Adding upstream version 247.3.upstream/247.3upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1312 insertions, 0 deletions

diff --git a/src/nspawn/nspawn-mount.c b/src/nspawn/nspawn-mount.c
new file mode 100644
index 0000000..2ea1bed
--- /dev/null
+++ b/src/nspawn/nspawn-mount.c
@@ -0,0 +1,1312 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <sys/mount.h>
+#include <linux/magic.h>
+
+#include "alloc-util.h"
+#include "escape.h"
+#include "fd-util.h"
+#include "format-util.h"
+#include "fs-util.h"
+#include "label.h"
+#include "mkdir.h"
+#include "mount-util.h"
+#include "mountpoint-util.h"
+#include "nspawn-mount.h"
+#include "parse-util.h"
+#include "path-util.h"
+#include "rm-rf.h"
+#include "set.h"
+#include "sort-util.h"
+#include "stat-util.h"
+#include "string-util.h"
+#include "strv.h"
+#include "tmpfile-util.h"
+#include "user-util.h"
+
+CustomMount* custom_mount_add(CustomMount **l, size_t *n, CustomMountType t) {
+        CustomMount *c, *ret;
+
+        assert(l);
+        assert(n);
+        assert(t >= 0);
+        assert(t < _CUSTOM_MOUNT_TYPE_MAX);
+
+        c = reallocarray(*l, *n + 1, sizeof(CustomMount));
+        if (!c)
+                return NULL;
+
+        *l = c;
+        ret = *l + *n;
+        (*n)++;
+
+        *ret = (CustomMount) {
+                .type = t
+        };
+
+        return ret;
+}
+
+void custom_mount_free_all(CustomMount *l, size_t n) {
+        size_t i;
+
+        for (i = 0; i < n; i++) {
+                CustomMount *m = l + i;
+
+                free(m->source);
+                free(m->destination);
+                free(m->options);
+
+                if (m->work_dir) {
+                        (void) rm_rf(m->work_dir, REMOVE_ROOT|REMOVE_PHYSICAL);
+                        free(m->work_dir);
+                }
+
+                if (m->rm_rf_tmpdir) {
+                        (void) rm_rf(m->rm_rf_tmpdir, REMOVE_ROOT|REMOVE_PHYSICAL);
+                        free(m->rm_rf_tmpdir);
+                }
+
+                strv_free(m->lower);
+                free(m->type_argument);
+        }
+
+        free(l);
+}
+
+static int custom_mount_compare(const CustomMount *a, const CustomMount *b) {
+        int r;
+
+        r = path_compare(a->destination, b->destination);
+        if (r != 0)
+                return r;
+
+        return CMP(a->type, b->type);
+}
+
+static bool source_path_is_valid(const char *p) {
+        assert(p);
+
+        if (*p == '+')
+                p++;
+
+        return path_is_absolute(p);
+}
+
+static char *resolve_source_path(const char *dest, const char *source) {
+
+        if (!source)
+                return NULL;
+
+        if (source[0] == '+')
+                return path_join(dest, source + 1);
+
+        return strdup(source);
+}
+
+static int allocate_temporary_source(CustomMount *m) {
+        assert(m);
+        assert(!m->source);
+        assert(!m->rm_rf_tmpdir);
+
+        m->rm_rf_tmpdir = strdup("/var/tmp/nspawn-temp-XXXXXX");
+        if (!m->rm_rf_tmpdir)
+                return log_oom();
+
+        if (!mkdtemp(m->rm_rf_tmpdir)) {
+                m->rm_rf_tmpdir = mfree(m->rm_rf_tmpdir);
+                return log_error_errno(errno, "Failed to acquire temporary directory: %m");
+        }
+
+        m->source = path_join(m->rm_rf_tmpdir, "src");
+        if (!m->source)
+                return log_oom();
+
+        if (mkdir(m->source, 0755) < 0)
+                return log_error_errno(errno, "Failed to create %s: %m", m->source);
+
+        return 0;
+}
+
+int custom_mount_prepare_all(const char *dest, CustomMount *l, size_t n) {
+        size_t i;
+        int r;
+
+        /* Prepare all custom mounts. This will make source we know all temporary directories. This is called in the
+         * parent process, so that we know the temporary directories to remove on exit before we fork off the
+         * children. */
+
+        assert(l || n == 0);
+
+        /* Order the custom mounts, and make sure we have a working directory */
+        typesafe_qsort(l, n, custom_mount_compare);
+
+        for (i = 0; i < n; i++) {
+                CustomMount *m = l + i;
+
+                /* /proc we mount in the inner child, i.e. when we acquired CLONE_NEWPID. All other mounts we mount
+                 * already in the outer child, so that the mounts are already established before CLONE_NEWPID and in
+                 * particular CLONE_NEWUSER. This also means any custom mounts below /proc also need to be mounted in
+                 * the inner child, not the outer one. Determine this here. */
+                m->in_userns = path_startswith(m->destination, "/proc");
+
+                if (m->type == CUSTOM_MOUNT_BIND) {
+                        if (m->source) {
+                                char *s;
+
+                                s = resolve_source_path(dest, m->source);
+                                if (!s)
+                                        return log_oom();
+
+                                free_and_replace(m->source, s);
+                        } else {
+                                /* No source specified? In that case, use a throw-away temporary directory in /var/tmp */
+
+                                r = allocate_temporary_source(m);
+                                if (r < 0)
+                                        return r;
+                        }
+                }
+
+                if (m->type == CUSTOM_MOUNT_OVERLAY) {
+                        char **j;
+
+                        STRV_FOREACH(j, m->lower) {
+                                char *s;
+
+                                s = resolve_source_path(dest, *j);
+                                if (!s)
+                                        return log_oom();
+
+                                free_and_replace(*j, s);
+                        }
+
+                        if (m->source) {
+                                char *s;
+
+                                s = resolve_source_path(dest, m->source);
+                                if (!s)
+                                        return log_oom();
+
+                                free_and_replace(m->source, s);
+                        } else {
+                                r = allocate_temporary_source(m);
+                                if (r < 0)
+                                        return r;
+                        }
+
+                        if (m->work_dir) {
+                                char *s;
+
+                                s = resolve_source_path(dest, m->work_dir);
+                                if (!s)
+                                        return log_oom();
+
+                                free_and_replace(m->work_dir, s);
+                        } else {
+                                r = tempfn_random(m->source, NULL, &m->work_dir);
