ END
 EOSUDO
 }
 
+unshare_configure_ccache() {
+    # ccache must be below /build for file permissions to work properly
+    cat <<'EOF' >> ${SBUILD_CONFIG}
+$path = "/usr/lib/ccache:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games";
+EOF
+}
+
 sbuild_dpkg_log_export() {
     export dpkg_partial_log="${1}"
 
@@ -152,3 +160,17 @@ sbuild_dpkg_log_export() {
     cat ${dpkg_partial_log} >> ${SCHROOT_DIR}/tmp/dpkg_common.log
     )  9>"${SCHROOT_DIR}/tmp/dpkg_common.log.lock"
 }
+
+# additional mounts managed by sbuild
+sbuild_add_unshare_mounts() {
+    dpkg_prepare_unshare_ccache
+
+    cat <<'EOF' >> ${SBUILD_CONFIG}
+$unshare_bind_mounts = [
+    { directory => '${WORKDIR}/rootfs', mountpoint => '${PP}/rootfs' },
+    { directory => '${WORKDIR}/isar-apt/${DISTRO}-${DISTRO_ARCH}/apt/${DISTRO}', mountpoint => '/isar-apt' },
+    { directory => '${REPO_BASE_DIR}', mountpoint => '/base-apt' },
+    { directory => "${CCACHE_DIR}", mountpoint => "/ccache" }
+];
+EOF
+}
diff --git a/meta/classes-recipe/sdk.bbclass b/meta/classes-recipe/sdk.bbclass
index 074f5ef8..6086e9ce 100644
--- a/meta/classes-recipe/sdk.bbclass
+++ b/meta/classes-recipe/sdk.bbclass
@@ -74,13 +74,17 @@ rootfs_configure_isar_apt_dir() {
 
 ROOTFS_POSTPROCESS_COMMAND:prepend:class-sdk = "sdkchroot_configscript "
 sdkchroot_configscript () {
-    run_in_chroot ${ROOTFSDIR} /configscript.sh ${DISTRO_ARCH}
+    run_privileged_heredoc <<'EOF'
+        set -e
+        ${@insert_isar_mounts(d, d.getVar('ROOTFSDIR'), d.getVar('ROOTFS_MOUNTS')) if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''}
+        cp -rL /etc/resolv.conf '${ROOTFSDIR}/etc'
+        chroot ${ROOTFSDIR} /configscript.sh ${DISTRO_ARCH}
+EOF
 }
 
 ROOTFS_POSTPROCESS_COMMAND:append:class-sdk = " sdkchroot_finalize"
 sdkchroot_finalize() {
-
-    rootfs_do_umounts
+    rootfs_do_umounts_priv
 
     # Remove setup scripts
     run_privileged rm -f ${ROOTFSDIR}/chroot-setup.sh ${ROOTFSDIR}/configscript.sh
diff --git a/meta/conf/bitbake.conf b/meta/conf/bitbake.conf
index 0f84e715..1c3a6f4a 100644
--- a/meta/conf/bitbake.conf
+++ b/meta/conf/bitbake.conf
@@ -72,7 +72,7 @@ KERNEL_FILE:arm64 ?= "vmlinux"
 
 MACHINEOVERRIDES ?= "${MACHINE}"
 DISTROOVERRIDES ?= "${DISTRO}"
-OVERRIDES = "${PACKAGE_ARCH}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${BASE_DISTRO_CODENAME}:forcevariable"
+OVERRIDES = "${PACKAGE_ARCH}:${MACHINEOVERRIDES}:${DISTROOVERRIDES}:${BASE_DISTRO_CODENAME}:${ISAR_CHROOT_MODE}:forcevariable"
 FILESOVERRIDES = "${PACKAGE_ARCH}:${MACHINE}"
 
 # Setting default QEMU_ARCH variables for different DISTRO_ARCH:
@@ -151,6 +151,10 @@ ISAR_APT_RETRIES ??= "${@'10' if bb.utils.to_boolean(d.getVar('ISAR_USE_APT_SNAP
 ISAR_APT_DELAY_MAX ??= "${@'600' if bb.utils.to_boolean(d.getVar('ISAR_USE_APT_SNAPSHOT')) else ''}"
 ISAR_APT_SNAPSHOT_TIMESTAMP ??= "${SOURCE_DATE_EPOCH}"
 
+# Rootless build execution
+ISAR_ROOTLESS ??= "0"
+ISAR_CHROOT_MODE ??= "${@'unshare' if bb.utils.to_boolean(d.getVar('ISAR_ROOTLESS')) else 'schroot'}"
+
 # Default parallelism and resource usage for xz
 XZ_MEMLIMIT ?= "50%"
 XZ_THREADS ?= "${@oe.utils.cpu_count(at_least=2)}"
@@ -206,6 +210,7 @@ CCACHE_DEBUG ?= "0"
 # Variables for tasks marking
 # Long term TODO: get rid of sudo marked tasks
 TASK_USE_NETWORK = "1"
+# nested namespacing requires this as well
 TASK_USE_SUDO = "1"
 TASK_USE_NETWORK_AND_SUDO = "1"
 
diff --git a/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc b/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc
index ea63ec79..de6c43bf 100644
--- a/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc
+++ b/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc
@@ -177,6 +177,7 @@ do_bootstrap() {
                          umount \$1/$base_apt_tmp && rm ${WORKDIR}/mmtmpdir && \
                          umount $base_apt_tmp && rm -rf --one-file-system $base_apt_tmp"
     else
+        # prepare dl_dir for access from both sides (local and rootfs)
         deb_dl_dir_import "${WORKDIR}/dl_dir" "${BOOTSTRAP_BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
 
