[RFC 11/12] 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                       | 29 +++++++
 doc/user_manual.md                            |  2 +
 meta/classes-global/base.bbclass              | 67 ++++++++++++++-
 meta/classes-recipe/deb-dl-dir.bbclass        |  9 +-
 meta/classes-recipe/dpkg-base.bbclass         | 16 +++-
 meta/classes-recipe/dpkg.bbclass              | 14 +++-
 .../image-locales-extension.bbclass           |  9 +-
 .../image-tools-extension.bbclass             | 82 +++++++++++++++++++
 meta/classes-recipe/rootfs.bbclass            | 53 +++++++++---
 meta/classes-recipe/sbuild.bbclass            | 27 +++++-
 meta/classes-recipe/sdk.bbclass               | 11 ++-
 meta/conf/bitbake.conf                        |  7 +-
 .../isar-mmdebstrap/isar-mmdebstrap.inc       | 12 ++-
 .../sbuild-chroot/sbuild-chroot.inc           | 24 +++++-
 15 files changed, 332 insertions(+), 32 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 f80630a0..29bf7590 100644
--- a/RECIPE-API-CHANGELOG.md
+++ b/RECIPE-API-CHANGELOG.md
@@ -990,3 +990,32 @@ rootless builds. For that, the deployment of images happens in two steps:
 
 Conversion commands need to follow this strategy as well, but can read the image
 (prior to conversion) from `${IMAGE_FILE_CHROOT}`.
+
+### 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"
+```
+
+On many systems, setting the following settings is sufficent, but no general guidance
+can be provided.
+
+```bash
+echo 0 | sudo tee -a /proc/sys/kernel/apparmor_restrict_unprivileged_userns
+echo 1 | sudo tee -a /proc/sys/kernel/unprivileged_userns_clone
+```
+
+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/meta/classes-global/base.bbclass b/meta/classes-global/base.bbclass
index 16939f64..26d1ee22 100644
--- a/meta/classes-global/base.bbclass
+++ b/meta/classes-global/base.bbclass
@@ -388,11 +388,54 @@ do_unpack[prefuncs] += "deprecation_checking"
 # Helpers for privileged execution. Only the non-underscore functions
 # shall be used outside of this class.
 
+def get_subid_range(idmap, d):
+    with open(idmap, 'r') as f:
+        entries = f.readlines()
+    for e in entries:
+        user, base, cnt = e.split(':')
+        if user == os.getuid() or user == os.getlogin():
+            return base, 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':
+        noone_id = 65534
+        uid_base = int(d.getVar('UNSHARE_SUBUID_BASE'))
+        uid_cnt = int(d.getVar('UNSHARE_SUBUID_COUNT')) - 2
+        nobody_subid = uid_base + uid_cnt
+        gid_base = int(d.getVar('UNSHARE_SUBGID_BASE'))
+        gid_cnt = int(d.getVar('UNSHARE_SUBGID_COUNT')) - 2
+        nogroup_subid = gid_base + gid_cnt
+        cmd = 'unshare --mount --user --pid' \
+              ' --setuid 0 --setgid 0 --fork' \
+              f' --map-users  0:{uid_base}:{uid_cnt}' \
+              f' --map-groups 0:{gid_base}:{gid_cnt}'
+        if uid_cnt < noone_id:
+            cmd += f' --map-users  {noone_id}:{nobody_subid}:1'
+        if gid_cnt < noone_id:
+            cmd += f' --map-groups {noone_id}:{nogroup_subid}:1'
+    else:
+        cmd = 'sudo -E'
     bb.debug(1, "privileged cmd: %s" % cmd)
     return cmd
 
+UNSHARE_SUBUID_BASE  := "${@get_subid_range('/etc/subuid', d)[0]}"
+UNSHARE_SUBUID_COUNT := "${@get_subid_range('/etc/subuid', d)[1]}"
+UNSHARE_SUBGID_BASE  := "${@get_subid_range('/etc/subgid', d)[0]}"
+UNSHARE_SUBGID_COUNT := "${@get_subid_range('/etc/subgid', d)[1]}"
+# store in variable to only compute once and make available to fetcher
 RUN_PRIVILEGED_CMD := "${@run_privileged_cmd(d)}"
 
