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Add a section about writing GLib applications

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For now, this includes some information about threads and security.
This commit is contained in:
Matthias Clasen
2012-09-11 22:41:18 -04:00
parent 6a50dc511b
commit fc7dc33113
2 changed files with 124 additions and 8 deletions
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11
docs/reference/glib/glib-docs.xml
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GLib is a general-purpose utility library, which provides many useful
data types, macros, type conversions, string utilities, file utilities,
a mainloop abstraction, and so on. It works on many UNIX-like platforms,
Windows, OS/2 and BeOS. GLib is released under the GNU Library General
Public License (GNU LGPL).
</para>
<para>
The general policy of GLib is that all functions are invisibly threadsafe
with the exception of data structure manipulation functions, where, if
you have two threads manipulating the <emphasis>same</emphasis> data
structure, they must use a lock to synchronize their operation.
as well as Windows and OS X. GLib is released under the GNU Library
General Public License (GNU LGPL).
</para>
<xi:include href="building.xml" />
<xi:include href="cross.xml" />
<xi:include href="programming.xml" />
<xi:include href="xml/compiling.xml" />
<xi:include href="running.xml" />
<xi:include href="changes.xml" />

121
docs/reference/glib/programming.xml Normal file
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<?xml version="1.0"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
<refentry id="glib-programming">
<refmeta>
<refentrytitle>Writing GLib Applications</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GLib Library</refmiscinfo>
</refmeta>
<refnamediv>
<refname>Writing GLib Applications</refname>
<refpurpose>
General considerations when programming with GLib
</refpurpose>
</refnamediv>
<refsect1>
<title>Writing GLib Applications</title>
<refsect2>
<title>Threads</title>
<para>
The general policy of GLib is that all functions are invisibly threadsafe
with the exception of data structure manipulation functions, where, if
you have two threads manipulating the <emphasis>same</emphasis> data
structure, they must use a lock to synchronize their operation.
</para>
<para>
GLib is creating a worker thread for its own purposes, so GLib applications
will always have at least 2 threads.
</para>
<para>
See the sections on <link linkend="glib-Threads">threads</link> and
<link linkend="glib-Thread-Pools">threadpools</link> for GLib APIs that
support multithreaded applications.
</para>
</refsect2>
<refsect2>
<title>Security</title>
<para>
When your program needs to carry out some privileged operation (say,
create a new user account), there are various ways in which you can go
about this:
<itemizedlist>
<listitem><para>
Implement a daemon that offers the privileged operation. A convenient
way to do this is as a D-Bus system-bus service. The daemon will probably
need ways to check the identity and authorization of the caller before
executing the operation. <ulink url="http://www.freedesktop.org/software/polkit/docs/latest/polkit.8.html">polkit</ulink> is a framework that allows this.
</para></listitem>
<listitem><para>
Use a small helper that is executed with elevated privileges via
pkexec. <ulink url="http://www.freedesktop.org/software/polkit/docs/latest/pkexec.1.html">pkexec</ulink> is a small program launcher that is part of polkit.
</para></listitem>
<listitem><para>
Use a small helper that is executed with elevated privileges by
being suid root.
</para></listitem>
</itemizedlist>
None of these approaches is the clear winner, they all have their
advantages and disadvantages.
</para>
<para>
When writing code that runs with elevated privileges, it is important
to follow some basic rules of secure programming. David Wheeler has an
excellent book on this topic,
<ulink url="http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html">Secure Programming for Linux and Unix HOWTO</ulink>.
</para>
<para>
When it comes to GLib and its associated libraries, GLib and
GObject are generally fine to use in code that runs with elevated
privileges; they don't load modules (executable code in shared objects)
or run other programs 'behind your back'.
</para>
<para>
When using GIO, you have to be more careful, since GIO has extension
points whose implementations get loaded from modules. However, GIO will
never load modules from your home-directory except when explictly asked
to do so via an environment variable.
</para>
<para>
In most cases, your helper program should be so small that you don't
need GIO, whose APIs are largely designed to support full-blown desktop
applications. If you can't resist the convenience of these APIs, here
are some steps you should take:
<itemizedlist>
<listitem><para>
Clear the environment, e.g. using the <function>clearenv()</function>
function.
David Wheeler has a good <ulink url="http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/environment-variables.html">explanation</ulink> for why it is
important to sanitize the environment.
See the GIO <ulink url="http://developer.gnome.org/gio/stable/ch03.html">documentation</ulink>
for a list of all environment variables affecting GIO. In particular,
<envar>PATH</envar> (used to locate binaries), <envar>GIO_EXTRA_MODULES</envar> (used to locate loadable modules) and <envar>DBUS_{SYSTEM,SESSION}_BUS_ADDRESS</envar> (used to locate the D-Bus system and session bus) are important.
</para></listitem>
<listitem><para>
Don't use GVfs, by setting <envar>GIO_USE_VFS=local</envar> in the environment.
The reason to avoid GVfs in security-sensitive programs is that it uses
many libraries which have not necessarily been audited for security problems.
Gvfs is also heavily distributed and relies on a session bus to be present.
</para></listitem>
</itemizedlist>
</para>
</refsect2>
</refsect1>
</refentry>