fstab-generator: add support for volatile boots

This adds support for a new kernel command line option "systemd.volatile=" that
provides the same functionality that systemd-nspawn's --volatile= switch
provides, but for host systems (i.e. systems booting with a kernel).

It takes the same parameter and has the same effect.

In order to implement systemd.volatile=yes a new service
systemd-volatile-root.service is introduced that only runs in the initrd and
rearranges the root directory as needed to become a tmpfs instance. Note that
systemd.volatile=state is implemented different: it simply generates a
var.mount unit file that is part of the normal boot and has no effect on the
initrd execution.

The way this is implemented ensures that other explicit configuration for /var
can always override the effect of these options.  Specifically, the var.mount
unit is generated in the "late" generator directory, so that it only is in
effect if nothing else overrides it.

14 files changed, 456 insertions, 19 deletions

diff --git a/.gitignore b/.gitignore
index ec4b7bd672..f246d3e6d5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -126,6 +126,7 @@
 /systemd-update-utmp
 /systemd-user-sessions
 /systemd-vconsole-setup
+/systemd-volatile-root
 /tags
 /test-acd
 /test-acl-util
diff --git a/Makefile-man.am b/Makefile-man.am
index 5e6eee5e32..27660ef1c2 100644
--- a/Makefile-man.am
+++ b/Makefile-man.am
@@ -142,6 +142,7 @@ MANPAGES += \
 	man/systemd-tty-ask-password-agent.1 \
 	man/systemd-udevd.service.8 \
 	man/systemd-update-done.service.8 \
+	man/systemd-volatile-root.service.8 \
 	man/systemd.1 \
 	man/systemd.automount.5 \
 	man/systemd.device.5 \
@@ -482,6 +483,7 @@ MANPAGES_ALIAS += \
 	man/systemd-udevd.8 \
 	man/systemd-update-done.8 \
 	man/systemd-user.conf.5 \
+	man/systemd-volatile-root.8 \
 	man/udev_device_get_action.3 \
 	man/udev_device_get_devlinks_list_entry.3 \
 	man/udev_device_get_devnode.3 \
@@ -837,6 +839,7 @@ man/systemd-udevd-kernel.socket.8: man/systemd-udevd.service.8
 man/systemd-udevd.8: man/systemd-udevd.service.8
 man/systemd-update-done.8: man/systemd-update-done.service.8
 man/systemd-user.conf.5: man/systemd-system.conf.5
+man/systemd-volatile-root.8: man/systemd-volatile-root.service.8
 man/udev_device_get_action.3: man/udev_device_get_syspath.3
 man/udev_device_get_devlinks_list_entry.3: man/udev_device_has_tag.3
 man/udev_device_get_devnode.3: man/udev_device_get_syspath.3
@@ -1790,6 +1793,9 @@ man/systemd-update-done.html: man/systemd-update-done.service.html
 man/systemd-user.conf.html: man/systemd-system.conf.html
 	$(html-alias)
 
+man/systemd-volatile-root.html: man/systemd-volatile-root.service.html
+	$(html-alias)
+
 man/udev_device_get_action.html: man/udev_device_get_syspath.html
 	$(html-alias)
 
@@ -2804,6 +2810,7 @@ EXTRA_DIST += \
 	man/systemd-update-utmp.service.xml \
 	man/systemd-user-sessions.service.xml \
 	man/systemd-vconsole-setup.service.xml \
+	man/systemd-volatile-root.service.xml \
 	man/systemd.automount.xml \
 	man/systemd.device.xml \
 	man/systemd.exec.xml \
diff --git a/Makefile.am b/Makefile.am
index 56b8aa3fe8..92a3680461 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -397,6 +397,7 @@ rootlibexec_PROGRAMS = \
 	systemd-initctl \
 	systemd-shutdown \
 	systemd-remount-fs \
+	systemd-volatile-root \
 	systemd-reply-password \
 	systemd-fsck \
 	systemd-ac-power \
@@ -538,6 +539,7 @@ nodist_systemunit_DATA = \
 	units/system-update-cleanup.service \
 	units/systemd-initctl.service \
 	units/systemd-remount-fs.service \
+	units/systemd-volatile-root.service \
 	units/systemd-ask-password-wall.service \
 	units/systemd-ask-password-console.service \
 	units/systemd-sysctl.service \
@@ -602,6 +604,7 @@ EXTRA_DIST += \
 	units/system-update-cleanup.service.in \
 	units/systemd-initctl.service.in \
 	units/systemd-remount-fs.service.in \
+	units/systemd-volatile-root.service.in \
 	units/systemd-update-utmp.service.in \
 	units/systemd-update-utmp-runlevel.service.in \
 	units/systemd-ask-password-wall.service.in \
@@ -3068,6 +3071,13 @@ systemd_remount_fs_LDADD = \
 	libsystemd-shared.la
 