 run_privileged() {
@@ -404,9 +447,29 @@ run_privileged_here() {
 }
 
 # create a directory that is suitable to be the
-# parent of a rootfs
+# parent of a rootfs. In unshare mode, we further need to
+# give the inner user the right to create a directory there.
+# This is needed, as the inner user needs to extract the
+# rootfs tarball and owns the '.' dir.
+create_chroot_parent_dir() {
+    mkdir -p "$@"
+    if [ "${ISAR_CHROOT_MODE}" = "unshare" ]; then
+        setfacl -m u:${UNSHARE_SUBUID_BASE}:rwX "$@"
+    fi
+}
+
+# get the base of the calling users subuid range
+get_base_subuid() {
+    grep $(whoami) /etc/subuid | cut -d ':' -f 2
+}
+
+# create the directory and prepare permissions to untar
+# a rootfs into an inner directory
 create_chroot_parent_dir() {
     mkdir -p "$@"
+    if [ "${ISAR_CHROOT_MODE}" = "unshare" ]; then
+        setfacl -m u:$(get_base_subuid):rwX "$@"
+    fi
 }
 
 run_in_chroot() {
diff --git a/meta/classes-recipe/deb-dl-dir.bbclass b/meta/classes-recipe/deb-dl-dir.bbclass
index fc0cd915..2020cdd0 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_here << '    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 5841d6ee..e4e72f80 100644
--- a/meta/classes-recipe/dpkg-base.bbclass
+++ b/meta/classes-recipe/dpkg-base.bbclass
@@ -168,12 +168,24 @@ 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
+}
+
 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..9fda58af 100644
--- a/meta/classes-recipe/dpkg.bbclass
+++ b/meta/classes-recipe/dpkg.bbclass
@@ -78,6 +78,7 @@ dpkg_runbuild() {
         distro="${HOST_BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
     fi
 
+    create_chroot_parent_dir ${WORKDIR}
     deb_dl_dir_import "${WORKDIR}/rootfs" "${distro}"
 
     deb_dir="/var/cache/apt/archives"
@@ -85,7 +86,10 @@ 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="${@ isar_deb_build_profiles(d)}"
@@ -110,12 +114,13 @@ dpkg_runbuild() {
     DSC_FILE=$(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 {} +" \
@@ -125,7 +130,10 @@ dpkg_runbuild() {
         --finished-build-commands="cp /var/log/dpkg.log ${ext_root}/dpkg_partial.log" \
         --build-path="" --build-dir=${WORKDIR} --dist="${DEBDISTRONAME}" ${DSC_FILE}
 
-    sbuild_dpkg_log_export "${WORKDIR}/rootfs/dpkg_partial.log"
+    # TODO: unclear if needed under unshare
+    if [ "${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 f4eb3718..f0683996 100644
--- a/meta/classes-recipe/image-locales-extension.bbclass
+++ b/meta/classes-recipe/image-locales-extension.bbclass
@@ -29,8 +29,15 @@ 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_here <<'EOF'
+    set -e
+    if [ "${ISAR_CHROOT_MODE}" = "unshare" ]; then
+        mount -o bind,private '${REPO_ISAR_DIR}/${DISTRO}' '${ROOTFSDIR}/isar-apt'
+    fi
+
+    chroot ${ROOTFSDIR} \
         /usr/bin/apt-get ${ROOTFS_APT_ARGS} -oDebug::NoLocking=1 --download-only localepurge
+EOF
 }
 
