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In this article from the final issue (June/July) of Game Developer magazine, game developers share their hacks, tricks, and war stories about getting the game out the door however they had to. (The complete issue is available as a free download here.)
The article is being highlighted as one of Gamasutra's best stories of 2013.
Joe Valenzuela, Insomniac Games
This trick was on the PS3: We on the Insomniac engine team had some textures that we wanted distributed with our engine/tools release. These were things like noise textures and source input for full-screen filter effects. For some unimportant reasons, we didn't want to distribute these as actual asset files, so instead we converted them to binary arrays and compiled them into the executable. There was one downside, though -- we wanted these to be in a different chunk of memory (RSX visible), so we would end up copying them out and just wasting the memory for the source.
The PS3 toolchain had a link feature to put certain sections in RSX memory, but it requires using 1MB pages, and in our case it would have wasted 700k. Instead, we added a new data section in the executable that aliased the bss (the "bss alias" or "balias" section). We had something like 3MB of bss in our final builds, so there was more than enough space to hide some texture assets. We ran some code as early as we could in the crt initialization to initialize the destination memory, copy the assets out, and then re-initialize the bss to 0.
Believe it or not, it worked! There was a little tweaking to accommodate hidden bss use and toolchain updates, but overall it was pretty straightforward.
Brett Douville, LucasArts
In early 2002, we were readying Star Wars: Jedi Starfighter for submission to Sony. One niggling TCR bug remained, which was that the controller's analog stick functionality would shut off while we were loading our post-mission cutscenes, causing the red light in the center of the controller to go off as well. This bug had shown up when we updated to a library version required by Sony, and the programmer who had originally written both the movie-loading code and the IOP logic for the controller itself had left LucasArts some months before.
In the hopes of narrowing down the problem quickly in code I didn't understand, I inserted seven screen clears in different colors in the code I determined to be the likely source of the problem, hoping that I could at least narrow it down to one or two sections of code by checking what color the screen was when the analog controls turned off. But when I then tried to reproduce the bug, it had disappeared.
It's an old programming adage that if you don't understand the cause, you can't be said to have fixed the bug. But in this case, we were two or three days away from our intended submission date and missing it would have been a big deal. So I changed all the screen clears to black, marked the bug fixed, and called it a day. We shipped on time, with no TCR showstoppers.
Jonathan Garrett, Insomniac Games
Ratchet and Clank: Up Your Arsenal was an online title that shipped without the ability to patch either code or data. Which was unfortunate.
The game downloads and displays an End User License Agreement each time it's launched. This is an ascii string stored in a static buffer. This buffer is filled from the server without checking that the size is within the buffer's capacity.
We exploited this fact to cause the EULA download to overflow the static buffer far enough to also overwrite a known global variable. This variable happened to be the function callback handler for a specific network packet. Once this handler was installed, we could send the network packet to cause a jump to the address in the overwritten global. The address was a pointer to some payload code that was stored earlier in the EULA data.
Valuable data existed between the real end of the EULA buffer and the overwritten global, so the first job of the payload code was to restore this trashed data. Once that was done things were back to normal and the actual patching work could be done.
One complication is that the EULA text is copied with strcpy. And strcpy ends when it finds a 0 byte (which is usually the end of the string). Our string contained code which often contains 0 bytes. So we mutated the compiled code such that it contained no zero bytes and had a carefully crafted piece of bootstrap asm to un-mutate it.
By the end, the hack looked like this:
1. Send oversized EULA
2. Overflow EULA buffer, miscellaneous data, callback handler pointer
3. Send packet to trigger handler
4. Game jumps to bootstrap code pointed to by handler
5. Bootstrap decodes payload data
6. Payload downloads and restores stomped miscellaneous data
7. Patch executes
Takeaways: Include patching code in your shipped game, and don't use unbounded strcpy.