 # ------------------------------------------------------------------------------
+systemd_volatile_root_SOURCES = \
+	src/volatile-root/volatile-root.c
+
+systemd_volatile_root_LDADD = \
+	libsystemd-shared.la
+
+# ------------------------------------------------------------------------------
 systemd_cgroups_agent_SOURCES = \
 	src/cgroups-agent/cgroups-agent.c
 
diff --git a/man/kernel-command-line.xml b/man/kernel-command-line.xml
index 78e45e66a9..7e1d408ded 100644
--- a/man/kernel-command-line.xml
+++ b/man/kernel-command-line.xml
@@ -125,6 +125,28 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>systemd.volatile=</varname></term>
+        <listitem>
+          <para>This parameter controls whether the system shall boot up in volatile mode. Takes a boolean argument, or
+          the special value <literal>state</literal>. If false (the default), normal boot mode is selected, the root
+          directory and <filename>/var</filename> are mounted as specified on the kernel command line or
+          <filename>/etc/fstab</filename>, or otherwise configured. If true, full state-less boot mode is selected. In
+          this case the root directory is mounted as volatile memory file system (<literal>tmpfs</literal>), and only
+          <filename>/usr</filename> is mounted from the file system configured as root device, in read-only mode. This
+          enables fully state-less boots were the vendor-supplied OS is used as shipped, with only default
+          configuration and no stored state in effect, as <filename>/etc</filename> and <filename>/var</filename> (as
+          well as all other resources shipped in the root file system) are reset at boot and lost on shutdown. If this
+          setting is set to <literal>state</literal> the root file system is mounted as usual, however
+          <filename>/var</filename> is mounted as a volatile memory file system (<literal>tmpfs</literal>), so that the
+          system boots up with the normal configuration applied, but all state reset at boot and lost at shutdown. For details,
+          see
+          <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+          and
+          <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>quiet</varname></term>
         <listitem>
           <para>Parameter understood by both the kernel and the system
@@ -382,6 +404,7 @@
         <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+        <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-modules-load.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-backlight@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
         <citerefentry><refentrytitle>systemd-rfkill.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
diff --git a/man/systemd-fstab-generator.xml b/man/systemd-fstab-generator.xml
index a971cb3675..5f37e9193e 100644
--- a/man/systemd-fstab-generator.xml
+++ b/man/systemd-fstab-generator.xml
@@ -89,12 +89,13 @@
 
         <listitem><para>Takes a boolean argument. Defaults to
         <literal>yes</literal>. If <literal>no</literal>, causes the
-        generator to ignore any mounts or swaps configured in
+        generator to ignore any mounts or swap devices configured in
         <filename>/etc/fstab</filename>. <varname>rd.fstab=</varname>
-        is honored only by initial RAM disk (initrd) while
+        is honored only by the initial RAM disk (initrd) while
         <varname>fstab=</varname> is honored by both the main system
         and the initrd.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>root=</varname></term>
 
@@ -102,6 +103,7 @@
         initrd. <varname>root=</varname> is honored by the
         initrd.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>rootfstype=</varname></term>
 
@@ -109,6 +111,7 @@
         passed to the mount command. <varname>rootfstype=</varname> is
         honored by the initrd.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>rootflags=</varname></term>
 