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to acquire working directory: %m");
+                        }
+
+                        (void) mkdir_label(m->work_dir, 0700);
+                }
+        }
+
+        return 0;
+}
+
+int bind_mount_parse(CustomMount **l, size_t *n, const char *s, bool read_only) {
+        _cleanup_free_ char *source = NULL, *destination = NULL, *opts = NULL;
+        const char *p = s;
+        CustomMount *m;
+        int r;
+
+        assert(l);
+        assert(n);
+
+        r = extract_many_words(&p, ":", EXTRACT_DONT_COALESCE_SEPARATORS, &source, &destination, NULL);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EINVAL;
+        if (r == 1) {
+                destination = strdup(source[0] == '+' ? source+1 : source);
+                if (!destination)
+                        return -ENOMEM;
+        }
+        if (r == 2 && !isempty(p)) {
+                opts = strdup(p);
+                if (!opts)
+                        return -ENOMEM;
+        }
+
+        if (isempty(source))
+                source = mfree(source);
+        else if (!source_path_is_valid(source))
+                return -EINVAL;
+
+        if (!path_is_absolute(destination))
+                return -EINVAL;
+
+        m = custom_mount_add(l, n, CUSTOM_MOUNT_BIND);
+        if (!m)
+                return -ENOMEM;
+
+        m->source = TAKE_PTR(source);
+        m->destination = TAKE_PTR(destination);
+        m->read_only = read_only;
+        m->options = TAKE_PTR(opts);
+
+        return 0;
+}
+
+int tmpfs_mount_parse(CustomMount **l, size_t *n, const char *s) {
+        _cleanup_free_ char *path = NULL, *opts = NULL;
+        const char *p = s;
+        CustomMount *m;
+        int r;
+
+        assert(l);
+        assert(n);
+        assert(s);
+
+        r = extract_first_word(&p, &path, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EINVAL;
+
+        if (isempty(p))
+                opts = strdup("mode=0755");
+        else
+                opts = strdup(p);
+        if (!opts)
+                return -ENOMEM;
+
+        if (!path_is_absolute(path))
+                return -EINVAL;
+
+        m = custom_mount_add(l, n, CUSTOM_MOUNT_TMPFS);
+        if (!m)
+                return -ENOMEM;
+
+        m->destination = TAKE_PTR(path);
+        m->options = TAKE_PTR(opts);
+
+        return 0;
+}
+
+int overlay_mount_parse(CustomMount **l, size_t *n, const char *s, bool read_only) {
+        _cleanup_free_ char *upper = NULL, *destination = NULL;
+        _cleanup_strv_free_ char **lower = NULL;
+        CustomMount *m;
+        int k;
+
+        k = strv_split_full(&lower, s, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
+        if (k < 0)
+                return k;
+        if (k < 2)
+                return -EADDRNOTAVAIL;
+        if (k == 2) {
+                /* If two parameters are specified, the first one is the lower, the second one the upper directory. And
+                 * we'll also define the destination mount point the same as the upper. */
+
+                if (!source_path_is_valid(lower[0]) ||
+                    !source_path_is_valid(lower[1]))
+                        return -EINVAL;
+
+                upper = TAKE_PTR(lower[1]);
+
+                destination = strdup(upper[0] == '+' ? upper+1 : upper); /* take the destination without "+" prefix */
+                if (!destination)
+                        return -ENOMEM;
+        } else {
+                char **i;
+
+                /* If more than two parameters are specified, the last one is the destination, the second to last one
+                 * the "upper", and all before that the "lower" directories. */
+
+                destination = lower[k - 1];
+                upper = TAKE_PTR(lower[k - 2]);
+
+                STRV_FOREACH(i, lower)
+                        if (!source_path_is_valid(*i))
+                                return -EINVAL;
+
+                /* If the upper directory is unspecified, then let's create it automatically as a throw-away directory
+                 * in /var/tmp */
+                if (isempty(upper))
+                        upper = mfree(upper);
+                else if (!source_path_is_valid(upper))
+                        return -EINVAL;
+
+                if (!path_is_absolute(destination))
+                        return -EINVAL;
+        }
+
+        m = custom_mount_add(l, n, CUSTOM_MOUNT_OVERLAY);
+        if (!m)
+                return -ENOMEM;
+
+        m->destination = TAKE_PTR(destination);
+        m->source = TAKE_PTR(upper);
+        m->lower = TAKE_PTR(lower);
+        m->read_only = read_only;
+
+        return 0;
+}
+
+int inaccessible_mount_parse(CustomMount **l, size_t *n, const char *s) {
+        _cleanup_free_ char *path = NULL;
+        CustomMount *m;
+
+        assert(l);
+        assert(n);
+        assert(s);
+
+        if (!path_is_absolute(s))
+                return -EINVAL;
+
+        path = strdup(s);
+        if (!path)
+                return -ENOMEM;
+
+        m = custom_mount_add(l, n, CUSTOM_MOUNT_INACCESSIBLE);
+        if (!m)
+                return -ENOMEM;
+
+        m->destination = TAKE_PTR(path);
+        return 0;
+}
+
+int tmpfs_patch_options(
+                const char *options,
+                uid_t uid_shift,
+                const char *selinux_apifs_context,
+                char **ret) {
+
+        char *buf = NULL;
+
+        if (uid_shift != UID_INVALID) {
+                if (asprintf(&buf, "%s%suid=" UID_FMT ",gid=" UID_FMT,
+                             strempty(options), options ? "," : "",
+                             uid_shift, uid_shift) < 0)
+                        return -ENOMEM;
+
+                options = buf;
+        }
+
+#if HAVE_SELINUX
+        if (selinux_apifs_context) {
+                char *t;
+
+                t = strjoin(strempty(options), options ? "," : "",
+                            "context=\"", selinux_apifs_context, "\"");
+                free(buf);
+                if (!t)
+                        return -ENOMEM;
+
+                buf = t;
+        }
+#endif
+
+        if (!buf && options) {
+                buf = strdup(options);
+                if (!buf)
+                        return -ENOMEM;
+        }
+        *ret = buf;
+
+        return !!buf;
+}
+
+int mount_sysfs(const char *dest, MountSettingsMask mount_settings) {
+        const char *full, *top, *x;
+        int r;
+        unsigned long extra_flags = 0;
+
+        top = prefix_roota(dest, "/sys");
+        r = path_is_fs_type(top, SYSFS_MAGIC);
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine filesystem type of %s: %m", top);
+        /* /sys might already be mounted as sysfs by the outer child in the
+         * !netns case. In this case, it's all good. Don't touch it because we
+         * don't have the right to do so, see https://github.com/systemd/systemd/issues/1555.
