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<h1>The International Obfuscated C Code Contest</h1>
<h2>2020/carlini - Best of Show - abuse of libc</h2>
<h3>printf machine</h3>
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<a class="Right" href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/prog.c">C code</a>
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<h2 id="author">Author:</h2>
<ul>
<li>Name: <a href="../../authors.html#Nicholas_Carlini">Nicholas Carlini</a><br>
Location: <a href="../../location.html#US">US</a> - <em>United States of America</em> (<em>United States</em>)</li>
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<h2 id="to-build">To build:</h2>
<pre><code> make</code></pre>
<h3 id="bugs-and-misfeatures">Bugs and (Mis)features:</h3>
<p>The current status of this entry is:</p>
<blockquote>
<p><strong>STATUS: INABIAF - please DO NOT fix</strong></p>
</blockquote>
<p>For more detailed information see <a href="../../bugs.html#2020_carlini">2020/carlini in bugs.html</a>.</p>
<h2 id="to-use">To use:</h2>
<pre><code> ./prog</code></pre>
<h2 id="try">Try:</h2>
<pre><code> ./try.sh</code></pre>
<p>Or if you actually want to play:</p>
<pre><code> # Play with a friend, real or imagined, P1 and P2, squares are numbered 1..9
./prog</code></pre>
<h2 id="judges-remarks">Judges’ remarks:</h2>
<p>This program consists of a single <code>printf(3)</code> statement wrapped in a <code>while</code>
loop. You would not think that this would amount to much, but you would be very,
very wrong. A clue to what is happening and how this works is encoded in the
ASCII art of the program source.</p>
<h2 id="authors-remarks">Author’s remarks:</h2>
<p>HOW ABOUT A NICE GAME OF <del>CHESS</del>TIC-TAC-TOE?</p>
<h3 id="usage">USAGE</h3>
<pre><code> cc -o prog prog.c
./prog</code></pre>
<p>Alternates between P1 and P2. Enter a digit [1-9] to move:</p>
<pre><code> 1 | 2 | 3
---------
4 | 5 | 6
---------
7 | 8 | 9</code></pre>
<p>The game ends if:</p>
<ul>
<li>A player completes three in a row; that player wins.</li>
<li>All squares are taken; neither player wins.</li>
<li>A player makes an illegal move; their opponent wins.</li>
</ul>
<h3 id="limitations">LIMITATIONS</h3>
<p>Bad things may happen if the entered move is outside of the range [1..9]</p>
<p>While the game will stop if one player tries to overwrite another player’s move,
it will happily allow a player to move on top of themself. However, given that
this is an effective no-op and guarantees a loss to a competent opponent, if you
do this you probably deserve to lose.</p>
<p>Your terminal may beep at you every turn. We’ll call this a feature that …
the program … is hard at work processing your move! That is! Please be
patient.</p>
<p>After macro expansion the format string is roughly 100K. This makes some
compilers unhappy, and others slow. (For example, passing GCC the flags <code>-Wall -Wextra -pedantic</code> will result in a warning about oversized strings.)</p>
<p>Under some terminals the program may exit early. The following should always work:</p>
<pre><code> echo "1 2 3 4 5 6 7" | ./prog</code></pre>
<h3 id="obfuscation">OBFUSCATION</h3>
<p>The entirety of the program consists of a single iterated call to <code>printf(3)</code>.</p>
<pre><code> int main() {
while(*d) printf(fmt, arg);
}</code></pre>
<p>Here, <code>fmt</code> is a single string, and <code>arg</code> is a series of arguments to
<code>printf(3)</code>.</p>
<p>While its primary purpose is to serve as The One True Debugger, <code>printf(3)</code> also happens
to be Turing complete. (See “Control-Flow Bending: On the Effectiveness of Control-Flow
Integrity” where we introduced this in an actual, published, academic paper. The
