machine-id-setup: add a machine_id_commit call to commit on disk a transient machine-id

If /etc was read only at boot time with an empty /etc/machine-id, the latter
will be mounted as a tmpfs and get reset at each boot. If the system becomes rw
later, this functionality enables to commit in a race-free manner the
transient machine-id to disk.

src/core/machine-id-setup.c

@@ -188,6 +188,85 @@ static int write_machine_id(int fd, char id[34]) {
         return -errno;
 }
 
+int machine_id_commit(const char *root) {
+        _cleanup_close_ int fd = -1, initial_mntns_fd = -1;
+        const char *etc_machine_id;
+        char id[34]; /* 32 + \n + \0 */
+        int r;
+
+        if (isempty(root))
+                etc_machine_id = "/etc/machine-id";
+        else {
+                char *x;
+
+                x = strappenda(root, "/etc/machine-id");
+                etc_machine_id = path_kill_slashes(x);
+        }
+
+        r = path_is_mount_point(etc_machine_id, false);
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine wether %s is a mount point: %m", etc_machine_id);
+        if (r == 0) {
+                log_debug("%s is is not a mount point. Nothing to do.", etc_machine_id);
+                return 0;
+        }
+
+        /* Read existing machine-id */
+        fd = open(etc_machine_id, O_RDONLY|O_CLOEXEC|O_NOCTTY);
+        if (fd < 0)
+                return log_error_errno(errno, "Cannot open %s: %m", etc_machine_id);
+
+        r = get_valid_machine_id(fd, id);
+        if (r < 0)
+                return log_error_errno(r, "We didn't find a valid machine ID in %s.", etc_machine_id);
+
+        r = is_fd_on_temporary_fs(fd);
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine whether %s is on a temporary file system: %m", etc_machine_id);
+        if (r == 0) {
+                log_error("%s is not on a temporary file system.", etc_machine_id);
+                return -EROFS;
+        }
+
+        fd = safe_close(fd);
+
+        /* Store current mount namespace */
+        r = namespace_open(0, NULL, &initial_mntns_fd, NULL, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Can't fetch current mount namespace: %m");
+
+        /* Switch to a new mount namespace, isolate ourself and unmount etc_machine_id in our new namespace */
+        if (unshare(CLONE_NEWNS) < 0)
+                return log_error_errno(errno, "Failed to enter new namespace: %m");
+
+        if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
+                return log_error_errno(errno, "Couldn't make-rslave / mountpoint in our private namespace: %m");
+
+        if (umount(etc_machine_id) < 0)
+                return log_error_errno(errno, "Failed to unmount transient %s file in our private namespace: %m", etc_machine_id);
+
+        /* Update a persistent version of etc_machine_id */
+        fd = open(etc_machine_id, O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444);
+        if (fd < 0)
+                return log_error_errno(errno, "Cannot open for writing %s. This is mandatory to get a persistent machine-id: %m", etc_machine_id);
+
+        r = write_machine_id(fd, id);
+        if (r < 0)
+                return log_error_errno(r, "Cannot write %s: %m", etc_machine_id);
+
+        fd = safe_close(fd);
+
+        /* Return to initial namespace and proceed a lazy tmpfs unmount */
+        r = namespace_enter(-1, initial_mntns_fd, -1, -1);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to switch back to initial mount namespace: %m.\nWe'll keep transient %s file until next reboot.", etc_machine_id);
+
+        if (umount2(etc_machine_id, MNT_DETACH) < 0)
+                return log_warning_errno(errno, "Failed to unmount transient %s file: %m.\nWe keep that mount until next reboot.", etc_machine_id);
+
+        return 0;
+}
+
 int machine_id_setup(const char *root) {
         const char *etc_machine_id, *run_machine_id;
         _cleanup_close_ int fd = -1;

src/core/machine-id-setup.h

@@ -21,4 +21,5 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+int machine_id_commit(const char *root);
 int machine_id_setup(const char *root);

src/shared/util.c

@@ -3052,6 +3052,15 @@ _pure_ static int is_temporary_fs(struct statfs *s) {
                F_TYPE_EQUAL(s->f_type, RAMFS_MAGIC);
 }
 
+int is_fd_on_temporary_fs(int fd) {
+        struct statfs s;
+
+        if (fstatfs(fd, &s) < 0)
+                return -errno;
+
+        return is_temporary_fs(&s);
+}
+
 int rm_rf_children(int fd, bool only_dirs, bool honour_sticky, struct stat *root_dev) {
         struct statfs s;
 

src/shared/util.h

@@ -454,6 +454,8 @@ int get_ctty(pid_t, dev_t *_devnr, char **r);
 int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid);
 int fchmod_and_fchown(int fd, mode_t mode, uid_t uid, gid_t gid);
 
+int is_fd_on_temporary_fs(int fd);
+
 int rm_rf_children(int fd, bool only_dirs, bool honour_sticky, struct stat *root_dev);
 int rm_rf_children_dangerous(int fd, bool only_dirs, bool honour_sticky, struct stat *root_dev);
 int rm_rf(const char *path, bool only_dirs, bool delete_root, bool honour_sticky);