Austin Adams

Acing a C Homework Using Inline Assembly

February 7, 2019

In courses taught in Java (such as a data structures class), you can build pretty robust autograders for homeworks — after all, the student’s code runs in the JVM, which prevents it from doing spooky things like corrupting memory or branching to weird places. So you can run the grader instantly after submission on a service like Gradescope and boom, they’ve got their grade. But C homeworks provide students a lot more opportunities to write… unconventional solutions.

I’ve posted an example homework/POC on GitHub; clone it if you want. The assignment asks students to complete fib() in assignment.c, like the following:

int fib(int n) {
    if (n < 0)
        return -1;

    int *arr = malloc(sizeof (int) * (n + 1));
    if (!arr)
        return -1;

    arr[0] = 0;
    if (n > 0)
        arr[1] = 1;

    for (int i = 2; i <= n; i++)
        arr[i] = arr[i - 1] + arr[i - 2];

    int ret = arr[n];


    return ret;

The test suite assignment_suite.c tests their implementation (don’t worry about this too much, it’s libcheck stuff):

static int input[] =  {-200, 0, 1, 2, 3, 5, 10,  15,     27};
static int output[] = {  -1, 0, 1, 1, 2, 5, 55, 610, 196418};

START_TEST(test_fib) {
    ck_assert_int_eq(fib(input[_i]), output[_i]);

But let’s be honest, sometimes you’re a little stressed and you just don’t have time for that homework. That’s 19 whole lines of code! This leaves you with two options:

  1. Copy your fraternity brother’s homework
  2. Use inline assembly to jump past the failing assertions back into the grader — specifically, to the end of the test_fib function

I’ll write a post on how to do #1 later, but first I’ll stub out fib() as follows:

int fib(int n) {
    (void)n; // don't complain about how this is unused
    return -1;

and then run it in gdb and step instruction-by-instruction until we’re back in test_fib():

$ make run-gdb
(gdb) layout asm
(gdb) b fib
(gdb) r
(gdb) si
(gdb) si
(gdb) si

Then gdb shows the following, which shows the instruction immediately following the instruction that calls fib() is 65 bytes after the beginning of test_fib():

 |0x555555555d4d <test_fib+55>    mov    (%rdx,%rax,1),%eax
 |0x555555555d50 <test_fib+58>    mov    %eax,%edi
 |0x555555555d52 <test_fib+60>    callq  0x555555555bb0 <fib>
>|0x555555555d57 <test_fib+65>    cltq
 |0x555555555d59 <test_fib+67>    mov    %rax,-0x8(%rbp)
 |0x555555555d5d <test_fib+71>    mov    -0x14(%rbp),%eax
 |0x555555555d60 <test_fib+74>    cltq

Why do we care? Well, this means the return address pushed onto the stack when calling fib() must be 65 bytes after the beginning of test_fib(). If we grab this return address off the stack, we can add some predetermined offset to it and perform an indirect jump to skip to the end of the test function, past any nasty failing assertions!

Looking at the disassembly of the test suite object file assignment_suite.o (assignment_suite.asm in the repository), we can see the teardown for test_fib() begins at 0xc3 = 195 bytes after the beginning of test_fib():

  b2:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # b9 <test_fib+0xb9>
  b9:   b8 00 00 00 00          mov    $0x0,%eax
  be:   e8 00 00 00 00          callq  c3 <test_fib+0xc3>
  c3:   c9                      leaveq 
  c4:   c3                      retq   

So we need to add 195-65 = 130 bytes to the return address to jump to the end of test_fib(). Now we can write our cooked solution using inline assembly:

int fib(int n) {
    __asm__ ("leave\n\t"
             "popq %rax\n\t"
             "addq $130, %rax\n\t"
             "jmp *%rax");

    return -1;

Each instruction does the following:

  1. leave will restore the frame pointer of the caller so it doesn’t get confused
  2. popq %rax pops the return return address off the stack and puts it in %rax
  3. addq $130, %rax adds our precalculated offset to the return address
  4. jmp *%rax jumps to the location we want, the end of test_fib(), past all the assertions

And behold, it “passes” all tests!

$ make run-tests
Running suite(s): fun assignment
100%: Checks: 9, Failures: 0, Errors: 0


You’d think you could avoid this by passing -fno-asm to gcc, but it turns out this only disables asm, not __asm__ which still works according to gcc(1):

   Do not recognize "asm", "inline" or "typeof" as a
   keyword, so that code can use these words as
   identifiers.  You can use the keywords "__asm__",
   "__inline__" and "__typeof__" instead.

You could also pass -D__asm__=YOUREABADBOY to gcc, but this seems to break standard library headers, and the student could thwart this anyway by simply saying #undef __asm__.

So if you ask me, the best way to prevent this form of cheating is to tweak the Makefile rule that builds .c files:

%.o: %.c $(HFILES)
    sed -i 's/\<__asm__\>/YOUREABADBOY/g' $<
    $(CC) $(CFLAGS) -c $< -o $@

But, of course, the dream is to run student code in a separate address space entirely, like QEMU or something. This way, they can’t mess with the autograder’s memory — a viable option I didn’t even try exploring.