Re: [PATCH v2 7/8] dracut: add support for cross initrd generation

[Zhihang Wei <wzh@ilbers.de>]

On 9/24/26 09:23, 'Felix Moessbauer' via isar-users wrote:
> For cross-initrd generation, a cross variant of the ldd tool needs to
> be provided. As there is no such tool, but this is THE performance
> critical part (it needs to scan each binary and all transitive dependencies),
> we provide a custom one which uses readelf to determine the shared library
> dependencies.
>
> Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
> ---
>   meta/classes-recipe/host-tooling.bbclass      |  4 +-
>   meta/classes-recipe/initrd-dracut.bbclass     | 38 ++++++++++-
>   meta/classes-recipe/rootfs.bbclass            |  4 +-
>   .../dracut-cross-ldd/dracut-cross-ldd.bb      | 16 +++++
>   .../dracut-cross-ldd/files/dracut-cross-ldd   | 65 +++++++++++++++++++
>   5 files changed, 123 insertions(+), 4 deletions(-)
>   create mode 100644 meta/recipes-devtools/dracut-cross-ldd/dracut-cross-ldd.bb
>   create mode 100644 meta/recipes-devtools/dracut-cross-ldd/files/dracut-cross-ldd
>
> diff --git a/meta/classes-recipe/host-tooling.bbclass b/meta/classes-recipe/host-tooling.bbclass
> index b76f8898..5e81dfda 100644
> --- a/meta/classes-recipe/host-tooling.bbclass
> +++ b/meta/classes-recipe/host-tooling.bbclass
> @@ -5,8 +5,8 @@
>   LICENSE = "gpl-2.0"
>   LIC_FILES_CHKSUM = "file://${LAYERDIR_core}/licenses/COPYING.GPLv2;md5=751419260aa954499f7abaabaa882bbe"
>   
> -HOST_TOOLING_PACKAGES ?= ""
> -HOST_TOOLING_DEPENDS ?= ""
> +HOST_TOOLING_PACKAGES ?= "dracut-core binutils zstd dracut-cross-ldd"
> +HOST_TOOLING_DEPENDS ?= "dracut-cross-ldd"
>   
>   # dependency to build the host-tooling-chroot
>   HOST_TOOLING_DEP ?= "host-tooling-chroot:do_build"
> diff --git a/meta/classes-recipe/initrd-dracut.bbclass b/meta/classes-recipe/initrd-dracut.bbclass
> index 00999b91..e3c9f174 100644
> --- a/meta/classes-recipe/initrd-dracut.bbclass
> +++ b/meta/classes-recipe/initrd-dracut.bbclass
> @@ -56,8 +56,44 @@ def extend_dracut_cmdline(d):
>   ROOTFS_INITRAMFS_GENERATOR_CMDLINE = "dracut --force --kver $kernel_version -L 5 --reproducible"
>   ROOTFS_INITRAMFS_GENERATOR_CMDLINE:append = " ${@ extend_dracut_cmdline(d)}"
>   
> +# Cross-assemble the initramfs, requires preparation of dracut module definitions
> +def dracut_cross_default(d):
> +    return bb.utils.to_boolean(d.getVar('ISAR_CROSS_COMPILE')) \
> +        and bb.utils.to_boolean(d.getVar('ROOTFS_USE_DRACUT')) \
> +        and d.getVar('ROOTFS_ARCH') != d.getVar('HOST_ARCH')
> +
> +ROOTFS_DRACUT_CROSS ??= "${@ '1' if dracut_cross_default(d) else '0' }"
> +ROOTFS_DRACUT_CROSS:bookworm = "0"
> +OVERRIDES:append = "${@':dracut-cross' if bb.utils.to_boolean(d.getVar('ROOTFS_DRACUT_CROSS')) else ''}"
> +
> +ROOTFS_INSTALL_DEPENDS:append:dracut-cross = " ${@bb.utils.contains('ROOTFS_INSTALL_COMMAND', 'rootfs_generate_initramfs', '${HOST_TOOLING_DEP}', '', d)}"
> +ROOTFS_INITRAMFS_GENERATOR_CMDLINE:append:dracut-cross = " --sysroot /mnt/rootfs"
> +DRACUT_MOUNTS = ""
> +DRACUT_MOUNTS:append:dracut-cross = " ${ROOTFSDIR}:/mnt/rootfs"
> +DRACUT_ROOTFS = "${ROOTFSDIR}"
> +DRACUT_ROOTFS:dracut-cross = "${WORKDIR}/host-tooling"
> +DRACUT_ENV = "DRACUT_ARCH=${QEMU_ARCH}"
> +DRACUT_ENV:append:dracut-cross = " \
> +    DRACUT_INSTALL=/usr/lib/dracut/dracut-install \
> +    DRACUT_LDD=/usr/libexec/dracut-cross-ldd \
> +"
> +
>   run_initrd_generator() {
>       mods_total="$(find ${ROOTFSDIR}/usr/lib/dracut/modules.d/ -maxdepth 1 -type d | wc -l)"
>       echo "Total number of modules: $mods_total (dracut)"
> -    run_in_chroot "${ROOTFSDIR}" sh -c "${ROOTFS_INITRAMFS_GENERATOR_CMDLINE}"
> +    if [ "${ROOTFS_DRACUT_CROSS}" = "1" ]; then
> +        mkdir -p ${DRACUT_ROOTFS}
> +        run_privileged tar -xf ${HOST_TOOLING_CHROOT} -C ${DRACUT_ROOTFS}
> +        trap 'run_privileged rm -rf ${DRACUT_ROOTFS}' EXIT
> +    fi
> +    ${@ insert_isar_umounts(d, d.getVar('DRACUT_ROOTFS'), d.getVar('DRACUT_MOUNTS')) }
> +    run_privileged_heredoc <<'EOF'
> +        ${@ insert_isar_mounts(d, d.getVar('DRACUT_ROOTFS'), d.getVar('DRACUT_MOUNTS')) }
> +        export ${DRACUT_ENV}
> +        chroot ${DRACUT_ROOTFS} ${ROOTFS_INITRAMFS_GENERATOR_CMDLINE}
> +EOF
>   }
> +