+         */
+        if (r > 0)
+                return 0;
+
+        full = prefix_roota(top, "/full");
+
+        (void) mkdir(full, 0755);
+
+        if (FLAGS_SET(mount_settings, MOUNT_APPLY_APIVFS_RO))
+                extra_flags |= MS_RDONLY;
+
+        r = mount_nofollow_verbose(LOG_ERR, "sysfs", full, "sysfs",
+                                   MS_NOSUID|MS_NOEXEC|MS_NODEV|extra_flags, NULL);
+        if (r < 0)
+                return r;
+
+        FOREACH_STRING(x, "block", "bus", "class", "dev", "devices", "kernel") {
+                _cleanup_free_ char *from = NULL, *to = NULL;
+
+                from = path_join(full, x);
+                if (!from)
+                        return log_oom();
+
+                to = path_join(top, x);
+                if (!to)
+                        return log_oom();
+
+                (void) mkdir(to, 0755);
+
+                r = mount_nofollow_verbose(LOG_ERR, from, to, NULL, MS_BIND, NULL);
+                if (r < 0)
+                        return r;
+
+                r = mount_nofollow_verbose(LOG_ERR, NULL, to, NULL,
+                                           MS_BIND|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT|extra_flags, NULL);
+                if (r < 0)
+                        return r;
+        }
+
+        r = umount_verbose(LOG_ERR, full, UMOUNT_NOFOLLOW);
+        if (r < 0)
+                return r;
+
+        if (rmdir(full) < 0)
+                return log_error_errno(errno, "Failed to remove %s: %m", full);
+
+        /* Create mountpoint for cgroups. Otherwise we are not allowed since we
+         * remount /sys read-only.
+         */
+        x = prefix_roota(top, "/fs/cgroup");
+        (void) mkdir_p(x, 0755);
+
+        return mount_nofollow_verbose(LOG_ERR, NULL, top, NULL,
+                                      MS_BIND|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT|extra_flags, NULL);
+}
+
+int mount_all(const char *dest,
+              MountSettingsMask mount_settings,
+              uid_t uid_shift,
+              const char *selinux_apifs_context) {
+
+#define PROC_INACCESSIBLE_REG(path)                                     \
+        { "/run/systemd/inaccessible/reg", (path), NULL, NULL, MS_BIND, \
+          MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO }, /* Bind mount first ... */ \
+        { NULL, (path), NULL, NULL, MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, \
+          MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO } /* Then, make it r/o */
+
+#define PROC_READ_ONLY(path)                                            \
+        { (path), (path), NULL, NULL, MS_BIND,                          \
+          MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO }, /* Bind mount first ... */ \
+        { NULL,   (path), NULL, NULL, MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, \
+          MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO } /* Then, make it r/o */
+
+        typedef struct MountPoint {
+                const char *what;
+                const char *where;
+                const char *type;
+                const char *options;
+                unsigned long flags;
+                MountSettingsMask mount_settings;
+        } MountPoint;
+
+        static const MountPoint mount_table[] = {
+                /* First we list inner child mounts (i.e. mounts applied *after* entering user namespacing) */
+                { "proc",            "/proc",           "proc",  NULL,        MS_NOSUID|MS_NOEXEC|MS_NODEV,
+                  MOUNT_FATAL|MOUNT_IN_USERNS|MOUNT_MKDIR|MOUNT_FOLLOW_SYMLINKS }, /* we follow symlinks here since not following them requires /proc/ already being mounted, which we don't have here. */
+
+                { "/proc/sys",       "/proc/sys",       NULL,    NULL,        MS_BIND,
+                  MOUNT_FATAL|MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO },                          /* Bind mount first ... */
+
+                { "/proc/sys/net",   "/proc/sys/net",   NULL,    NULL,        MS_BIND,
+                  MOUNT_FATAL|MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO|MOUNT_APPLY_APIVFS_NETNS }, /* (except for this) */
+
+                { NULL,              "/proc/sys",       NULL,    NULL,        MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT,
+                  MOUNT_FATAL|MOUNT_IN_USERNS|MOUNT_APPLY_APIVFS_RO },                          /* ... then, make it r/o */
+
+                /* Make these files inaccessible to container payloads: they potentially leak information about kernel
+                 * internals or the host's execution environment to the container */
+                PROC_INACCESSIBLE_REG("/proc/kallsyms"),
+                PROC_INACCESSIBLE_REG("/proc/kcore"),
+                PROC_INACCESSIBLE_REG("/proc/keys"),
+                PROC_INACCESSIBLE_REG("/proc/sysrq-trigger"),
+                PROC_INACCESSIBLE_REG("/proc/timer_list"),
+
+                /* Make these directories read-only to container payloads: they show hardware information, and in some
+                 * cases contain tunables the container really shouldn't have access to. */
+                PROC_READ_ONLY("/proc/acpi"),
+                PROC_READ_ONLY("/proc/apm"),
+                PROC_READ_ONLY("/proc/asound"),
+                PROC_READ_ONLY("/proc/bus"),
+                PROC_READ_ONLY("/proc/fs"),
+                PROC_READ_ONLY("/proc/irq"),
+                PROC_READ_ONLY("/proc/scsi"),
+
+                { "mqueue",                 "/dev/mqueue",                  "mqueue", NULL,                            MS_NOSUID|MS_NOEXEC|MS_NODEV,
+                  MOUNT_IN_USERNS|MOUNT_MKDIR },
+
+                /* Then we list outer child mounts (i.e. mounts applied *before* entering user namespacing) */