things you can get away with sometimes.)</p>
<p>We <del>ab</del>use this fact to implement the logic of tic-tac-toe entirely within
this one <code>printf(3)</code> call (and a call to <code>scanf(3)</code> to read user input).</p>
<p>Here’s (briefly) how it works.</p>
<h4 id="preliminaries">Preliminaries</h4>
<p>This program uses three <code>printf(3)</code> format specifiers.</p>
<ul>
<li><p><code>%d</code> takes an integer argument and prints it.</p></li>
<li><p><code>%s</code> takes a string argument and prints it.</p></li>
<li><p><code>%n</code> takes a pointer and writes (!!) the number of bytes printed so far.</p></li>
</ul>
<p>Okay, everyone probably knows this. Let’s get a bit more advanced.</p>
<p>Format specifiers can take extra “arguments”.</p>
<ul>
<li><p><code>"%hhn"</code>: store the number of bytes written mod 256 to the <code>char</code> pointer.</p></li>
<li><p><code>"%2$d"</code>: print argument 2 to <code>printf(3)</code> (and not the sequentially next argument).</p></li>
<li><p><code>"%8d"</code>: pad the printed integer to 8 characters.</p></li>
<li><p><code>"%3$.*4$d"</code>: print argument 3 to <code>printf(3)</code> with as many zeros as in argument 4.</p></li>
</ul>
<p>For example, the following expression</p>
<pre><code> printf("%1$.*2$d%3$hhn", 5, 10, &x)</code></pre>
<p>will have the same effect as if we had written</p>
<pre><code> x = 10;</code></pre>
<p>because it will print out <code>0000000005</code> (5 padded to size 10) and then write the
number of bytes written to <code>x</code>.</p>
<h4 id="printf3-oriented-programming"><code>printf(3)</code> Oriented Programming</h4>
<p>Alright, now we can get to the real fun.</p>
<p>We perform arbitrary computation with <code>printf(3)</code> treating memory as a binary
array–one bit per pair of bytes:</p>
<ul>
<li><p>The zero bit is represented by the sequence <code>00 00</code>.</p></li>
<li><p>The one-bit is represented by the sequence <code>xx 00</code> where <code>xx</code> is any non-zero byte.</p></li>
</ul>
<p>We can use format strings to compute the OR/NOT of arbitrary “bits”.</p>
<p>We’ll start with the simplest, OR:</p>
<pre><code> printf("%1$s%2$s%3$hhn", a, b, c)</code></pre>
<p>will compute</p>
<pre><code> *c = strlen(a) + strlen(b)</code></pre>
<p>but given that <code>strlen(x)</code> is <code>1</code> for a 1-bit and <code>0</code> for a 0-bit, we have</p>
<pre><code> *c = a | b</code></pre>
<p>Computing the NOT of a single value is also easy:</p>
<pre><code> printf("%1$255d%1$s%hhn", a, b)</code></pre>
<p>will compute</p>
<pre><code> *b = (strlen(a)+255)%256 = strlen(a)-1</code></pre>
<p>and again, because <code>strlen(x)</code> is either <code>1</code> or <code>0</code> we have</p>
<pre><code> *c = !b</code></pre>
<p>From here we can compute any binary circuit. Doing something efficient,
though, still takes work.</p>
<h4 id="tic-tac-toe">Tic-Tac-Toe</h4>
<p>The game itself is represented as a board of 18 bits, 9 bits per player, along
with a turn counter that alternates between player 1 and player 2.</p>
<p>To detect who has won, we implement the following logic. Let <code>A</code>, <code>B</code>, and <code>C</code> be
pointers to three squares in a row to test, and <code>D</code> be where to save if there is a
win or not.</p>
<pre><code> "%A$s%B$s%C$s%1$253d%11$hhn" // r11 = !(*A & *B & *C)
ZERO
"%11$s%1$255d%11hhn" // r11 = !r11
ZERO
"%11$s%D$s%D$hhn" // *D = *D | r11</code></pre>
<p>That is, we set <code>*D</code> to <code>1</code> if there is a three-in-a-row. We repeat this for all
possible three-in-a-row configurations, for both players.</p>
<p>The ZERO macro ensures that the number of bytes written out is 0 mod 256 with
the following expression</p>
