Rating:
1. Reversing the interpreter 1
1 is a small statically linked ELF64 with no section headers. The entry point opens the path supplied as argv[1], reads the whole file into memory, treats it as an array of 64-bit little-endian cells, and starts executing at VM program counter 0.
Each instruction is a triple of cells:
[a, b, c]
The VM logic is:
uint64_t *mem = read_entire_file(argv[1]);
int64_t pc = 0;
while (0 <= pc && pc < cell_count) {
uint64_t a = mem[pc + 0];
uint64_t b = mem[pc + 1];
uint64_t c = mem[pc + 2];
if ((int64_t)a == -1) {
// input opcode
unsigned char ch;
read(0, &ch, 1);
mem[b] = ch;
pc = c;
} else if ((int64_t)a == -2) {
// output opcode
print_hex_without_0x_or_leading_zeroes(mem[b]);
write(1, " ", 1);
pc = c;
} else {
// add-and-branch opcode
mem[b] += mem[a];
pc = ((int64_t)mem[b] > 0) ? pc + 3 : c;
}
}
The important detail is the signed branch after the addition. The bytecode uses that one arithmetic instruction to implement loops, table indexing, byte arithmetic, copies, and the final hash.
A direct VM trace of the correct input runs for roughly 3,975,036,377 VM instructions and performs:
128 input opcodes
32 output opcodes
