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<?xml-stylesheet type="text/xsl" href="assets/xml/rss.xsl" media="all"?><rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:atom="http://www.w3.org/2005/Atom"><channel><title>M-x laser</title><link>https://vanlaser.github.io/</link><description>VanLaser's blog</description><atom:link href="https://vanlaser.github.io/rss.xml" rel="self" type="application/rss+xml"></atom:link><language>en</language><copyright>Contents © 2017 VanLaser </copyright><lastBuildDate>Sun, 24 Sep 2017 11:55:10 GMT</lastBuildDate><generator>Nikola (getnikola.com)</generator><docs>http://blogs.law.harvard.edu/tech/rss</docs><item><title>LD_PRELOAD for fun and profit</title><link>https://vanlaser.github.io/posts/ld_preload-for-fun-and-profit/</link><dc:creator>VanLaser</dc:creator><description>&lt;div id="outline-container-orge2affee" class="outline-2"&gt;
&lt;h2 id="orge2affee"&gt;Intro&lt;/h2&gt;
&lt;div class="outline-text-2" id="text-orge2affee"&gt;
&lt;p&gt;
There is a great, old, unique-kind-of strategy game called "Majesty", that was
ported to Linux a long time ago. The game &lt;i&gt;can&lt;/i&gt; be installed on modern systems
– even 64bit, if the relevant 32bit libraries are installed on the Linux distro
you're using – e.g. with Arch Linux, they go to the &lt;code&gt;/usr/lib32&lt;/code&gt; folder, and
assuming you can dig up the game CD-ROM. There are some caveats though:
&lt;/p&gt;

&lt;ul class="org-ul"&gt;
&lt;li&gt;when running in full-screen mode, the resolution is messed up on &lt;i&gt;both&lt;/i&gt;
monitors, and it's also not restored after the game exits.&lt;/li&gt;
&lt;li&gt;when using the SDL libraries that come with the game, the sound system tries
to open &lt;code&gt;/dev/dsp&lt;/code&gt;, which nowadays (with Alsa and Pulse) does not exist
anymore.&lt;/li&gt;
&lt;li&gt;by default, the game scrolling speed is way too fast, even when with &lt;i&gt;Fast
Scrolling&lt;/i&gt; set to &lt;code&gt;Off&lt;/code&gt; (the default).&lt;/li&gt;
&lt;/ul&gt;

&lt;p&gt;
The first issue can be solved by passing the &lt;code&gt;-w&lt;/code&gt; flag to the game executable,
at least once. The sound problem can be avoided either by using the old OSS:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ sudo modprobe snd_seq_oss snd_pcm_oss snd_mixer_oss
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
 … or, the preferable way I think is to use a newer version of the SDL 1.2
library (of course, you have to install it first), instead of its own:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ &lt;span class="nv"&gt;LD_LIBRARY_PATH&lt;/span&gt;&lt;span class="o"&gt;=&lt;/span&gt;/usr/lib32 &lt;span class="nv"&gt;$HOME&lt;/span&gt;/games/majesty/majx -w
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
… since the newer SDL library knows to switch to Alsa or Pulse, and can even
be controlled using environment variables.
&lt;/p&gt;

&lt;p&gt;
The in-game scrolling speed can be adjusted by editing directly the game's
configuration file: &lt;code&gt;.lgp/majx/majxprefs&lt;/code&gt; (that's also possible with Majesty
Gold in Windows, where there's an &lt;code&gt;.INI&lt;/code&gt; file instead). I find a value of 5 to
be just right:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;&amp;lt;ScrollSpeed&amp;gt;5&amp;lt;/ScrollSpeed&amp;gt;
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
The &lt;b&gt;fun&lt;/b&gt; part starts when trying to play the game in windowed mode: the fixed
800x600 resolution, on 1920x1080 monitors, it's not great, but can be easily
circumvented, for example, by &lt;i&gt;scaling&lt;/i&gt; the monitor output with &lt;code&gt;xrandr&lt;/code&gt;, for
the duration of the game. You can get the name (e.g. &lt;code&gt;VGA1&lt;/code&gt;) and the available
resolutions for your monitor(s) using &lt;code&gt;xrandr -q&lt;/code&gt;. Then, to scale the output &lt;b&gt;as
if&lt;/b&gt; you're switching to another resolution (say 1440x900), you can use something
like:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ xrandr --output VGA1 --mode 1920x1080 --scale-from 1440x900
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
(where the &lt;code&gt;mode&lt;/code&gt; is the current monitor resolution, one you're trying to keep)
… and presto!, the screen got scaled and you can better see your small window.
&lt;/p&gt;

&lt;p&gt;
Restoring the scale (after the game is over):
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ xrandr --output VGA1 --mode 1920x1080 --scale 1x1
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;/div&gt;