<pre><code> "%1$hhn%1$s" (repeated 256 times)</code></pre>
<p>where argument 1 is a pointer to a temporary variable followed by a NUL byte.</p>
<p>This works because if the current count is 0 mod 256, then <code>"%1$hhn"</code> will write
zero to argument 1 and then <code>"%1$s"</code> will never emit any text. If, on the other
hand, the count is not 0 mod 256, a length-1 string will be written to argument
1, and then <code>"%1$s"</code> will increment the count by one. By repeating this 256
times we’re eventually going to reach 0 mod 256.</p>
<p>Checking if there has been an invalid move is achieved similarly.</p>
<p>In order to decide what to print out, we have to cast the “in-memory” array of
bits to Xs and Os to print out. This is actually rather straightforward. Given
in <code>1$</code> the pointer to player 1’s square, and <code>2$</code> the pointer to player 2’s, and
in <code>3$</code> the pointer to the board string, we can compute</p>
<pre><code> "%1$s" (repeated 47 times) "%2$s" (repeated 56 times) %1$32d%3$hhn"</code></pre>
<p>which will, in effect, compute</p>
<pre><code> *r3 = (*r1) * 47 + (*r2) * 56 + 32</code></pre>
<p>which will output <code>' '</code> if neither are true, <code>'X'</code> if <code>r1</code> is true, or <code>'O'</code> if
<code>r2</code> is true.</p>
<h4 id="further-obfuscations">Further Obfuscations</h4>
<p>In order to be able to finally display the board, while still only using one
<code>printf(3)</code> statement, we finish the statement with</p>
<pre><code> "\n\033[2J\n%26$s"</code></pre>
<p>which is the escape sequence to clear the screen, and then prints argument 26.
Argument 26 is a pointer to a <code>char*</code> in memory, that initially is undefined,
but within the <code>printf(3)</code> statement we will construct this string to look like a
tic-tac-toe board.</p>
<p>After the board, we need to print one of the following strings:</p>
<pre><code> P1>_
P2>_
P1 WINS
P2 WINS
P1 TIES
P2 TIES</code></pre>
<p>Depending on if it’s P1 or P2’s turn to move, the game is over and someone
won, or the game is over and it is a draw.</p>
<p>This turns out not to be as hard as it might look. Using the same trick as
before, we set byte four to be</p>
<pre><code> *byte4 = is_win * 'W' + is_tie * 'T'</code></pre>
<p>The byte <code>'I'</code> and <code>'S'</code> can always be the same, and we do the same for <code>'E'</code>/<code>'N'</code>.</p>
<p>We do this same on-the-fly creation of the <code>scanf(3)</code> format string, but for a
different reason. We first want to run <code>printf(3)</code> to show the first board, and
then alternate between runs to <code>scanf(3)</code> and <code>printf(3)</code> reading and then
displaying moves. Importantly, we <em>do not</em> want a final <code>scanf(3)</code> when the game
ends. It should just exit.</p>
<p>One option would be to implement the logic as</p>
<pre><code> printf()
while (*ok) {
scanf();
printf();
}</code></pre>
<p>but this would DOUBLE the number of calls to <code>printf(3)</code> we require. So instead we
implement it like this</p>
<pre><code> while (*ok) {
scanf();
printf();
}</code></pre>
<p>(In reality we actually pass <code>scanf(3)</code> as an argument to avoid the extra
statement, but it has the same effect.)</p>
<p>Notice there is now no initial <code>printf(3)</code>. In order make sure the program doesn’t
block before the first <code>printf(3)</code>, we initialize the <code>scanf(3)</code> format to the NUL
string so that it returns right away without blocking. The first time the
<code>printf(3)</code> call runs, it writes out <code>"%hhd"</code> to create the <code>scanf(3)</code> format string.</p>
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