+                { "tmpfs",                  "/tmp",                         "tmpfs", "mode=1777" NESTED_TMPFS_LIMITS,  MS_NOSUID|MS_NODEV|MS_STRICTATIME,
+                  MOUNT_FATAL|MOUNT_APPLY_TMPFS_TMP|MOUNT_MKDIR },
+                { "tmpfs",                  "/sys",                         "tmpfs", "mode=555" TMPFS_LIMITS_SYS,      MS_NOSUID|MS_NOEXEC|MS_NODEV,
+                  MOUNT_FATAL|MOUNT_APPLY_APIVFS_NETNS|MOUNT_MKDIR },
+                { "sysfs",                  "/sys",                         "sysfs", NULL,                             MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV,
+                  MOUNT_FATAL|MOUNT_APPLY_APIVFS_RO|MOUNT_MKDIR },    /* skipped if above was mounted */
+                { "sysfs",                  "/sys",                         "sysfs", NULL,                             MS_NOSUID|MS_NOEXEC|MS_NODEV,
+                  MOUNT_FATAL|MOUNT_MKDIR },                          /* skipped if above was mounted */
+                { "tmpfs",                  "/dev",                         "tmpfs", "mode=755" TMPFS_LIMITS_DEV,      MS_NOSUID|MS_STRICTATIME,
+                  MOUNT_FATAL|MOUNT_MKDIR },
+                { "tmpfs",                  "/dev/shm",                     "tmpfs", "mode=1777" NESTED_TMPFS_LIMITS,  MS_NOSUID|MS_NODEV|MS_STRICTATIME,
+                  MOUNT_FATAL|MOUNT_MKDIR },
+                { "tmpfs",                  "/run",                         "tmpfs", "mode=755" TMPFS_LIMITS_RUN,      MS_NOSUID|MS_NODEV|MS_STRICTATIME,
+                  MOUNT_FATAL|MOUNT_MKDIR },
+                { "/run/host",              "/run/host",                    NULL,    NULL,                             MS_BIND,
+                  MOUNT_FATAL|MOUNT_MKDIR|MOUNT_PREFIX_ROOT }, /* Prepare this so that we can make it read-only when we are done */
+                { "/etc/os-release",        "/run/host/os-release",         NULL,    NULL,                             MS_BIND,
+                  MOUNT_TOUCH }, /* As per kernel interface requirements, bind mount first (creating mount points) and make read-only later */
+                { "/usr/lib/os-release",    "/run/host/os-release",         NULL,    NULL,                             MS_BIND,
+                  MOUNT_FATAL }, /* If /etc/os-release doesn't exist use the version in /usr/lib as fallback */
+                { NULL,                     "/run/host/os-release",         NULL,    NULL,                             MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT,
+                  MOUNT_FATAL },
+                { NULL,                     "/run/host",                    NULL,    NULL,                             MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT,
+                  MOUNT_FATAL|MOUNT_IN_USERNS },
+#if HAVE_SELINUX
+                { "/sys/fs/selinux",        "/sys/fs/selinux",              NULL,    NULL,                             MS_BIND,
+                  MOUNT_MKDIR },  /* Bind mount first (mkdir/chown the mount point in case /sys/ is mounted as minimal skeleton tmpfs) */
+                { NULL,                     "/sys/fs/selinux",              NULL,    NULL,                             MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT,
+                  0 },            /* Then, make it r/o (don't mkdir/chown the mount point here, the previous entry already did that) */
+#endif
+        };
+
+        bool use_userns = FLAGS_SET(mount_settings, MOUNT_USE_USERNS);
+        bool netns = FLAGS_SET(mount_settings, MOUNT_APPLY_APIVFS_NETNS);
+        bool ro = FLAGS_SET(mount_settings, MOUNT_APPLY_APIVFS_RO);
+        bool in_userns = FLAGS_SET(mount_settings, MOUNT_IN_USERNS);
+        bool tmpfs_tmp = FLAGS_SET(mount_settings, MOUNT_APPLY_TMPFS_TMP);
+        size_t k;
+        int r;
+
+        for (k = 0; k < ELEMENTSOF(mount_table); k++) {
+                _cleanup_free_ char *where = NULL, *options = NULL, *prefixed = NULL;
+                bool fatal = FLAGS_SET(mount_table[k].mount_settings, MOUNT_FATAL);
+                const char *o;
+
+                if (in_userns != FLAGS_SET(mount_table[k].mount_settings, MOUNT_IN_USERNS))
+                        continue;
+
+                if (!netns && FLAGS_SET(mount_table[k].mount_settings, MOUNT_APPLY_APIVFS_NETNS))
+                        continue;
+
+                if (!ro && FLAGS_SET(mount_table[k].mount_settings, MOUNT_APPLY_APIVFS_RO))
+                        continue;
+
+                if (!tmpfs_tmp && FLAGS_SET(mount_table[k].mount_settings, MOUNT_APPLY_TMPFS_TMP))
+                        continue;
+
+                r = chase_symlinks(mount_table[k].where, dest, CHASE_NONEXISTENT|CHASE_PREFIX_ROOT, &where, NULL);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, mount_table[k].where);
+
+                /* Skip this entry if it is not a remount. */
+                if (mount_table[k].what) {
+                        r = path_is_mount_point(where, NULL, 0);
+                        if (r < 0 && r != -ENOENT)
+                                return log_error_errno(r, "Failed to detect whether %s is a mount point: %m", where);
+                        if (r > 0)
+                                continue;
+                }
+
+                if ((mount_table[k].mount_settings & (MOUNT_MKDIR|MOUNT_TOUCH)) != 0) {
+                        uid_t u = (use_userns && !in_userns) ? uid_shift : UID_INVALID;
+
+                        if (FLAGS_SET(mount_table[k].mount_settings, MOUNT_TOUCH))
+                                r = mkdir_parents_safe(dest, where, 0755, u, u, 0);
+                        else
+                                r = mkdir_p_safe(dest, where, 0755, u, u, 0);
+                        if (r < 0 && r != -EEXIST) {