@@ -116,6 +119,7 @@
         use. <varname>rootflags=</varname> is honored by the
         initrd.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>mount.usr=</varname></term>
 
@@ -133,6 +137,7 @@
         <para><varname>mount.usr=</varname> is honored by the initrd.
         </para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>mount.usrfstype=</varname></term>
 
@@ -150,6 +155,7 @@
         <para><varname>mount.usrfstype=</varname> is honored by the
         initrd.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><varname>mount.usrflags=</varname></term>
 
@@ -166,6 +172,39 @@
         <para><varname>mount.usrflags=</varname> is honored by the
         initrd.</para></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>systemd.volatile=</varname></term>
+
+        <listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
+        special value <option>state</option>.</para>
+
+        <para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
+        normal mode.</para>
+
+        <para>If true the generator ensures
+        <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+        is run as part of the initial RAM disk ("initrd"). This service changes the mount table before transitioning to
+        the host system, so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory,
+        with only <filename>/usr</filename> mounted into it from the configured root file system, in read-only
+        mode. This way the system operates in fully stateless mode, with all configuration and state reset at boot and
+        lost at shutdown, as <filename>/etc</filename> and <filename>/var</filename> will be served from the (initially
+        unpopulated) volatile memory file system.</para>
+
+        <para>If set to <option>state</option> the generator will leave the root
+        directory mount point unaltered, however will mount a <literal>tmpfs</literal> file system to
+        <filename>/var</filename>. In this mode the normal system configuration (i.e the contents of
+        <literal>/etc</literal>) is in effect (and may be modified during system runtime), however the system state
+        (i.e. the contents of <literal>/var</literal>) is reset at boot and lost at shutdown.</para>
+
+        <para>Note that in none of these modes the root directory, <filename>/etc</filename>, <filename>/var</filename>
+        or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
+        that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
+
+        <para>Note that enabling this setting will only work correctly on operating systems that can boot up with only
+        <filename>/usr</filename> mounted, and are able to automatically populate <filename>/etc</filename>, and also
+        <filename>/var</filename> in case of <literal>systemd.volatile=yes</literal>.</para></listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
 
@@ -176,7 +215,8 @@
       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-      <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+      <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
     </para>
   </refsect1>
 
diff --git a/man/systemd-nspawn.xml b/man/systemd-nspawn.xml
index 2bc81ea1aa..f6b3f57fc7 100644
--- a/man/systemd-nspawn.xml
+++ b/man/systemd-nspawn.xml
@@ -939,12 +939,15 @@
         <option>no</option> (the default), the whole OS tree is made
         available writable.</para>
 
-        <para>Note that setting this to <option>yes</option> or
-        <option>state</option> will only work correctly with
-        operating systems in the container that can boot up with only
-        <filename>/usr</filename> mounted, and are able to populate
-        <filename>/var</filename> automatically, as
-        needed.</para></listitem>
+        <para>This option provides similar functionality for containers as the <literal>systemd.volatile=</literal>
+        kernel command line switch provides for host systems. See
+        <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry> for
+        details.</para>
+
+        <para>Note that enabling this setting will only work correctly with operating systems in the container that can
+        boot up with only <filename>/usr</filename> mounted, and are able to automatically populate
+        <filename>/var</filename>, and also <filename>/etc</filename> in case of
+        <literal>--volatile=yes</literal>.</para></listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/man/systemd-volatile-root.service.xml b/man/systemd-volatile-root.service.xml
new file mode 100644
index 0000000000..b90a3261fa
--- /dev/null
+++ b/man/systemd-volatile-root.service.xml
@@ -0,0 +1,79 @@
+<?xml version="1.0"?>
+<!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!--
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+<refentry id="systemd-volatile-root.service">
+
+  <refentryinfo>
+    <title>systemd-volatile-root.service</title>
+    <productname>systemd</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>Developer</contrib>
+        <firstname>Lennart</firstname>
+        <surname>Poettering</surname>
+        <email>lennart@poettering.net</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-volatile-root.service</refentrytitle>
+    <manvolnum>8</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-volatile-root.service</refname>
+    <refname>systemd-volatile-root</refname>
+    <refpurpose>Make the root file system volatile</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename>systemd-volatile-root.service</filename></para>
+    <para><filename>/usr/lib/systemd/systemd-volatile-root</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><filename>systemd-volatile-root.service</filename> is a service that replaces the root directory with a
+    volatile memory file system (<literal>tmpfs</literal>), mounting the original (non-volatile)
+    <filename>/usr</filename> inside it read-only. This way, vendor data from <filename>/usr</filename> is available as
+    usual, but all configuration data in <filename>/etc</filename>, all state data in <filename>/var</filename> and all
+    other resources stored directly under the root directory are reset on boot and lost at shutdown, enabling fully
+    stateless systems.</para>
+
+    <para>This service is only enabled if full volatile mode is selected, for example by specifying
+    <literal>systemd.volatile=yes</literal> on the kernel command line. This service runs only in the initial RAM disk
+    ("initrd"), before the system transitions to the host's root directory. Note that this service is not used if
+    <literal>systemd.volatile=state</literal> is used, as in that mode the root directory is non-volatile.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>
diff --git a/src/fstab-generator/fstab-generator.c b/src/fstab-generator/fstab-generator.c
index 84163abbc5..f58aa27df2 100644
--- a/src/fstab-generator/fstab-generator.c
+++ b/src/fstab-generator/fstab-generator.c
@@ -42,8 +42,10 @@
 #include "unit-name.h"
 #include "util.h"
 #include "virt.h"
+#include "volatile-util.h"
 