         bootstrap_list="${WORKDIR}/sources.list.d/bootstrap.list"
@@ -196,6 +197,7 @@ do_bootstrap() {
                                  -o Dir::State="$1/var/lib/apt" \
                                  -o Dir::Etc="$1/etc/apt" \
                                  -o Dir::Cache="$1/var/cache/apt" \
+                                 ${@'-o APT::Sandbox::User=root' if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''} \
                                  -o Apt::Architecture="${BOOTSTRAP_DISTRO_ARCH}" \
                                  ${@get_apt_opts(d, '-o')}'
         extra_essential="$extra_essential && $syncout"
@@ -213,13 +215,14 @@ do_bootstrap() {
 
     # Cleanup mounts if fails
     trap 'exit 1' INT HUP QUIT TERM ALRM USR1
-    trap 'bootstrap_cleanup' EXIT
+    trap ${@'true' if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else 'bootstrap_cleanup'} EXIT
 
     # Create lock file so that it is owned by the user running the build (not root)
     mkdir -p ${DEBDIR}
     touch ${DEB_DL_LOCK}
 
-    run_privileged TMPDIR="${BOOTSTRAP_TMPDIR}" mmdebstrap $bootstrap_args \
+    ${@'' if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else 'run_privileged'} \
+    TMPDIR="${BOOTSTRAP_TMPDIR}" mmdebstrap $bootstrap_args \
                    $arch_param \
                    --mode=unshare \
                    ${MMHOOKS} \
@@ -238,6 +241,7 @@ do_bootstrap() {
                    --customize-hook='sed -i "/en_US.UTF-8 UTF-8/s/^#//g" "$1/etc/locale.gen"' \
                    --customize-hook='chroot "$1" /usr/sbin/locale-gen' \
                    --customize-hook='chroot "$1" /usr/bin/apt-get -y clean' \
+                   ${@'--skip=output/dev' if d.getVar('ISAR_CHROOT_MODE') == 'unshare' else ''} \
                    --skip=cleanup/apt \
                    --skip=download/empty \
                    ${@get_apt_opts(d, '--aptopt')} \
@@ -251,7 +255,8 @@ do_bootstrap() {
 
     if [ "${ISAR_USE_CACHED_BASE_REPO}" != "1" ]; then
         deb_dl_dir_export "${WORKDIR}/dl_dir" "${BOOTSTRAP_BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
-        run_privileged rm -rf --one-file-system "${WORKDIR}/dl_dir"
+        run_privileged find ${WORKDIR}/dl_dir -maxdepth 1 -mindepth 1 -exec rm -rf --one-file-system "{}" \;
+        rmdir ${WORKDIR}/dl_dir
     fi
 }
 addtask bootstrap before do_build after do_generate_keyrings
diff --git a/meta/recipes-devtools/sbuild-chroot/sbuild-chroot.inc b/meta/recipes-devtools/sbuild-chroot/sbuild-chroot.inc
index 61d37760..7a778d8c 100644
--- a/meta/recipes-devtools/sbuild-chroot/sbuild-chroot.inc
+++ b/meta/recipes-devtools/sbuild-chroot/sbuild-chroot.inc
@@ -66,8 +66,28 @@ ROOTFS_POSTPROCESS_COMMAND:remove = "rootfs_cleanup_base_apt"
 
 DEPLOY_SCHROOT = "${@d.getVar('SCHROOT_' + d.getVar('SBUILD_VARIANT').upper() + '_DIR')}${SBUILD_SCHROOT_SUFFIX}"
 
-do_sbuildchroot_deploy[dirs] = "${DEPLOY_DIR}/schroot-${SBUILD_VARIANT}"
-do_sbuildchroot_deploy() {
+sbuildchroot_deploy_tree() {
     ln -Tfsr "${ROOTFSDIR}" "${DEPLOY_SCHROOT}"
 }
+sbuildchroot_deploy_tar() {
+    lopts="--one-file-system --exclude=var/cache/apt/archives --exclude=isar-apt"
+    # we cannot use pzstd, as this results in a different magic
+    # (zstd skippable frame) which is not detected by sbuild
+    # https://salsa.debian.org/debian/sbuild/-/blob/d975d388a98627a0d7d112791e441c27a6d529df/lib/Sbuild/ChrootUnshare.pm#L608
+    ZSTD="zstd -${SSTATE_ZSTD_CLEVEL} -T${ZSTD_THREADS}"
+    run_privileged \
+        tar -C ${ROOTFSDIR} -cpS $lopts ${ROOTFS_TAR_ATTR_FLAGS} . \
+            | $ZSTD > ${DEPLOY_SCHROOT}.tar.zst
+    # cleanup extracted rootfs
+    run_privileged rm -rf ${ROOTFSDIR}
+}
+
+do_sbuildchroot_deploy[network] = "${TASK_USE_SUDO}"
+do_sbuildchroot_deploy[dirs] += "${DEPLOY_DIR}/schroot-${SBUILD_VARIANT}"
+python do_sbuildchroot_deploy() {
+    if d.getVar('ISAR_CHROOT_MODE') == 'unshare':
+        bb.build.exec_func('sbuildchroot_deploy_tar', d)
+    else:
+        bb.build.exec_func('sbuildchroot_deploy_tree', d)
+}
 addtask sbuildchroot_deploy before do_build after do_rootfs
-- 
2.51.0

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