Cody Boone Ferguson
22e483723c
Instead of code blocks the bug / (mis)features statuses are quoted rather than in a code block. In the case that there is more than one a line break was added at the end of all but the last one. A bug status was renamed to be slightly more correct (maybe more than one was renamed for the same purpose but at least one was). The bug statuses in the quotes are now bold to let them stand out better.
353 lines
9.0 KiB
Markdown
353 lines
9.0 KiB
Markdown
## To build:
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``` <!---sh-->
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make
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```
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### Bugs and (Mis)features:
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The current status of this entry is:
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> **STATUS: INABIAF - please DO NOT fix**
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For more detailed information see [2020/carlini in bugs.html](../../bugs.html#2020_carlini).
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## To use:
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``` <!---sh-->
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./prog
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```
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## Try:
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``` <!---sh-->
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./try.sh
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```
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Or if you actually want to play:
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``` <!---sh-->
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# Play with a friend, real or imagined, P1 and P2, squares are numbered 1..9
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./prog
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```
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## Judges' remarks:
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This program consists of a single `printf(3)` statement wrapped in a `while`
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loop. You would not think that this would amount to much, but you would be very,
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very wrong. A clue to what is happening and how this works is encoded in the
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ASCII art of the program source.
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## Author's remarks:
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HOW ABOUT A NICE GAME OF <del>CHESS</del>TIC-TAC-TOE?
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### USAGE
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``` <!---sh-->
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cc -o prog prog.c
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./prog
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```
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Alternates between P1 and P2. Enter a digit [1-9] to move:
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```
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1 | 2 | 3
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---------
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4 | 5 | 6
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---------
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7 | 8 | 9
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```
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The game ends if:
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* A player completes three in a row; that player wins.
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* All squares are taken; neither player wins.
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* A player makes an illegal move; their opponent wins.
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### LIMITATIONS
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Bad things may happen if the entered move is outside of the range [1..9]
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While the game will stop if one player tries to overwrite another player's move,
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it will happily allow a player to move on top of themself. However, given that
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this is an effective no-op and guarantees a loss to a competent opponent, if you
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do this you probably deserve to lose.
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Your terminal may beep at you every turn. We'll call this a feature that ...
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the program ... is hard at work processing your move! That is! Please be
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patient.
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After macro expansion the format string is roughly 100K. This makes some
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compilers unhappy, and others slow. (For example, passing GCC the flags `-Wall
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-Wextra -pedantic` will result in a warning about oversized strings.)
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Under some terminals the program may exit early. The following should always work:
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``` <!---sh-->
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echo "1 2 3 4 5 6 7" | ./prog
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```
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### OBFUSCATION
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The entirety of the program consists of a single iterated call to `printf(3)`.
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``` <!---c-->
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int main() {
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while(*d) printf(fmt, arg);
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}
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```
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Here, `fmt` is a single string, and `arg` is a series of arguments to
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`printf(3)`.
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While its primary purpose is to serve as The One True Debugger, `printf(3)` also happens
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to be Turing complete. (See "Control-Flow Bending: On the Effectiveness of Control-Flow
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Integrity" where we introduced this in an actual, published, academic paper. The
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things you can get away with sometimes.)
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We <del>ab</del>use this fact to implement the logic of tic-tac-toe entirely within
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this one `printf(3)` call (and a call to `scanf(3)` to read user input).
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Here's (briefly) how it works.
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#### Preliminaries
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This program uses three `printf(3)` format specifiers.
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- `%d` takes an integer argument and prints it.
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- `%s` takes a string argument and prints it.
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- `%n` takes a pointer and writes (!!) the number of bytes printed so far.
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Okay, everyone probably knows this. Let's get a bit more advanced.
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Format specifiers can take extra "arguments".
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- `"%hhn"`: store the number of bytes written mod 256 to the `char` pointer.
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- `"%2$d"`: print argument 2 to `printf(3)` (and not the sequentially next argument).
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- `"%8d"`: pad the printed integer to 8 characters.
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- `"%3$.*4$d"`: print argument 3 to `printf(3)` with as many zeros as in argument 4.
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For example, the following expression
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``` <!---c-->
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printf("%1$.*2$d%3$hhn", 5, 10, &x)
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```
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will have the same effect as if we had written
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``` <!---c-->
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x = 10;
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```
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because it will print out `0000000005` (5 padded to size 10) and then write the
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number of bytes written to `x`.
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#### `printf(3)` Oriented Programming
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Alright, now we can get to the real fun.
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We perform arbitrary computation with `printf(3)` treating memory as a binary
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array--one bit per pair of bytes:
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- The zero bit is represented by the sequence `00 00`.
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- The one-bit is represented by the sequence `xx 00` where `xx` is any non-zero byte.
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We can use format strings to compute the OR/NOT of arbitrary "bits".
