Reminder: you must work in a group of two for this project. You don't need to have the same partner as the earlier projects.
This assignment will help you and your partner further hone your skills as elite hackers. Your goal is to write a program (a farmer) that plays the game of FarmVille. This game simultaneously tests your knowledge of computer systems, your programming skills, and your ability to protect programs from being hacked. Writing a top notch bot will require all the skills you have been developing so far in the course and, hopefully, be a fun and interesting challenge. At the end of the assignment, we will hold a tournament pitting all your farmers against each other and award, not just a sense of m4$$1V3 l337N3$$ (massive leetness), but actual bonus points on the assignment. Plus we will be holding a grad-student banquet (read: pizza, cookies, and soda) for all!
The year is 4096 AD. Mother Earth has become a wasteland, filled with industrial waste produced by humans for the last two thousand years. Healthy food has become a scarcity. A faction of refugees of the Earth known as the Ithacans flow to the "Outland", the home world of the Orcs, to find the next Eden. Unfortunately, the Orcs do not welcome the invaders from the Earth. One group, the Crimson Orcs, is especially hostile and eager to defend their homeland. They send teams of orcs to wait at the exit of the Dark Portal, and confront the Earth invaders with a proposal of a match - the game of FarmVille. Whoever wins the game is going to be the owner of soil at the Outland. The curtain of a brutal game is about to open...
The game is played by two teams of four farmers each. All eight farmers are placed in a single shared memory space, running on a simulated 8-core MIPS machine with a very small partitioned, direct-mapped data memory cache. The farmers are executed by the same simulator we have used previously, but modified to simulate a simple data cache, and to simulate 8 MIPS CPUs simultaneously: one instruction of core 0 then one instruction of core 1, and so forth.
Each team of farmers is assigned a produce, a particular byte of data. The goal of Farmville is to fill memory with their team's produce as much as possible, while simultaneously trying to impede the progress of the other team. All access to data memory goes through a very small cache, and there is a large penalty for missing the cache, a clever farmer will need to carefully optimize its algorithms for filling memory. Each farmer can normally only read and write its own team's half of the memory space using its own half of the data cache. However, a farmer can steal from the opponent in exploit mode, which enables the farmer to both read or write the opponent's half of memory space and to take advantage of the opponent's half of the data cache. Moreover, this is a hostile game, all farmers have been trained in the military academy and each side has a supply of 15 landmines and 30 grenades. Landmines and grenades are special bytes of data (0xF0 for a landmine, 0xF1 for a grenade). A farmer is able to plant landmines or throw grenades into their opponent’s memory space when a bot is in exploit mode. To plant a landmine, a farmer writes 0xF0 to a location in the opponent's memory. To disarm a landmine, a farmer should write 0xF2 to the address where he thinks a landmine might be present. Similarly, to throw a grenade into your opponent's half of memory, a farmer should write 0xF1 to a location in the opponent's memory.
Each team supplies a single MIPS executable program. At the beginning of the game, the simulator starts each farmer by calling the function
void __start(int core_id, int num_crashed, unsigned char produce);
Each of the four cores devoted to one team will invoke the function when it first boots, passing its core ID (a number from 0 to 3), and the team's produce (from 0 to 239). If a core ever crashes, it will simply reset and invoke the function again. The num_crashes parameter is 0 when a core first boots, and is incremented each time the core crashes.
A bot can enter exploit mode by calling
void exploit();
and can leave exploit mode by calling
void retreat();
exploit mode can potentially let a farmer fill memory more quickly, because it can take advantage of the opponent's memory cache, and can also be used to impede the progress of the opponent by interfering with the opponent's use of that same cache. However, these come with risks. Whenever a bot is in exploit mode, its core ID is written into a taunt[] array that is available to its opponent. Additionally, it is possible to detect the presence of a exploit farmer by looking at cache usage statistics and timing information. A farmer can accuse one of its opponent's cores of stealing by calling:
int accuse(int opponent_core_id);
If the opponent's core was indeed stealing at the time of the accusation, that core is killed off, i.e. permanently halted. To keep farmers honest, false accusations are penalized by having the accusing core stall for 100,000 cycles.