 static const char *arg_dest = "/tmp";
+static const char *arg_dest_late = "/tmp";
 static bool arg_fstab_enabled = true;
 static char *arg_root_what = NULL;
 static char *arg_root_fstype = NULL;
@@ -52,6 +54,7 @@ static int arg_root_rw = -1;
 static char *arg_usr_what = NULL;
 static char *arg_usr_fstype = NULL;
 static char *arg_usr_options = NULL;
+static VolatileMode arg_volatile_mode = _VOLATILE_MODE_INVALID;
 
 static int add_swap(
                 const char *what,
@@ -235,6 +238,7 @@ static int write_requires_mounts_for(FILE *f, const char *opts) {
 }
 
 static int add_mount(
+                const char *dest,
                 const char *what,
                 const char *where,
                 const char *fstype,
@@ -286,7 +290,7 @@ static int add_mount(
         if (r < 0)
                 return log_error_errno(r, "Failed to generate unit name: %m");
 
-        unit = strjoin(arg_dest, "/", name);
+        unit = strjoin(dest, "/", name);
         if (!unit)
                 return log_oom();
 
@@ -318,7 +322,7 @@ static int add_mount(
         }
 
         if (passno != 0) {
-                r = generator_write_fsck_deps(f, arg_dest, what, where, fstype);
+                r = generator_write_fsck_deps(f, dest, what, where, fstype);
                 if (r < 0)
                         return r;
         }
@@ -334,7 +338,7 @@ static int add_mount(
         if (!isempty(fstype) && !streq(fstype, "auto"))
                 fprintf(f, "Type=%s\n", fstype);
 
-        r = generator_write_timeouts(arg_dest, what, where, opts, &filtered);
+        r = generator_write_timeouts(dest, what, where, opts, &filtered);
         if (r < 0)
                 return r;
 
@@ -350,7 +354,7 @@ static int add_mount(
                 return log_error_errno(r, "Failed to write unit file %s: %m", unit);
 
         if (!noauto && !automount) {
-                lnk = strjoin(arg_dest, "/", post, nofail ? ".wants/" : ".requires/", name);
+                lnk = strjoin(dest, "/", post, nofail ? ".wants/" : ".requires/", name);
                 if (!lnk)
                         return log_oom();
 
@@ -364,7 +368,7 @@ static int add_mount(
                 if (r < 0)
                         return log_error_errno(r, "Failed to generate unit name: %m");
 
-                automount_unit = strjoin(arg_dest, "/", automount_name);
+                automount_unit = strjoin(dest, "/", automount_name);
                 if (!automount_unit)
                         return log_oom();
 