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We'll start with the simplest, OR:
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``` <!---c-->
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printf("%1$s%2$s%3$hhn", a, b, c)
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```
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will compute
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``` <!---c-->
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*c = strlen(a) + strlen(b)
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```
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but given that `strlen(x)` is `1` for a 1-bit and `0` for a 0-bit, we have
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``` <!---c-->
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*c = a | b
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```
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Computing the NOT of a single value is also easy:
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``` <!---c-->
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printf("%1$255d%1$s%hhn", a, b)
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```
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will compute
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``` <!---c-->
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*b = (strlen(a)+255)%256 = strlen(a)-1
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```
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and again, because `strlen(x)` is either `1` or `0` we have
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``` <!---c-->
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*c = !b
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```
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From here we can compute any binary circuit. Doing something efficient,
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though, still takes work.
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#### Tic-Tac-Toe
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The game itself is represented as a board of 18 bits, 9 bits per player, along
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with a turn counter that alternates between player 1 and player 2.
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To detect who has won, we implement the following logic. Let `A`, `B`, and `C` be
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pointers to three squares in a row to test, and `D` be where to save if there is a
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win or not.
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``` <!---c-->
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"%A$s%B$s%C$s%1$253d%11$hhn" // r11 = !(*A & *B & *C)
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ZERO
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"%11$s%1$255d%11hhn" // r11 = !r11
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ZERO
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"%11$s%D$s%D$hhn" // *D = *D | r11
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```
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That is, we set `*D` to `1` if there is a three-in-a-row. We repeat this for all
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possible three-in-a-row configurations, for both players.
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The ZERO macro ensures that the number of bytes written out is 0 mod 256 with
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the following expression
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``` <!---c-->
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"%1$hhn%1$s" (repeated 256 times)
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```
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where argument 1 is a pointer to a temporary variable followed by a NUL byte.
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This works because if the current count is 0 mod 256, then `"%1$hhn"` will write
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zero to argument 1 and then `"%1$s"` will never emit any text. If, on the other
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hand, the count is not 0 mod 256, a length-1 string will be written to argument
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1, and then `"%1$s"` will increment the count by one. By repeating this 256
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times we're eventually going to reach 0 mod 256.
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Checking if there has been an invalid move is achieved similarly.
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In order to decide what to print out, we have to cast the "in-memory" array of
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bits to Xs and Os to print out. This is actually rather straightforward. Given
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in `1$` the pointer to player 1's square, and `2$` the pointer to player 2's, and
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in `3$` the pointer to the board string, we can compute
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``` <!---c-->
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"%1$s" (repeated 47 times) "%2$s" (repeated 56 times) %1$32d%3$hhn"
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```
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which will, in effect, compute
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``` <!---c-->
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*r3 = (*r1) * 47 + (*r2) * 56 + 32
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```
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which will output `' '` if neither are true, `'X'` if `r1` is true, or `'O'` if
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`r2` is true.
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#### Further Obfuscations
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In order to be able to finally display the board, while still only using one
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`printf(3)` statement, we finish the statement with
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``` <!---c-->
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"\n\033[2J\n%26$s"
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```
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which is the escape sequence to clear the screen, and then prints argument 26.
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Argument 26 is a pointer to a `char*` in memory, that initially is undefined,
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but within the `printf(3)` statement we will construct this string to look like a
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tic-tac-toe board.
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After the board, we need to print one of the following strings:
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```
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P1>_
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P2>_
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P1 WINS
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P2 WINS
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P1 TIES
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P2 TIES
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```
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Depending on if it's P1 or P2's turn to move, the game is over and someone
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won, or the game is over and it is a draw.
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This turns out not to be as hard as it might look. Using the same trick as
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before, we set byte four to be
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``` <!---c-->
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*byte4 = is_win * 'W' + is_tie * 'T'
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```
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The byte `'I'` and `'S'` can always be the same, and we do the same for `'E'`/`'N'`.
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We do this same on-the-fly creation of the `scanf(3)` format string, but for a
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different reason. We first want to run `printf(3)` to show the first board, and
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then alternate between runs to `scanf(3)` and `printf(3)` reading and then
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displaying moves. Importantly, we *do not* want a final `scanf(3)` when the game
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ends. It should just exit.
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One option would be to implement the logic as
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``` <!---c-->
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printf()
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while (*ok) {
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scanf();
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printf();
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}
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```
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but this would DOUBLE the number of calls to `printf(3)` we require. So instead we
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implement it like this
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``` <!---c-->
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while (*ok) {
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scanf();
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printf();
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}
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```
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(In reality we actually pass `scanf(3)` as an argument to avoid the extra
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statement, but it has the same effect.)
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Notice there is now no initial `printf(3)`. In order make sure the program doesn't
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block before the first `printf(3)`, we initialize the `scanf(3)` format to the NUL
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string so that it returns right away without blocking. The first time the
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`printf(3)` call runs, it writes out `"%hhd"` to create the `scanf(3)` format string.
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<!--
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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For more information, see:
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https://creativecommons.org/licenses/by-sa/4.0/
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-->
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