Commit d84f291c authored by David Zeuthen's avatar David Zeuthen
Browse files

Rewrite the "Writing polkit applications" chapter


Signed-off-by: default avatarDavid Zeuthen <davidz@redhat.com>
parent da49ff95
......@@ -20,45 +20,149 @@
<chapter id="polkit-apps">
<title>Writing polkit applications</title>
<para>
polkit applications are privileged mechanisms using the
polkit authority as a decider component. To do this, a
<emphasis>mechanism</emphasis> use either
the <link linkend="ref-api">GObject API</link>,
the <link linkend="ref-dbus-api">D-Bus API</link> or
the <link linkend="pkcheck.1">pkcheck</link> command to
communicate with the polkit Authority.
polkit applications are privileged mechanisms (typically) using
the polkit authority as a decider component. They do this by
installing a <filename class='extension'>.policy</filename> file
into the
<filename class='directory'>/usr/share/polkit-1/actions</filename>
directory and communicating with the polkit authority at runtime
(either via the
<link linkend="ref-dbus-api">D-Bus API</link> or indirectly
via the
<link linkend="ref-api">libpolkit-gobject-1</link> library or the
<link linkend="pkcheck.1">pkcheck</link> command).
</para>
<para>
Note that <emphasis>clients</emphasis> normally doesn't use the
polkit API directly – it is intended for privileged
<emphasis>mechanisms</emphasis>. If a client needs to disable,
modify or remove UI elements to e.g. convey to the user that a
certain action cannot be carried out (because e.g. the user is
not authorized) or authentication is needed (by e.g. displaying
a padlock icon in the UI), it is usually better to have the
mechanism provide an API for this.
</para>
<para>
If a polkit application wants to handle the case where no
authentication agent exists (for example if the app is launched
via a
<citerefentry><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>
login), it is trivial for the application to use the <link
linkend="PolkitAgentTextListener">PolkitAgentTextListener</link>
class to spawn its own authentication agent as
needed. Alternatively, the <xref linkend="pkttyagent.1"/>
helper can be used to do this.
</para>
<para>
As an example of code using the GObject API, see <xref linkend="cancel-example"/>.
For an example using the D-Bus API, see <xref linkend="polkit-raw-dbus-py"/>.
</para>
<example id="cancel-example"><title>Querying the Authority</title>
<programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../src/examples/cancel.c"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting>
</example>
<example id="polkit-raw-dbus-py"><title>Accessing the Authority via D-Bus</title>
<programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../src/examples/polkit-raw-dbus.py"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting>
</example>
<simplesect id="polkit-apps-best-practices">
<title>Best practices</title>
<itemizedlist mark='opencircle' spacing='compact'>
<listitem><para>
<emphasis role='bold'>DO</emphasis> use polkit if you are writing a mechanism that is intended to be used by unprivileged programs.
</para></listitem>
<listitem><para>
<emphasis role='bold'>DO</emphasis> carefully consider what actions to define. In many cases there is not a 1:1 mapping between operations and polkit actions (often a polkit action has more to do with the object the operation is acting on than the operation itself). It is important to strike the right balance between too fine-grained and too coarse-grained.
</para></listitem>
<listitem><para>
<emphasis role='bold'>DO</emphasis> try to pick actions and implicit authorizations so applications using your mechanism will work out-of-the box for users logged in at the console (e.g. without interrupting the user with authentication dialogs).
</para></listitem>
<listitem><para>
<emphasis role='bold'>DO</emphasis> pass polkit variables along with
<link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.CheckAuthorization">CheckAuthorization()</link>
requests so it's possible to write <emphasis>authorization rules</emphasis> matching on these. Also document these variables in your documentation (for example, see the
<ulink url="http://udisks.freedesktop.org/docs/latest/udisks-polkit-actions.html">udisks2 actions and variables</ulink>).
</para></listitem>
<listitem><para>
<emphasis role='bold'>DO</emphasis> pass a customized authentication message (using the <literal>polkit.message</literal> and <literal>polkit.gettext_domain</literal> variables) that includes more detailed information about the request than whatever is declared in the <filename class='extension'>.policy</filename> file's <literal>message</literal> element. For example, it's better to show <quote>Authentication is needed to format INTEL SSDSA2MH080G1GC (/dev/sda)</quote> than just <quote>Authentication is needed to format the device</quote>.
</para></listitem>
<listitem><para>
<emphasis role='bold'>DON'T</emphasis> use polkit if your program isn't intended to be used by unprivileged programs. For example, if you are writing developer tools or low-level core OS command it's fine to just require the user to be root. Users can always run your tool through e.g.
<citerefentry><refentrytitle>sudo</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<link linkend="pkexec.1"><citerefentry><refentrytitle>pkexec</refentrytitle><manvolnum>1</manvolnum></citerefentry></link>
or write a simple polkit-using mechanism that allows access to a (safe) subset of your tool.
