224-15b-final-exam-examples-1

CS 224 Final Exam Example Problems Consider the following C code and corresponding X86 assembly code. void bob() { int x = 0x11111111; short y = 0x2222; char z = 0x88; char buffer[2]; gets(buffer); printf("%x %x %s\n", y, z, buffer); scanf("%x %hx %hhx\n", &x, &y, &z); int i = 0; while (scanf("%x", &x) != EOF) buffer[i++] = (char)x; } bob: pushq %rbp movq %rsp, %rbp subq $16, %rsp movl $286331153, -8(%rbp) movw $8738, -10(%rbp) movb $-120, -11(%rbp) leaq -13(%rbp), %rax movq %rax, %rdi movl $0, %eax call gets@PLT movzbl -11(%rbp), %eax movsbl %al, %edx movzwl -10(%rbp), %eax cwtl leaq -13(%rbp), %rcx movl %eax, %esi leaq .LC0(%rip), %rdi movl $0, %eax call printf@PLT leaq -11(%rbp), %rcx leaq -10(%rbp), %rdx leaq -8(%rbp), %rax movq %rax, %rsi leaq .LC1(%rip), %rdi movl $0, %eax call __isoc99_scanf@PLT movl $0, -4(%rbp) jmp .L4 .L5: movl -8(%rbp), %ecx movl -4(%rbp), %eax leal 1(%rax), %edx movl %edx, -4(%rbp) movl %ecx, %edx cltq movb %dl, -13(%rbp,%rax) .L4: leaq -8(%rbp), %rax

movq %rax, %rsi leaq .LC2(%rip), %rdi movl $0, %eax call __isoc99_scanf@PLT cmpl $-1, %eax jne .L5 nop nop leave ret Problem 1 After the 'subq' instruction function in 'bob' completes, the stack contains a number of items listed below. Determine where on the stack each item is located. Give the byte offset of each item relative to %rsp. Express these as decimal offsets (positive or negative) relative to register %rsp: return address saved %rbp i x y z buffer Problem 2 Consider the case when 'bob' is called and we type 'ABCDEFGH' in response to gets. For each item in the following list, indicate which are/are not corrupted by the response from gets, i.e., their values were altered by some action within the call to gets. return address saved %rbp i x y z buffer What will the printf function print for the following: y z buffer Problem 3 You will now design an input string that,