+                                if (fatal && r != -EROFS)
+                                        return log_error_errno(r, "Failed to create directory %s: %m", where);
+
+                                log_debug_errno(r, "Failed to create directory %s: %m", where);
+
+                                /* If we failed mkdir() or chown() due to the root directory being read only,
+                                 * attempt to mount this fs anyway and let mount_verbose log any errors */
+                                if (r != -EROFS)
+                                        continue;
+                        }
+                }
+
+                if (FLAGS_SET(mount_table[k].mount_settings, MOUNT_TOUCH)) {
+                        r = touch(where);
+                        if (r < 0 && r != -EEXIST) {
+                                if (fatal && r != -EROFS)
+                                        return log_error_errno(r, "Failed to create file %s: %m", where);
+
+                                log_debug_errno(r, "Failed to create file %s: %m", where);
+                                if (r != -EROFS)
+                                        continue;
+                        }
+                }
+
+                o = mount_table[k].options;
+                if (streq_ptr(mount_table[k].type, "tmpfs")) {
+                        r = tmpfs_patch_options(o, in_userns ? 0 : uid_shift, selinux_apifs_context, &options);
+                        if (r < 0)
+                                return log_oom();
+                        if (r > 0)
+                                o = options;
+                }
+
+                if (FLAGS_SET(mount_table[k].mount_settings, MOUNT_PREFIX_ROOT)) {
+                        /* Optionally prefix the mount source with the root dir. This is useful in bind
+                         * mounts to be created within the container image before we transition into it. Note
+                         * that MOUNT_IN_USERNS is run after we transitioned hence prefixing is not ncessary
+                         * for those. */
+                        r = chase_symlinks(mount_table[k].what, dest, CHASE_PREFIX_ROOT, &prefixed, NULL);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, mount_table[k].what);
+                }
+
+                r = mount_verbose_full(
+                                fatal ? LOG_ERR : LOG_DEBUG,
+                                prefixed ?: mount_table[k].what,
+                                where,
+                                mount_table[k].type,
+                                mount_table[k].flags,
+                                o,
+                                FLAGS_SET(mount_table[k].mount_settings, MOUNT_FOLLOW_SYMLINKS));
+                if (r < 0 && fatal)
+                        return r;
+        }
+
+        return 0;
+}
+
+static int parse_mount_bind_options(const char *options, unsigned long *mount_flags, char **mount_opts) {
+        const char *p = options;
+        unsigned long flags = *mount_flags;
+        char *opts = NULL;
+        int r;
+
+        assert(options);
+
+        for (;;) {
+                _cleanup_free_ char *word = NULL;
+
+                r = extract_first_word(&p, &word, ",", 0);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to extract mount option: %m");
+                if (r == 0)
+                        break;
+
+                if (streq(word, "rbind"))
+                        flags |= MS_REC;
+                else if (streq(word, "norbind"))
+                        flags &= ~MS_REC;
+                else {
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "Invalid bind mount option: %s",
+                                               word);
+                }
+        }
+
+        *mount_flags = flags;
+        /* in the future mount_opts will hold string options for mount(2) */
+        *mount_opts = opts;
+
+        return 0;
+}
+
+static int mount_bind(const char *dest, CustomMount *m) {
+        _cleanup_free_ char *mount_opts = NULL, *where = NULL;
+        unsigned long mount_flags = MS_BIND | MS_REC;
+        struct stat source_st, dest_st;
+        int r;
+
+        assert(dest);
+        assert(m);
+
+        if (m->options) {
+                r = parse_mount_bind_options(m->options, &mount_flags, &mount_opts);
+                if (r < 0)
+                        return r;
+        }
+
+        if (stat(m->source, &source_st) < 0)
+                return log_error_errno(errno, "Failed to stat %s: %m", m->source);
+
+        r = chase_symlinks(m->destination, dest, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &where, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, m->destination);
+        if (r > 0) { /* Path exists already? */
+
+                if (stat(where, &dest_st) < 0)
+                        return log_error_errno(errno, "Failed to stat %s: %m", where);
+
+                if (S_ISDIR(source_st.st_mode) && !S_ISDIR(dest_st.st_mode))
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "Cannot bind mount directory %s on file %s.",
+                                               m->source, where);
+
+                if (!S_ISDIR(source_st.st_mode) && S_ISDIR(dest_st.st_mode))
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "Cannot bind mount file %s on directory %s.",
+                                               m->source, where);
+
+        } else { /* Path doesn't exist yet? */
+                r = mkdir_parents_label(where, 0755);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to make parents of %s: %m", where);
+
+                /* Create the mount point. Any non-directory file can be
+                * mounted on any non-directory file (regular, fifo, socket,
+                * char, block).