@@ -406,7 +410,7 @@ static int add_mount(
                         return log_error_errno(r, "Failed to write unit file %s: %m", automount_unit);
 
                 free(lnk);
-                lnk = strjoin(arg_dest, "/", post, nofail ? ".wants/" : ".requires/", automount_name);
+                lnk = strjoin(dest, "/", post, nofail ? ".wants/" : ".requires/", automount_name);
                 if (!lnk)
                         return log_oom();
 
@@ -479,7 +483,8 @@ static int parse_fstab(bool initrd) {
                         else
                                 post = SPECIAL_LOCAL_FS_TARGET;
 
-                        k = add_mount(what,
+                        k = add_mount(arg_dest,
+                                      what,
                                       where,
                                       me->mnt_type,
                                       me->mnt_opts,
@@ -540,7 +545,8 @@ static int add_sysroot_mount(void) {
                         return r;
         }
 
-        return add_mount(what,
+        return add_mount(arg_dest,
+                         what,
                          "/sysroot",
                          arg_root_fstype,
                          opts,
@@ -593,7 +599,8 @@ static int add_sysroot_usr_mount(void) {
                 opts = arg_usr_options;
 
         log_debug("Found entry what=%s where=/sysroot/usr type=%s", what, strna(arg_usr_fstype));
-        return add_mount(what,
+        return add_mount(arg_dest,
+                         what,
                          "/sysroot/usr",
                          arg_usr_fstype,
                          opts,
@@ -605,6 +612,46 @@ static int add_sysroot_usr_mount(void) {
                          "/proc/cmdline");
 }
 
+static int add_volatile_root(void) {
+        const char *from, *to;
+
+        if (arg_volatile_mode != VOLATILE_YES)
+                return 0;
+
+        /* Let's add in systemd-remount-volatile.service which will remount the root device to tmpfs if this is
+         * requested, leaving only /usr from the root mount inside. */
+
+        from = strjoina(SYSTEM_DATA_UNIT_PATH "/systemd-volatile-root.service");
+        to = strjoina(arg_dest, "/" SPECIAL_INITRD_ROOT_FS_TARGET, ".requires/systemd-volatile-root.service");
+
+        (void) mkdir_parents(to, 0755);
+
+        if (symlink(from, to) < 0)
+                return log_error_errno(errno, "Failed to hook in volatile remount service: %m");
+
+        return 0;
+}
+
+static int add_volatile_var(void) {
+
+        if (arg_volatile_mode != VOLATILE_STATE)
+                return 0;
+
+        /* If requested, mount /var as tmpfs, but do so only if there's nothing else defined for this. */
+
+        return add_mount(arg_dest_late,
+                         "tmpfs",
+                         "/var",
+                         "tmpfs",
+                         "mode=0755",
+                         0,
+                         false,
+                         false,
+                         false,
+                         SPECIAL_LOCAL_FS_TARGET,
+                         "/proc/cmdline");
+}
+
 static int parse_proc_cmdline_item(const char *key, const char *value, void *data) {
         int r;
 
@@ -686,6 +733,18 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
                 arg_root_rw = true;
         else if (streq(key, "ro") && !value)
                 arg_root_rw = false;
+        else if (streq(key, "systemd.volatile")) {
+                VolatileMode m;
+
+                if (value) {
+                        m = volatile_mode_from_string(value);
+                        if (m < 0)
+                                log_warning("Failed to parse systemd.volatile= argument: %s", value);
+                        else
+                                arg_volatile_mode = m;
+                } else
+                        arg_volatile_mode = VOLATILE_YES;
+        }
 
         return 0;
 }
@@ -700,6 +759,8 @@ int main(int argc, char *argv[]) {
 
         if (argc > 1)
                 arg_dest = argv[1];
+        if (argc > 3)
+                arg_dest_late = argv[3];
 
         log_set_target(LOG_TARGET_SAFE);
         log_parse_environment();
@@ -720,8 +781,12 @@ int main(int argc, char *argv[]) {
                 k = add_sysroot_usr_mount();
                 if (k < 0)
                         r = k;
+
+                k = add_volatile_root();
+                if (k < 0)
+                        r = k;
         } else
-                r = 0;
+                r = add_volatile_var();
 