</para></listitem>
<listitem><para>
<emphasis role='bold'>DON'T</emphasis> use polkit unless you actually have to. In other words, not every single privileged program providing a service to an unprivileged programs has to use polkit. For example, if you have a small well-written <ulink url="http://en.wikipedia.org/wiki/Setuid">setuid</ulink> helper to help deal with some implementation-detail of the OS (such as elevating the priority of the sound server process to real-time for sessions on local seats) it's not really helpful to define a polkit action for this since no-one is going to choose to not grant the privilige (in the example, no-one is going run the sound server process without real-time priority).
</para></listitem>
<listitem><para>
<emphasis role='bold'>DON'T</emphasis> call
<link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.CheckAuthorization">CheckAuthorization()</link>
for all your actions every time the authority emits the
<link linkend="eggdbus-signal-org.freedesktop.PolicyKit1.Authority::Changed">Changed</link> signal. Not only is this a waste of resources, the result may also be inaccurate as authorization rules can return whatever they want, whenever they want.
</para></listitem>
<listitem><para>
<emphasis role='bold'>DON'T</emphasis> block the main thread in your mechanism (e.g. the one used to service IPC requests from unprivileged programs) while waiting for the authority to reply - calls to
<link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.CheckAuthorization">CheckAuthorization()</link>
may take a very long time (seconds, even minutes) to complete as user interaction may be involved.
Instead, use either the <link linkend="polkit-authority-check-authorization">asynchronous API</link> or
a dedicated thread with the <link linkend="polkit-authority-check-authorization-sync">synchronous API</link>.
</para></listitem>
</itemizedlist>
</simplesect>
<simplesect id="polkit-apps-unprivileged">
<title>Usage in unprivileged programs</title>
<para>
Unprivileged programs consuming privileged mechanisms normally
does use polkit directly - they simply call into the mechanism
and the mechanism either renders service (or refuses the
request) after checking with polkit (which may include
presenting an authentication dialog). In either case, the
unprivileged program in question is oblivious to the fact that
polkit is being used - it simply just waits for the privileged
mechanism to carry out the request (which, if authentication
dialogs are involved may take many seconds).
</para>
<para>
Note that unprivileged programs often need to disable, modify
or remove UI elements to e.g. convey to the user that a certain
action cannot be carried out (because e.g. the user is not
authorized) or authentication is needed (by e.g. displaying a
padlock icon in the UI).
In this case, the best approach is
usually to have the unprivileged program get this information
from the privileged mechanism - especially because there often
is no reliable way that the unprivileged program can know what
polkit action is going to be used. In general, there is no
guarantee that operations (such as D-Bus methods) map 1:1: to
polkit action - for example, a disk manager service's
<literal>Format()</literal> method may check for the action
<literal>net.company.diskmanager.format-removable</literal> if
the disk is removable and
<literal>net.company.diskmanager.format-fixed</literal>
otherwise.
</para>
<para>
However, in certain cases, for example when using the
<emphasis>org.freedesktop.policykit.imply</emphasis> annotation
(see the
<link linkend="polkit.8"><citerefentry><refentrytitle>polkit</refentrytitle><manvolnum>8</manvolnum></citerefentry></link> man page),
it does make sense for an unprivileged program to query the
polkit authority (to e.g. update UI elements) and it is
in fact allowed to do so as long as the unprivileged program doesn't pass any variables along with the
<link linkend="eggdbus-method-org.freedesktop.PolicyKit1.Authority.CheckAuthorization">CheckAuthorization()</link>
call (otherwise it would be easy to spoof authentication dialogs and bypass authorization rules).
In fact, since this use-case is so common,
<link linkend="ref-api">libpolkit-gobject-1</link> provides the
<link linkend="PolkitPermission"><type>PolkitPermission</type></link> type (which is derived from
<ulink url="http://developer.gnome.org/gio/unstable/GPermission.html"><type>GPermission</type></ulink>)
that can be used together with
<ulink url="http://developer.gnome.org/gtk3/unstable/GtkLockButton.html"><type>GtkLockButton</type></ulink>
(often used to implement an
<ulink url="http://developer.gnome.org/hig-book/3.2/hig-book.html#windows-instant-apply">instant apply</ulink>
paradigm).
Note that for <type>GtkLockButton</type> to work well, the
polkit action backing it should use <literal>auth_self_keep</literal> or
<literal>auth_admin_keep</literal> for its implicit authorizations.
</para>
</simplesect>
<simplesect id="polkit-apps-no-auth-agent">
<title>No authentication agent</title>
<para>
If a polkit application wants to handle the case where no
authentication agent exists (for example if the app is launched
via a
<citerefentry><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>
login), the application can use the <link
linkend="PolkitAgentTextListener">PolkitAgentTextListener</link>
type to spawn its own authentication agent as
needed. Alternatively, the <xref linkend="pkttyagent.1"/>
helper can be used to do this.
</para>
</simplesect>
</chapter>
<chapter id="polkit-agents">
......
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