 ROOTFS_INSTALL_COMMAND += "image_install_localepurge_install"
diff --git a/meta/classes-recipe/image-tools-extension.bbclass b/meta/classes-recipe/image-tools-extension.bbclass
index 2eac3619..addc514a 100644
--- a/meta/classes-recipe/image-tools-extension.bbclass
+++ b/meta/classes-recipe/image-tools-extension.bbclass
@@ -16,6 +16,9 @@ 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() {
     IMAGE_STAGE_DIR=$(dirname $IMAGE_STAGE_HOST)
     create_chroot_parent_dir $IMAGE_STAGE_DIR
@@ -114,3 +117,82 @@ 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()}"
+
+    create_chroot_parent_dir $(realpath -m "${ROOTFS_IMAGETOOLS}/..")
+
+    run_privileged_here <<'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
+            mount -o bind,private ${REPO_ISAR_DIR}/${DISTRO} ${ROOTFS_IMAGETOOLS}/isar-apt
+            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_here <<'EOF'
+            set -e
+            mount -o bind,private ${REPO_ISAR_DIR}/${DISTRO} ${ROOTFS_IMAGETOOLS}/isar-apt
+            chroot ${ROOTFS_IMAGETOOLS} apt-get -o Debug::pkgProblemResolver=yes --no-install-recommends -y \
+                --allow-unauthenticated --allow-downgrades install \
+                $local_install
+EOF
+    fi
+
+    run_privileged_here <<'EOF' "$@"
+        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/rootfs.bbclass b/meta/classes-recipe/rootfs.bbclass
index 5a7c2a93..4d1f542f 100644
--- a/meta/classes-recipe/rootfs.bbclass
+++ b/meta/classes-recipe/rootfs.bbclass
@@ -164,7 +164,12 @@ 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_here <<'EOSUDO'
         set -e
         mountpoint -q '${ROOTFSDIR}/dev' || \
@@ -187,7 +192,12 @@ rootfs_do_mounts() {
 EOSUDO
 }
 
-rootfs_do_umounts() {
+python rootfs_do_umounts() {
+    if d.getVar('ISAR_CHROOT_MODE') == 'schroot':
+        bb.build.exec_func('rootfs_do_umounts_priv', d)
+}
+
+rootfs_do_umounts_priv() {
     run_privileged_here <<'EOSUDO'
         set -e
 
@@ -234,7 +244,12 @@ ROOTFS_EXTRA_IMPORTED := "${@rootfs_extra_import(d)}"
 
 rootfs_prepare[weight] = "25"
 rootfs_prepare(){
-    run_privileged tar -xf "${BOOTSTRAP_SRC}" -C "${ROOTFSDIR}" --exclude="./dev/console"
+    create_chroot_parent_dir $(realpath "${ROOTFSDIR}/..")
+    rm -rf ${ROOTFSDIR}
+    run_privileged_here << 'EOF'
+        mkdir -p ${ROOTFSDIR}
+        tar -xf "${BOOTSTRAP_SRC}" -C "${ROOTFSDIR}" --exclude="./dev/console"
+EOF
 
     # setup chroot
     run_privileged "${ROOTFSDIR}/chroot-setup.sh" "setup" "${ROOTFSDIR}"
@@ -304,10 +319,14 @@ 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_here <<'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"
@@ -335,9 +354,12 @@ 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_here <<'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 ??= ""
@@ -364,8 +386,12 @@ 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_here <<'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"
@@ -669,8 +695,11 @@ 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
+        create_chroot_parent_dir $(realpath -m "${ROOTFSDIR}/..")
+        run_privileged_here <<'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 f0757891..d2d333ed 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 +145,14 @@ END
 EOSUDO
 }
 
+unshare_configure_ccache() {
+    # ccache must be below /build for file permissions to work properly
+    cat <<'EOF' >> ${SBUILD_CONFIG}
+$build_environment = { "CCACHE_DIR" => "/ccache" };
+$path = "/usr/lib/ccache:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games";
+EOF
+}
+
 sbuild_dpkg_log_export() {
     export dpkg_partial_log="${1}"
 
@@ -152,3 +161,19 @@ 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() {
+    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}"
+
+    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..64a501d2 100644
--- a/meta/classes-recipe/sdk.bbclass
+++ b/meta/classes-recipe/sdk.bbclass
@@ -74,13 +74,20 @@ 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_here <<'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
+    if [ "${ISAR_CHROOT_MODE}" = "schroot" ]; then
+        rootfs_do_umounts_priv
+    fi
 
     # 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 8ca295b7..c90bc59c 100644
--- a/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc
+++ b/meta/recipes-core/isar-mmdebstrap/isar-mmdebstrap.inc
@@ -178,6 +178,8 @@ 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)
+        create_chroot_parent_dir ${WORKDIR}/dl_dir
         deb_dl_dir_import "${WORKDIR}/dl_dir" "${BOOTSTRAP_BASE_DISTRO}-${BASE_DISTRO_CODENAME}"
 
         bootstrap_list="${WORKDIR}/sources.list.d/bootstrap.list"
@@ -197,6 +199,7 @@ 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"
@@ -214,13 +217,14 @@ 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} \
@@ -239,6 +243,7 @@ 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')} \
@@ -252,7 +257,8 @@ 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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