+                */
+                if (S_ISDIR(source_st.st_mode))
+                        r = mkdir_label(where, 0755);
+                else
+                        r = touch(where);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to create mount point %s: %m", where);
+        }
+
+        r = mount_nofollow_verbose(LOG_ERR, m->source, where, NULL, mount_flags, mount_opts);
+        if (r < 0)
+                return r;
+
+        if (m->read_only) {
+                r = bind_remount_recursive(where, MS_RDONLY, MS_RDONLY, NULL);
+                if (r < 0)
+                        return log_error_errno(r, "Read-only bind mount failed: %m");
+        }
+
+        return 0;
+}
+
+static int mount_tmpfs(const char *dest, CustomMount *m, uid_t uid_shift, const char *selinux_apifs_context) {
+
+        const char *options;
+        _cleanup_free_ char *buf = NULL, *where = NULL;
+        int r;
+
+        assert(dest);
+        assert(m);
+
+        r = chase_symlinks(m->destination, dest, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &where, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, m->destination);
+        if (r == 0) { /* Doesn't exist yet? */
+                r = mkdir_p_label(where, 0755);
+                if (r < 0)
+                        return log_error_errno(r, "Creating mount point for tmpfs %s failed: %m", where);
+        }
+
+        r = tmpfs_patch_options(m->options, uid_shift == 0 ? UID_INVALID : uid_shift, selinux_apifs_context, &buf);
+        if (r < 0)
+                return log_oom();
+        options = r > 0 ? buf : m->options;
+
+        return mount_nofollow_verbose(LOG_ERR, "tmpfs", where, "tmpfs", MS_NODEV|MS_STRICTATIME, options);
+}
+
+static char *joined_and_escaped_lower_dirs(char **lower) {
+        _cleanup_strv_free_ char **sv = NULL;
+
+        sv = strv_copy(lower);
+        if (!sv)
+                return NULL;
+
+        strv_reverse(sv);
+
+        if (!strv_shell_escape(sv, ",:"))
+                return NULL;
+
+        return strv_join(sv, ":");
+}
+
+static int mount_overlay(const char *dest, CustomMount *m) {
+        _cleanup_free_ char *lower = NULL, *where = NULL, *escaped_source = NULL;
+        const char *options;
+        int r;
+
+        assert(dest);
+        assert(m);
+
+        r = chase_symlinks(m->destination, dest, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &where, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, m->destination);
+        if (r == 0) { /* Doesn't exist yet? */
+                r = mkdir_label(where, 0755);
+                if (r < 0)
+                        return log_error_errno(r, "Creating mount point for overlay %s failed: %m", where);
+        }
+
+        (void) mkdir_p_label(m->source, 0755);
+
+        lower = joined_and_escaped_lower_dirs(m->lower);
+        if (!lower)
+                return log_oom();
+
+        escaped_source = shell_escape(m->source, ",:");
+        if (!escaped_source)
+                return log_oom();
+
+        if (m->read_only)
+                options = strjoina("lowerdir=", escaped_source, ":", lower);
+        else {
+                _cleanup_free_ char *escaped_work_dir = NULL;
+
+                escaped_work_dir = shell_escape(m->work_dir, ",:");
+                if (!escaped_work_dir)
+                        return log_oom();
+
+                options = strjoina("lowerdir=", lower, ",upperdir=", escaped_source, ",workdir=", escaped_work_dir);
+        }
+
+        return mount_nofollow_verbose(LOG_ERR, "overlay", where, "overlay", m->read_only ? MS_RDONLY : 0, options);
+}
+
+static int mount_inaccessible(const char *dest, CustomMount *m) {
+        _cleanup_free_ char *where = NULL, *source = NULL;
+        struct stat st;
+        int r;
+
+        assert(dest);
+        assert(m);
+
+        r = chase_symlinks_and_stat(m->destination, dest, CHASE_PREFIX_ROOT, &where, &st, NULL);
+        if (r < 0) {
+                log_full_errno(m->graceful ? LOG_DEBUG : LOG_ERR, r, "Failed to resolve %s/%s: %m", dest, m->destination);
+                return m->graceful ? 0 : r;
+        }
+
+        r = mode_to_inaccessible_node(NULL, st.st_mode, &source);
+        if (r < 0)
+                return m->graceful ? 0 : r;
+
+        r = mount_nofollow_verbose(m->graceful ? LOG_DEBUG : LOG_ERR, source, where, NULL, MS_BIND, NULL);
+        if (r < 0)
+                return m->graceful ? 0 : r;
+
+        r = mount_nofollow_verbose(m->graceful ? LOG_DEBUG : LOG_ERR, NULL, where, NULL, MS_BIND|MS_RDONLY|MS_REMOUNT, NULL);
+        if (r < 0) {
+                (void) umount_verbose(m->graceful ? LOG_DEBUG : LOG_ERR, where, UMOUNT_NOFOLLOW);
+                return m->graceful ? 0 : r;
+        }
+
+        return 0;
+}
+
+static int mount_arbitrary(const char *dest, CustomMount *m) {
+        _cleanup_free_ char *where = NULL;
+        int r;
+
+        assert(dest);
+        assert(m);
+
+        r = chase_symlinks(m->destination, dest, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &where, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to resolve %s/%s: %m", dest, m->destination);
+        if (r == 0) { /* Doesn't exist yet? */
+                r = mkdir_p_label(where, 0755);
+                if (r < 0)
+                        return log_error_errno(r, "Creating mount point for mount %s failed: %m", where);
+        }
+
+        return mount_nofollow_verbose(LOG_ERR, m->source, where, m->type_argument, 0, m->options);
+}
+
+int mount_custom(
+                const char *dest,
+                CustomMount *mounts, size_t n,
+                uid_t uid_shift,
+                const char *selinux_apifs_context,
+                MountSettingsMask mount_settings) {
+
+        size_t i;
+        int r;
+
+        assert(dest);
+
+        for (i = 0; i < n; i++) {
+                CustomMount *m = mounts + i;
+
+                if (FLAGS_SET(mount_settings, MOUNT_IN_USERNS) != m->in_userns)
+                        continue;
+
+                if (FLAGS_SET(mount_settings, MOUNT_ROOT_ONLY) && !path_equal(m->destination, "/"))
+                        continue;
+
+                if (FLAGS_SET(mount_settings, MOUNT_NON_ROOT_ONLY) && path_equal(m->destination, "/"))
+                        continue;
+
+                switch (m->type) {
+
+                case CUSTOM_MOUNT_BIND:
+                        r = mount_bind(dest, m);
+                        break;
+
+                case CUSTOM_MOUNT_TMPFS:
+                        r = mount_tmpfs(dest, m, uid_shift, selinux_apifs_context);