         /* Honour /etc/fstab only when that's enabled */
         if (arg_fstab_enabled) {
diff --git a/src/shared/volatile-util.c b/src/shared/volatile-util.c
index 1329b51f4e..e7e9721411 100644
--- a/src/shared/volatile-util.c
+++ b/src/shared/volatile-util.c
@@ -17,8 +17,10 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
+#include "alloc-util.h"
 #include "macro.h"
 #include "parse-util.h"
+#include "proc-cmdline.h"
 #include "string-util.h"
 #include "volatile-util.h"
 
@@ -39,3 +41,28 @@ VolatileMode volatile_mode_from_string(const char *s) {
 
         return _VOLATILE_MODE_INVALID;
 }
+
+int query_volatile_mode(VolatileMode *ret) {
+        _cleanup_free_ char *mode = NULL;
+        VolatileMode m = VOLATILE_NO;
+        int r;
+
+        r = proc_cmdline_get_key("systemd.volatile", PROC_CMDLINE_VALUE_OPTIONAL, &mode);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                goto finish;
+
+        if (mode) {
+                m = volatile_mode_from_string(mode);
+                if (m < 0)
+                        return -EINVAL;
+        } else
+                m = VOLATILE_YES;
+
+        r = 1;
+
+finish:
+        *ret = m;
+        return r;
+}
diff --git a/src/shared/volatile-util.h b/src/shared/volatile-util.h
index d012940c76..17930ba6ae 100644
--- a/src/shared/volatile-util.h
+++ b/src/shared/volatile-util.h
@@ -28,3 +28,5 @@ typedef enum VolatileMode {
 } VolatileMode;
 