&lt;div id="outline-container-orge804664" class="outline-2"&gt;
&lt;h2 id="orge804664"&gt;Grabbing input on request&lt;/h2&gt;
&lt;div class="outline-text-2" id="text-orge804664"&gt;
&lt;p&gt;
The remaining (very annoying!) issue is that, when playing in windowed mode, the
cursor can leave the game area, and can be moved to another windows or to
another monitor etc. This isn't bad &lt;i&gt;per se&lt;/i&gt;, because from time to time one may
need to do some other stuff on the computer (and the game can be paused). But
the darn thing is that, as in many strategy games, moving the cursor to the
edges of the game screen is associated with in-game &lt;i&gt;scrolling&lt;/i&gt;, which is a
pretty important and so-often used action. Or, when moving the cursor just a
little too fast over the game window edge, the scrolling action doesn't get
triggered, and … nothing happens instead!
&lt;/p&gt;

&lt;p&gt;
So, how to solve this? Certainly one solution is to somehow make the game grab
the mouse input and confine it to the game area, but this only happens in
full-screen mode (which we're not using). Also, grabbing the input inside the
game window should be something that can be done and undone (a toggle action),
since as mentioned above one may need to move the mouse, or change window focus,
or type in another window, on another monitor etc.
&lt;/p&gt;

&lt;p&gt;
Since the source code is not available … &lt;code&gt;LD_PRELOAD&lt;/code&gt; to the rescue! The
game is SDL-based, and the trick here is to pre-load (i.e. inject) a library that
overrides the &lt;code&gt;SDL_GetKeyState&lt;/code&gt; function, in order to perform some &lt;b&gt;extra&lt;/b&gt;
actions, when called:
&lt;/p&gt;

&lt;ul class="org-ul"&gt;
&lt;li&gt;check if the user pressed either &lt;code&gt;RCTRL&lt;/code&gt; (VirtualBox-inspired) or (since not
all keyboards have a right control key! e.g. HHKB) the &lt;code&gt;LCTRL-SPACE&lt;/code&gt; combo.
If so:&lt;/li&gt;
&lt;li&gt;grab input if it's not grabbed, release it if it's grabbed (the toggle).&lt;/li&gt;
&lt;/ul&gt;

&lt;p&gt;
But how does one knows which SDL functions the game uses? Running:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ strings majx &lt;span class="p"&gt;|&lt;/span&gt; grep SDL
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
… to get the list of functions, and reading some of the SDL 1.2 API docs is
enough.
&lt;/p&gt;

&lt;p&gt;
The code:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;&lt;span class="cp"&gt;#define _GNU_SOURCE&lt;/span&gt;
&lt;span class="cp"&gt;#include&lt;/span&gt; &lt;span class="cpf"&gt;&amp;lt;dlfcn.h&amp;gt;&lt;/span&gt;&lt;span class="cp"&gt;&lt;/span&gt;
&lt;span class="cp"&gt;#include&lt;/span&gt; &lt;span class="cpf"&gt;&amp;lt;SDL/SDL.h&amp;gt;&lt;/span&gt;&lt;span class="cp"&gt;&lt;/span&gt;

&lt;span class="c1"&gt;// pointer to the real function&lt;/span&gt;
&lt;span class="n"&gt;Uint8&lt;/span&gt;&lt;span class="o"&gt;*&lt;/span&gt; &lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="o"&gt;*&lt;/span&gt;&lt;span class="n"&gt;real_SDL_GetKeyState&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt; &lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="kt"&gt;int&lt;/span&gt; &lt;span class="o"&gt;*&lt;/span&gt;&lt;span class="n"&gt;numkeys&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="nb"&gt;NULL&lt;/span&gt;&lt;span class="p"&gt;;&lt;/span&gt;

&lt;span class="kt"&gt;void&lt;/span&gt; &lt;span class="nf"&gt;__attribute__&lt;/span&gt; &lt;span class="p"&gt;((&lt;/span&gt;&lt;span class="n"&gt;constructor&lt;/span&gt;&lt;span class="p"&gt;))&lt;/span&gt; &lt;span class="n"&gt;jail_init&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="kt"&gt;void&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
&lt;span class="p"&gt;{&lt;/span&gt;
	&lt;span class="c1"&gt;// match the library name loaded by our binary target&lt;/span&gt;
	&lt;span class="n"&gt;dlopen&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="s"&gt;"libSDL-1.2.so.0"&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="n"&gt;RTLD_LAZY&lt;/span&gt;&lt;span class="p"&gt;);&lt;/span&gt;
	&lt;span class="n"&gt;real_SDL_GetKeyState&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;dlsym&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;RTLD_NEXT&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="s"&gt;"SDL_GetKeyState"&lt;/span&gt;&lt;span class="p"&gt;);&lt;/span&gt;
&lt;span class="p"&gt;}&lt;/span&gt;