If a farmer steps on a landmine while planting produce in his land, he will be stunned for 1,000,000 cycles. A farmer can disarm a landmine by writing a special byte to the landmine memory location; disarming a landmine takes 100 cycles. Planting a landmine also takes 100 cycles. In addition to planting landmines, farmers can also throw grenades. When a grenade is thrown into a half of memory, any enemy farmers who try to write to that half of memory for the next 100 cycles will be stunned for 300 cycles.
The game ends when one team manages to fill more than half of all data memory with its produce, or after a fixed number of cycles, whichever comes first. The team having the most memory filled with its produce wins the match and the honor to rule the Outland.
FarmVille is interesting because the farmers on each team must cooperate to make effective use of the limited data memory cache, and be resilient to interference from the opponent's farmers. A successful team will use clever strategies to fill memory quickly and try to slow the opponent's progress, while also ensuring that it is not vulnerable to the same types of interference.
The simulator makes it seem like all four of your team's farmers live in the lower half of the memory address space (addresses below 0x80000000). The first 1 MB of this memory is read-only, and is where the text segment of your MIPS executable is loaded. The next 31 MB is read-write memory, where global variables and stacks are placed. Global variables are typically placed near the bottom of this segment by the compiler, and the stacks are initialized to near the top of the segment and grow down. All four of the farmers on a team share this same space, and share the same global variables. Your opponent is mapped into the upper half of the memory address space (addresses starting at 0x80000000), with an identical layout.
Program stacks: The four program stacks for the four farmers on a team are initially spaced evenly near the top of the 31 MB read-write segment (though your code can set $sp to anything you like once it starts executing). The stacks are spaced 1 MB apart; this is plenty of space for most program, but if you invoke a deeply recursive function, the call stacks can overwrite each other. Note: The upper 4 MB of memory on each side are protected against landmines.
Status and cores: The simulator keeps track of the information about each team and each core, and maps this data into part of the 1 MB read-only memory region, where your bots can access it, and where your opponent can access it when stealing. This includes information about your current score (how many bytes of memory currently contain your team's produce), how many cycles are left remaining in the game, the contents of all four of your farmer's register files, and statistics about cache misses and stalls for each of your four farmers. Some of this information is available in other places as well: your team's produce is passed to __start(), for example; and cache statistics are also accessible via a system call.
Physical addresses: Both farmers view the lower half of memory as their own, and both access their own code, data, and stacks using addresses below 0x80000000. These, of course, are virtual addresses. In actuality, the first player's virtual addresses correspond directly to physical memory addresses, while the second player's virtual addresses are mapped in reverse: virtual address 0x00001234, for example, is mapped to physical addresses 0x80001234, and virtual address 0x80001234 is mapped to physical address 0x00001234. Except when stealing, this virtual to physical mapping is completely transparent to the program.
Caches: Instruction fetch is assumed to use an infinitely large cache with no miss penalty. All loads and stores to memory done by the program use a data cache. Cache hits are serviced in a single cycle, with no processor stalls. Cache misses cause roughly 100 cycles of processor stalling. The data cache is direct-mapped and physically tagged, with 4 cache lines, and a 256 byte block size, for a total of 1024 bytes of cache. However, the data cache is partitioned, with the first 2 lines used to cache the lower half of physical memory (the first player's half), and the last 2 lines used to cache the upper half of physical memory (the second player's half).
Cache Management: The version of MIPS we are using (MIPS-I) lacks instructions to manage the cache, but we have provided system calls instead. A farmer can call
void invalidate(void *ptr);
to remove from the cache any block containing the data pointed to by ptr. This only works if the farmer has access to the memory in question at the time the call is made. Similarly, when a farmer calls
void prefetch(void *ptr);
the cache will start to fetch the block containing the given address into the cache. The call returns immediately with no stalling, and roughly 100 cycles later the data will arrive in the cache and be available for use. This call only works if the farmer actually has access to the specified block of memory. For each cache line, there can be at most one outstanding fetch in progress. Prefetch requests are ignored if a fetch is already in progress for that line, and actual data fetch requests (e.g. from load/store instructions, rather than prefetch requests) supersede any prefetch requests that are in progress. There are additional system calls to read the current cache tags and the tags for lines that are being fetched. This same information is also available in the memory mapped data-structures.