 VolatileMode volatile_mode_from_string(const char *s);
+
+int query_volatile_mode(VolatileMode *ret);
diff --git a/src/volatile-root/Makefile b/src/volatile-root/Makefile
new file mode 120000
index 0000000000..d0b0e8e008
--- /dev/null
+++ b/src/volatile-root/Makefile
@@ -0,0 +1 @@
+../Makefile
\ No newline at end of file
diff --git a/src/volatile-root/volatile-root.c b/src/volatile-root/volatile-root.c
new file mode 100644
index 0000000000..3c0b6fa1de
--- /dev/null
+++ b/src/volatile-root/volatile-root.c
@@ -0,0 +1,157 @@
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <sys/mount.h>
+
+#include "alloc-util.h"
+#include "fs-util.h"
+#include "mkdir.h"
+#include "mount-util.h"
+#include "stat-util.h"
+#include "volatile-util.h"
+#include "string-util.h"
+#include "path-util.h"
+
+static int make_volatile(const char *path) {
+        _cleanup_free_ char *old_usr = NULL;
+        int r;
+
+        r = path_is_mount_point(path, NULL, AT_SYMLINK_FOLLOW);
+        if (r < 0)
+                return log_error_errno(r, "Couldn't determine whether %s is a mount point: %m", path);
+        if (r == 0) {
+                log_error("%s is not a mount point.", path);
+                return -EINVAL;
+        }
+
+        r = path_is_temporary_fs(path);
+        if (r < 0)
+                return log_error_errno(r, "Couldn't determine whether %s is a temporary file system: %m", path);
+        if (r > 0) {
+                log_info("%s already is a temporary file system.", path);
+                return 0;
+        }
+
+        r = chase_symlinks("/usr", path, CHASE_PREFIX_ROOT, &old_usr);
+        if (r < 0)
+                return log_error_errno(r, "/usr not available in old root: %m");
+
+        r = mkdir_p("/run/systemd/volatile-sysroot", 0700);
+        if (r < 0)
+                return log_error_errno(r, "Couldn't generate volatile sysroot directory: %m");
+
+        r = mount_verbose(LOG_ERR, "tmpfs", "/run/systemd/volatile-sysroot", "tmpfs", MS_STRICTATIME, "mode=755");
+        if (r < 0)
+                goto finish_rmdir;
+
+        if (mkdir("/run/systemd/volatile-sysroot/usr", 0755) < 0) {
+                r = -errno;
+                goto finish_umount;
+        }
+
+        r = mount_verbose(LOG_ERR, old_usr, "/run/systemd/volatile-sysroot/usr", NULL, MS_BIND|MS_REC, NULL);
+        if (r < 0)
+                goto finish_umount;
+
+        r = bind_remount_recursive("/run/systemd/volatile-sysroot/usr", true, NULL);
+        if (r < 0)
+                goto finish_umount;
+
+        r = umount_recursive(path, 0);
+        if (r < 0) {
+                log_error_errno(r, "Failed to unmount %s: %m", path);
+                goto finish_umount;
+        }
+
+        if (mount(NULL, "/", NULL, MS_SLAVE|MS_REC, NULL) < 0)
+                log_warning_errno(errno, "Failed to remount %s MS_SLAVE|MS_REC: %m", path);
+
+        r = mount_verbose(LOG_ERR, "/run/systemd/volatile-sysroot", path, NULL, MS_MOVE, NULL);
+
+finish_umount:
+        (void) umount_recursive("/run/systemd/volatile-sysroot", 0);
+
+finish_rmdir:
+        (void) rmdir("/run/systemd/volatile-sysroot");
+
+        return r;
+}
+
+int main(int argc, char *argv[]) {
+        VolatileMode m = _VOLATILE_MODE_INVALID;
+        const char *path;
+        int r;
+
+        log_set_target(LOG_TARGET_AUTO);
+        log_parse_environment();
+        log_open();
+
+        if (argc > 3) {
+                log_error("Too many arguments. Expected directory and mode.");
+                r = -EINVAL;
+                goto finish;
+        }
+
+        r = query_volatile_mode(&m);
+        if (r < 0) {
+                log_error_errno(r, "Failed to determine volatile mode from kernel command line.");
+                goto finish;
+        }
+        if (r == 0 && argc >= 2) {
+                /* The kernel command line always wins. However if nothing was set there, the argument passed here wins instead. */
+                m = volatile_mode_from_string(argv[1]);
+                if (m < 0) {
+                        log_error("Couldn't parse volatile mode: %s", argv[1]);
+                        r = -EINVAL;
+                        goto finish;
+                }
+        }
+
+        if (argc < 3)
+                path = "/sysroot";
+        else {
+                path = argv[2];
+
+                if (isempty(path)) {
+                        log_error("Directory name cannot be empty.");
+                        r = -EINVAL;
+                        goto finish;
+                }
+                if (!path_is_absolute(path)) {
+                        log_error("Directory must be specified as absolute path.");
+                        r = -EINVAL;
+                        goto finish;
+                }
+                if (path_equal(path, "/")) {
+                        log_error("Directory cannot be the root directory.");
+                        r = -EINVAL;
+                        goto finish;
+                }
+        }
+
+        if (m != VOLATILE_YES) {
+                r = 0;
+                goto finish;
+        }
+
+        r = make_volatile(path);
+
+finish:
+        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
+}
diff --git a/units/.gitignore b/units/.gitignore
index 8fdb6e9ab5..4398a59f91 100644
--- a/units/.gitignore
+++ b/units/.gitignore
@@ -75,5 +75,6 @@
 /systemd-update-utmp.service
 /systemd-user-sessions.service
 /systemd-vconsole-setup.service
+/systemd-volatile-root.service
 /tmp.mount
 /user@.service
diff --git a/units/systemd-volatile-root.service.in b/units/systemd-volatile-root.service.in
new file mode 100644
index 0000000000..cc4e604e4c
--- /dev/null
+++ b/units/systemd-volatile-root.service.in
@@ -0,0 +1,21 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Enforce Volatile Root File Systems
+Documentation=man:systemd-volatile-root.service(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+After=sysroot.mount
+Before=initrd-root-fs.target shutdown.target
+Conflicts=shutdown.target
+AssertPathExists=/etc/initrd-release
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=@rootlibexecdir@/systemd-volatile-root yes /sysroot