+                        break;
+
+                case CUSTOM_MOUNT_OVERLAY:
+                        r = mount_overlay(dest, m);
+                        break;
+
+                case CUSTOM_MOUNT_INACCESSIBLE:
+                        r = mount_inaccessible(dest, m);
+                        break;
+
+                case CUSTOM_MOUNT_ARBITRARY:
+                        r = mount_arbitrary(dest, m);
+                        break;
+
+                default:
+                        assert_not_reached("Unknown custom mount type");
+                }
+
+                if (r < 0)
+                        return r;
+        }
+
+        return 0;
+}
+
+bool has_custom_root_mount(const CustomMount *mounts, size_t n) {
+        size_t i;
+
+        for (i = 0; i < n; i++) {
+                const CustomMount *m = mounts + i;
+
+                if (path_equal(m->destination, "/"))
+                        return true;
+        }
+
+        return false;
+}
+
+static int setup_volatile_state(const char *directory, uid_t uid_shift, const char *selinux_apifs_context) {
+
+        _cleanup_free_ char *buf = NULL;
+        const char *p, *options;
+        int r;
+
+        assert(directory);
+
+        /* --volatile=state means we simply overmount /var with a tmpfs, and the rest read-only. */
+
+        r = bind_remount_recursive(directory, MS_RDONLY, MS_RDONLY, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to remount %s read-only: %m", directory);
+
+        p = prefix_roota(directory, "/var");
+        r = mkdir(p, 0755);
+        if (r < 0 && errno != EEXIST)
+                return log_error_errno(errno, "Failed to create %s: %m", directory);
+
+        options = "mode=755" TMPFS_LIMITS_VOLATILE_STATE;
+        r = tmpfs_patch_options(options, uid_shift == 0 ? UID_INVALID : uid_shift, selinux_apifs_context, &buf);
+        if (r < 0)
+                return log_oom();
+        if (r > 0)
+                options = buf;
+
+        return mount_nofollow_verbose(LOG_ERR, "tmpfs", p, "tmpfs", MS_STRICTATIME, options);
+}
+
+static int setup_volatile_yes(const char *directory, uid_t uid_shift, const char *selinux_apifs_context) {
+
+        bool tmpfs_mounted = false, bind_mounted = false;
+        char template[] = "/tmp/nspawn-volatile-XXXXXX";
+        _cleanup_free_ char *buf = NULL, *bindir = NULL;
+        const char *f, *t, *options;
+        struct stat st;
+        int r;
+
+        assert(directory);
+
+        /* --volatile=yes means we mount a tmpfs to the root dir, and the original /usr to use inside it, and
+         * that read-only. Before we start setting this up let's validate if the image has the /usr merge
+         * implemented, and let's output a friendly log message if it hasn't. */
+
+        bindir = path_join(directory, "/bin");
+        if (!bindir)
+                return log_oom();
+        if (lstat(bindir, &st) < 0) {
+                if (errno != ENOENT)
+                        return log_error_errno(errno, "Failed to stat /bin directory below image: %m");
+
+                /* ENOENT is fine, just means the image is probably just a naked /usr and we can create the
+                 * rest. */
+        } else if (S_ISDIR(st.st_mode))
+                return log_error_errno(SYNTHETIC_ERRNO(EISDIR),
+                                       "Sorry, --volatile=yes mode is not supported with OS images that have not merged /bin/, /sbin/, /lib/, /lib64/ into /usr/. "
+                                       "Please work with your distribution and help them adopt the merged /usr scheme.");
+        else if (!S_ISLNK(st.st_mode))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Error starting image: if --volatile=yes is used /bin must be a symlink (for merged /usr support) or non-existent (in which case a symlink is created automatically).");
+
+        if (!mkdtemp(template))
+                return log_error_errno(errno, "Failed to create temporary directory: %m");
+
+        options = "mode=755" TMPFS_LIMITS_ROOTFS;
+        r = tmpfs_patch_options(options, uid_shift == 0 ? UID_INVALID : uid_shift, selinux_apifs_context, &buf);
+        if (r < 0)
+                goto fail;
+        if (r > 0)
+                options = buf;
+
+        r = mount_nofollow_verbose(LOG_ERR, "tmpfs", template, "tmpfs", MS_STRICTATIME, options);
+        if (r < 0)
+                goto fail;
+
+        tmpfs_mounted = true;
+
+        f = prefix_roota(directory, "/usr");
+        t = prefix_roota(template, "/usr");
+
+        r = mkdir(t, 0755);
+        if (r < 0 && errno != EEXIST) {
+                r = log_error_errno(errno, "Failed to create %s: %m", t);
+                goto fail;
+        }
+
+        r = mount_nofollow_verbose(LOG_ERR, f, t, NULL, MS_BIND|MS_REC, NULL);
+        if (r < 0)
+                goto fail;
+
+        bind_mounted = true;
+
+        r = bind_remount_recursive(t, MS_RDONLY, MS_RDONLY, NULL);
+        if (r < 0) {
+                log_error_errno(r, "Failed to remount %s read-only: %m", t);
+                goto fail;
+        }
+
+        r = mount_nofollow_verbose(LOG_ERR, template, directory, NULL, MS_MOVE, NULL);
+        if (r < 0)
+                goto fail;
+
+        (void) rmdir(template);
+
+        return 0;
+
+fail:
+        if (bind_mounted)
+                (void) umount_verbose(LOG_ERR, t, UMOUNT_NOFOLLOW);
+
+        if (tmpfs_mounted)
+                (void) umount_verbose(LOG_ERR, template, UMOUNT_NOFOLLOW);
+
+        (void) rmdir(template);
+        return r;
+}
+
+static int setup_volatile_overlay(const char *directory, uid_t uid_shift, const char *selinux_apifs_context) {
+
+        _cleanup_free_ char *buf = NULL, *escaped_directory = NULL, *escaped_upper = NULL, *escaped_work = NULL;
+        char template[] = "/tmp/nspawn-volatile-XXXXXX";
+        const char *upper, *work, *options;
+        bool tmpfs_mounted = false;
+        int r;
+
+        assert(directory);
+
+        /* --volatile=overlay means we mount an overlayfs to the root dir. */
+
+        if (!mkdtemp(template))
+                return log_error_errno(errno, "Failed to create temporary directory: %m");
+
+        options = "mode=755" TMPFS_LIMITS_ROOTFS;
+        r = tmpfs_patch_options(options, uid_shift == 0 ? UID_INVALID : uid_shift, selinux_apifs_context, &buf);
+        if (r < 0)
+                goto finish;
+        if (r > 0)
+                options = buf;
+
+        r = mount_nofollow_verbose(LOG_ERR, "tmpfs", template, "tmpfs", MS_STRICTATIME, options);