We have provided a header file (described below) containing various useful macros for accessing your own code and data segments, and those of your opponent. The header also describes in detail the memory layout, cache organization, and available system calls.
Pointer arithmetic in C: When performing pointer arithmetic in C, the compiler implicitly multiplies by the size of the data type that is pointed to. For instance:
char *byteptr = (char *)0x1000; byteptr[3] = 0xFF; // store one byte at memory three BYTES after ptr, so address 0x1000+3 = 0x1003 byteptr += 5; // increments pointer by five BYTES, to address 0x1005 int *wordptr = (int *)0x1000; wordptr[3] = 0xFFFFFFFF; // store one word at memory three WORDS after ptr, so address 0x1000+12 = 0x100c wordptr += 5; // increments pointer by five WORDS, to address 0x1000+20 = 0x1014
From this, we can see that the following code is almost certainly a bug (two bugs, in fact), since the first statement advances by 12 cache lines instead of 3 cache lines, and the second statement advances by 8 cache lines instead of 2 cache lines:
int *ptr = (int *)HOME_DATA_START + 3*CACHE_LINE; pre += 2*CACHE_LINE;
To advance by 3 cache lines (first statement) or 2 cache lines (second statement) when using integer pointers, one could do:
int *ptr = (int *)(HOME_DATA_START + 3*CACHE_LINE); pre = (int *)((int)pre + 2*CACHE_LINE);
Or:
int *ptr = (int *)HOME_DATA_START + (3*CACHE_LINE)/4; pre += (2*CACHE_LINE)/4;
What can be said? Design code that makes efficient use of the data cache to fill memory quickly, and find clever ways to steal your opponent’s produce and interfere with your opponent's cache usage. And although you cannot directly modify your opponent's registers, you can modify their call stack and global variables. Coupled with what you know about exploiting programs, you may even be able to get your opponent to work for you. At the same time, you need to be on the lookout for stealers from your opponent's team. Because all of these tasks require processing time and access to memory, you will need to carefully plan how to coordinate use of these scarce resources.
To get started, we will supply several example teams that employ some basic strategies. Their source code can be found in the same directory as the FarmVille simulator. The example teams are:
Note: these teams do not do a particularly good job making effective use of the cache. For instance, most write a single byte at a time to memory, and take only a little care to ensure that their bots don't compete with each other for cache lines.
The FarmVille simulator and example teams can be found on the CSUG machines at:
/courses/cs3410/pa3/
Copy the whole directory to your home directory to get started.
$ cp -R /courses/cs3410/pa3 ~
Each team should create a single program, written in C and compiled into a MIPS binary executable. The four farmers on a team all share the same code (though your program can obviously do different things depending on the core_id parameter it receives from the simulator). The farmers directory contains all the teams listed above, and a Makefile to help compile them. Changing to that directory and typing make will compile them all. When you write your own farmer instance, you can add it to compilation list by editing the Makefile. Add the name of your farmer (minus the .c extension) to the list of TARGETS and run make to compile.
In the main directory you will also farmville, the simulator used to play the game. The simulator farmville takes two arguments, the names of the teams in the match, for example:
$ ./farmville farmers/greedy farmers/fbi
You will also find simple shell script farmvall which takes the name of a single team and pits it against every other team in turn. This lets you compare how one team does in comparison to all the others:
$ ./farmvall farmers/greedy farmers
Compilers don't always produce the code you expect them to, so you will likely want to read the compiled assembly. As you have seen before, you can decompile compiled code using objdump:
$ /courses/cs3410/mipsel-linux/bin/mipsel-linux-objdump -xdl farmers/greedy
Finally you have two tools at your disposition. First, you will want to read the farmville.h header file in the farmers directory. It is the only header file you should import into your farmer programs, besides any you might write yourself, but it is designed to be all you will need. It contains the basic FarmVille functions steal , accuse, retreat and so on, along with various other system calls like printf and rand(). But more importantly, its comment describes in detail how your program is laid out in memory and how simulator statistics and other data is mapped into your address space.
Your second major tool is the farmville simulator which has several useful command line options. Type farmville with no arguments to see a list.
Your grade for this assignment will have three parts.
The source code for your farmer, with comments, and a Makefile so we can build it. Good luck!
Peace, Law, Order!!