&lt;span class="c1"&gt;// the wrapper&lt;/span&gt;
&lt;span class="n"&gt;Uint8&lt;/span&gt;&lt;span class="o"&gt;*&lt;/span&gt; &lt;span class="nf"&gt;SDL_GetKeyState&lt;/span&gt; &lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="kt"&gt;int&lt;/span&gt; &lt;span class="o"&gt;*&lt;/span&gt;&lt;span class="n"&gt;numkeys&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
&lt;span class="p"&gt;{&lt;/span&gt;
	&lt;span class="n"&gt;Uint8&lt;/span&gt;&lt;span class="o"&gt;*&lt;/span&gt; &lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;;&lt;/span&gt;
	&lt;span class="n"&gt;SDL_GrabMode&lt;/span&gt; &lt;span class="n"&gt;mode&lt;/span&gt;&lt;span class="p"&gt;;&lt;/span&gt;

	&lt;span class="n"&gt;keys&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="o"&gt;*&lt;/span&gt;&lt;span class="n"&gt;real_SDL_GetKeyState&lt;/span&gt;&lt;span class="p"&gt;)(&lt;/span&gt;&lt;span class="n"&gt;numkeys&lt;/span&gt;&lt;span class="p"&gt;);&lt;/span&gt;

	&lt;span class="k"&gt;if&lt;/span&gt; &lt;span class="p"&gt;((&lt;/span&gt;&lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;[&lt;/span&gt;&lt;span class="n"&gt;SDLK_LCTRL&lt;/span&gt;&lt;span class="p"&gt;]&lt;/span&gt; &lt;span class="o"&gt;&amp;amp;&amp;amp;&lt;/span&gt; &lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;[&lt;/span&gt;&lt;span class="n"&gt;SDLK_SPACE&lt;/span&gt;&lt;span class="p"&gt;])&lt;/span&gt;&lt;span class="o"&gt;||&lt;/span&gt; &lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;[&lt;/span&gt;&lt;span class="n"&gt;SDLK_RCTRL&lt;/span&gt;&lt;span class="p"&gt;])&lt;/span&gt;
	&lt;span class="p"&gt;{&lt;/span&gt;
		&lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;[&lt;/span&gt;&lt;span class="n"&gt;SDLK_SPACE&lt;/span&gt;&lt;span class="p"&gt;]&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="mi"&gt;0&lt;/span&gt;&lt;span class="p"&gt;;&lt;/span&gt;
		&lt;span class="n"&gt;mode&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;SDL_WM_GrabInput&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;SDL_GRAB_QUERY&lt;/span&gt;&lt;span class="p"&gt;);&lt;/span&gt;
		&lt;span class="n"&gt;SDL_WM_GrabInput&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;mode&lt;/span&gt; &lt;span class="o"&gt;==&lt;/span&gt; &lt;span class="n"&gt;SDL_GRAB_OFF&lt;/span&gt; &lt;span class="o"&gt;?&lt;/span&gt; &lt;span class="nl"&gt;SDL_GRAB_ON&lt;/span&gt; &lt;span class="p"&gt;:&lt;/span&gt; &lt;span class="n"&gt;SDL_GRAB_OFF&lt;/span&gt;&lt;span class="p"&gt;);&lt;/span&gt;
	&lt;span class="p"&gt;}&lt;/span&gt;

	&lt;span class="k"&gt;return&lt;/span&gt; &lt;span class="n"&gt;keys&lt;/span&gt;&lt;span class="p"&gt;;&lt;/span&gt;
&lt;span class="p"&gt;}&lt;/span&gt;
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
Building on Arch Linux (64bit, &lt;code&gt;gcc-multilib&lt;/code&gt; package):
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;$ gcc -m32 -fPIC -shared -o libjail.so jail.c -ldl
&lt;/pre&gt;&lt;/div&gt;

&lt;p&gt;
And finally, to run the game, use the following script, placed
in a folder from your &lt;code&gt;PATH&lt;/code&gt;:
&lt;/p&gt;

&lt;div class="highlight"&gt;&lt;pre&gt;&lt;span&gt;&lt;/span&gt;&lt;span class="ch"&gt;#!/bin/sh&lt;/span&gt;
&lt;span class="nv"&gt;LD_LIBRARY_PATH&lt;/span&gt;&lt;span class="o"&gt;=&lt;/span&gt;/usr/lib32 &lt;span class="nv"&gt;LD_PRELOAD&lt;/span&gt;&lt;span class="o"&gt;=&lt;/span&gt;&lt;span class="nv"&gt;$HOME&lt;/span&gt;/games/majesty/libjail.so &lt;span class="nv"&gt;$HOME&lt;/span&gt;/games/majesty/majx -w
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;/div&gt;</description><category>C</category><category>LD_PRELOAD</category><category>SDL</category><guid>https://vanlaser.github.io/posts/ld_preload-for-fun-and-profit/</guid><pubDate>Sat, 23 Sep 2017 21:07:39 GMT</pubDate></item></channel></rss>