+        if (r < 0)
+                goto finish;
+
+        tmpfs_mounted = true;
+
+        upper = strjoina(template, "/upper");
+        work = strjoina(template, "/work");
+
+        if (mkdir(upper, 0755) < 0) {
+                r = log_error_errno(errno, "Failed to create %s: %m", upper);
+                goto finish;
+        }
+        if (mkdir(work, 0755) < 0) {
+                r = log_error_errno(errno, "Failed to create %s: %m", work);
+                goto finish;
+        }
+
+        /* And now, let's overmount the root dir with an overlayfs that uses the root dir as lower dir. It's kinda nice
+         * that the kernel allows us to do that without going through some mount point rearrangements. */
+
+        escaped_directory = shell_escape(directory, ",:");
+        escaped_upper = shell_escape(upper, ",:");
+        escaped_work = shell_escape(work, ",:");
+        if (!escaped_directory || !escaped_upper || !escaped_work) {
+                r = -ENOMEM;
+                goto finish;
+        }
+
+        options = strjoina("lowerdir=", escaped_directory, ",upperdir=", escaped_upper, ",workdir=", escaped_work);
+        r = mount_nofollow_verbose(LOG_ERR, "overlay", directory, "overlay", 0, options);
+
+finish:
+        if (tmpfs_mounted)
+                (void) umount_verbose(LOG_ERR, template, UMOUNT_NOFOLLOW);
+
+        (void) rmdir(template);
+        return r;
+}
+
+int setup_volatile_mode(
+                const char *directory,
+                VolatileMode mode,
+                uid_t uid_shift,
+                const char *selinux_apifs_context) {
+
+        switch (mode) {
+
+        case VOLATILE_YES:
+                return setup_volatile_yes(directory, uid_shift, selinux_apifs_context);
+
+        case VOLATILE_STATE:
+                return setup_volatile_state(directory, uid_shift, selinux_apifs_context);
+
+        case VOLATILE_OVERLAY:
+                return setup_volatile_overlay(directory, uid_shift, selinux_apifs_context);
+
+        default:
+                return 0;
+        }
+}
+
+/* Expects *pivot_root_new and *pivot_root_old to be initialised to allocated memory or NULL. */
+int pivot_root_parse(char **pivot_root_new, char **pivot_root_old, const char *s) {
+        _cleanup_free_ char *root_new = NULL, *root_old = NULL;
+        const char *p = s;
+        int r;
+
+        assert(pivot_root_new);
+        assert(pivot_root_old);
+
+        r = extract_first_word(&p, &root_new, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EINVAL;
+
+        if (isempty(p))
+                root_old = NULL;
+        else {
+                root_old = strdup(p);
+                if (!root_old)
+                        return -ENOMEM;
+        }
+
+        if (!path_is_absolute(root_new))
+                return -EINVAL;
+        if (root_old && !path_is_absolute(root_old))
+                return -EINVAL;
+
+        free_and_replace(*pivot_root_new, root_new);
+        free_and_replace(*pivot_root_old, root_old);
+
+        return 0;
+}
+
+int setup_pivot_root(const char *directory, const char *pivot_root_new, const char *pivot_root_old) {
+        _cleanup_free_ char *directory_pivot_root_new = NULL;
+        _cleanup_free_ char *pivot_tmp_pivot_root_old = NULL;
+        char pivot_tmp[] = "/tmp/nspawn-pivot-XXXXXX";
+        bool remove_pivot_tmp = false;
+        int r;
+
+        assert(directory);
+
+        if (!pivot_root_new)
+                return 0;
+
+        /* Pivot pivot_root_new to / and the existing / to pivot_root_old.
+         * If pivot_root_old is NULL, the existing / disappears.
+         * This requires a temporary directory, pivot_tmp, which is
+         * not a child of either.
+         *
+         * This is typically used for OSTree-style containers, where
+         * the root partition contains several sysroots which could be
+         * run. Normally, one would be chosen by the bootloader and
+         * pivoted to / by initramfs.
+         *
+         * For example, for an OSTree deployment, pivot_root_new
+         * would be: /ostree/deploy/$os/deploy/$checksum. Note that this
+         * code doesn’t do the /var mount which OSTree expects: use
+         * --bind +/sysroot/ostree/deploy/$os/var:/var for that.
+         *
+         * So in the OSTree case, we’ll end up with something like:
+         *  - directory = /tmp/nspawn-root-123456
+         *  - pivot_root_new = /ostree/deploy/os/deploy/123abc
+         *  - pivot_root_old = /sysroot
+         *  - directory_pivot_root_new =
+         *       /tmp/nspawn-root-123456/ostree/deploy/os/deploy/123abc
+         *  - pivot_tmp = /tmp/nspawn-pivot-123456
+         *  - pivot_tmp_pivot_root_old = /tmp/nspawn-pivot-123456/sysroot
+         *
+         * Requires all file systems at directory and below to be mounted
+         * MS_PRIVATE or MS_SLAVE so they can be moved.
+         */
+        directory_pivot_root_new = path_join(directory, pivot_root_new);
+        if (!directory_pivot_root_new)
+                return log_oom();
+
+        /* Remount directory_pivot_root_new to make it movable. */
+        r = mount_nofollow_verbose(LOG_ERR, directory_pivot_root_new, directory_pivot_root_new, NULL, MS_BIND, NULL);
+        if (r < 0)
+                goto done;
+
+        if (pivot_root_old) {
+                if (!mkdtemp(pivot_tmp)) {
+                        r = log_error_errno(errno, "Failed to create temporary directory: %m");
+                        goto done;
+                }
+
+                remove_pivot_tmp = true;
+                pivot_tmp_pivot_root_old = path_join(pivot_tmp, pivot_root_old);
+                if (!pivot_tmp_pivot_root_old) {
+                        r = log_oom();
+                        goto done;
+                }
+
+                r = mount_nofollow_verbose(LOG_ERR, directory_pivot_root_new, pivot_tmp, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+
+                r = mount_nofollow_verbose(LOG_ERR, directory, pivot_tmp_pivot_root_old, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+
+                r = mount_nofollow_verbose(LOG_ERR, pivot_tmp, directory, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+        } else {
+                r = mount_nofollow_verbose(LOG_ERR, directory_pivot_root_new, directory, NULL, MS_MOVE, NULL);
+                if (r < 0)
+                        goto done;
+        }
+
+done:
+        if (remove_pivot_tmp)
+                (void) rmdir(pivot_tmp);
+
+        return r;
+}