Hi Felix,

In privileged mode,mounts are left behind, e.g.:
[stdlog] 2026-09-24 14:46:59,368 avocado.test cibuilder        L0369 
INFO | WARNING: 
/isar/build/tmp/work/debian-trixie-arm64/isar-image-ci-qemuarm64/1.0-r0/host-tooling/mnt/rootfs 
left mounted, unmounting...
and
[stdlog] 2026-09-24 14:54:33,999 avocado.test cibuilder        L0369 
INFO | WARNING: 
/isar/build/tmp/work/debian-trixie-arm64/isar-dracut-qemuarm64/1.0-r0/host-tooling/mnt/rootfs 
left mounted, unmounting...

On Jenkins these are treated as errors.

Zhihang

> +HOST_TOOLING_DEP ??= ""
> +inherit_defer ${@'host-tooling' if bb.utils.to_boolean(d.getVar('ROOTFS_DRACUT_CROSS')) else ''}
> +do_generate_initramfs[depends] += "${HOST_TOOLING_DEP}"
> diff --git a/meta/classes-recipe/rootfs.bbclass b/meta/classes-recipe/rootfs.bbclass
> index 1de27d7b..cea85626 100644
> --- a/meta/classes-recipe/rootfs.bbclass
> +++ b/meta/classes-recipe/rootfs.bbclass
> @@ -452,6 +452,8 @@ rootfs_capture_apt_state() {
>           var/lib/dpkg/status
>   }
>   
> +ROOTFS_INSTALL_DEPENDS ?= ""
> +
>   do_rootfs_install[root_cleandirs] = "${ROOTFSDIR}"
>   do_rootfs_install[cleandirs] += "${DEPLOYDIR}"
>   do_rootfs_install[sstate-inputdirs] = "${DEPLOYDIR}"
> @@ -459,7 +461,7 @@ do_rootfs_install[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"
>   do_rootfs_install[vardeps] += "${ROOTFS_CONFIGURE_COMMAND} ${ROOTFS_INSTALL_COMMAND} ${ROOTFS_POSTPROCESS_COMMAND} ${ROOTFS_VARDEPS}"
>   do_rootfs_install[vardepsexclude] += "IMAGE_ROOTFS"
>   do_rootfs_install[depends] = "bootstrap-${@'target' if d.getVar('ROOTFS_ARCH') == d.getVar('DISTRO_ARCH') else 'host'}:do_build"
> -do_rootfs_install[depends] += "base-apt:do_cache isar-apt:do_cache_config"
> +do_rootfs_install[depends] += "base-apt:do_cache isar-apt:do_cache_config ${ROOTFS_INSTALL_DEPENDS}"
>   do_rootfs_install[deptask] = "do_deploy_deb"
>   do_rootfs_install[rdeptask] = "do_deploy_deb"
>   do_rootfs_install[network] = "${TASK_USE_SUDO}"
> diff --git a/meta/recipes-devtools/dracut-cross-ldd/dracut-cross-ldd.bb b/meta/recipes-devtools/dracut-cross-ldd/dracut-cross-ldd.bb
> new file mode 100644
> index 00000000..327a5c82
> --- /dev/null
> +++ b/meta/recipes-devtools/dracut-cross-ldd/dracut-cross-ldd.bb
> @@ -0,0 +1,16 @@
> +# This software is a part of Isar.
> +# Copyright (C) 2026 Siemens AG
> +
> +inherit dpkg-raw
> +
> +MAINTAINER = "isar-users <isar-users@googlegroups.com>"
> +DESCRIPTION = "Minimal ldd replacement for dracut cross builds"
> +
> +SRC_URI = "file://${BPN}"
> +
> +DEBIAN_DEPENDS .= ",python3, binutils"
> +
> +do_install[cleandirs] += "${D}/usr/libexec"
> +do_install() {
> +    install -v -m 755 "${WORKDIR}/${BPN}" "${D}/usr/libexec/${BPN}"
> +}
> diff --git a/meta/recipes-devtools/dracut-cross-ldd/files/dracut-cross-ldd b/meta/recipes-devtools/dracut-cross-ldd/files/dracut-cross-ldd
> new file mode 100644
> index 00000000..ef60b3fa
> --- /dev/null
> +++ b/meta/recipes-devtools/dracut-cross-ldd/files/dracut-cross-ldd
> @@ -0,0 +1,65 @@
> +#!/usr/bin/env python3
> +# This software is a part of Isar.
> +# Copyright (C) 2026 Siemens AG
> +import re
> +import subprocess
> +import sys
> +from pathlib import Path
> +
> +# glob patterns relative to the sysroot, including multiarch directories
> +LIB_DIRS = ["lib", "lib/*", "usr/lib", "usr/lib/*"]
> +
> +
> +def find_sysroot(binary):
> +    for parent in binary.parents:
> +        if (parent / "usr/lib").is_dir():
> +            return parent
> +    return Path("/")
> +
> +
> +def find_library(name, search_dirs, sysroot):
> +    for directory in search_dirs:
> +        for path in sysroot.glob(f"{directory.strip('/')}/{name}"):
> +            return "/" + str(path.relative_to(sysroot))
> +    return None
> +
> +
> +def read_dynamic_section(path):
> +    return subprocess.check_output(["readelf", "-d", path], text=True)
> +
> +
> +def get_needed(dynamic):
> +    runpaths = re.findall(r'\((?:RUNPATH|RPATH)\).*?\[(.*?)\]', dynamic)
> +    search_dirs = [d for paths in runpaths for d in paths.split(":")]
> +    needed = re.findall(r'\(NEEDED\).*?\[(.*?)\]', dynamic)
> +    return search_dirs + LIB_DIRS, needed
> +
> +
> +binary = Path(sys.argv[-1]).absolute()
> +try:
> +    dynamic = read_dynamic_section(binary)
> +except subprocess.SubprocessError:
> +    sys.exit(1)
> +
> +sysroot = find_sysroot(binary)
> +resolved = {}
> +pending = [get_needed(dynamic)]
> +
> +# ldd reports the whole dependency tree, so resolve it transitively
> +while pending:
> +    search_dirs, needed = pending.pop()
> +    for library in needed:
> +        if library in resolved:
> +            continue
> +        path = find_library(library, search_dirs, sysroot)
> +        resolved[library] = path
> +        if path:
> +            try:
> +                pending.append(get_needed(
> +                    read_dynamic_section(sysroot / path.strip("/"))))
> +            except subprocess.SubprocessError:
> +                pass
> +
> +for library, path in resolved.items():
> +    print(f"\t{library} => {path} (0x0000000000000000)" if path
> +          else f"\t{library} => not found")

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