
INFORMATION TECH.PROJECT...-W/MINDTAP
9th Edition
ISBN: 9781337586801
Author: SCHWALBE
Publisher: CENGAGE L
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Consider the following assembly code for a C for loop:
movl $0, %eax
jmp
.L2
.L3:
addq
$1, %rdi
addq
%rsi, %rax
subq
$1, %rsi
.L2:
cmpq
%rsi, %rdi
jl
.L3
addq
ret
%rdi, %rax
Based on the assembly code above, fill in the blanks below in its corresponding C
source code. Recall that registers %rdi and %rsi contain the first and second, respectively,
argument of a function. (Note: you may only use the symbolic variables x, y, and
result in your expressions below do not use register names.)
long loop (long x, long y)
{
long result;
}
for (
}
return result;
__; y--) {
In each of the following C code snippets, there are issues that can prevent the compiler
from applying certain optimizations. For each snippet:
Circle the line number that contains compiler optimization blocker.
⚫ Select the best modification to improve optimization.
1. Which line prevents compiler optimization? Circle one: 2 3 4 5 6
Suggested solution:
⚫ Remove printf or move it outside the loop.
Remove the loop.
• Replace arr[i] with a constant value.
1 int sum (int *arr, int n) {
2
int s = 0;
3
for (int i
=
0; i < n; i++) {
4
5
6
}
7
8 }
s = arr[i];
printf("%d\n", s);
return s;
234206
2. Which line prevents compiler optimization? Circle one: 2 3 4 5 6
Suggested solution:
Move or eliminate do_extra_work() if it's not necessary inside the loop.
Remove the loop (but what about scaling?).
⚫ Replace arr[i] *= factor; with arr[i] = 0; (why would that help?).
1 void scale (int *arr, int n, int factor) {
5
6 }
for (int i = 0; i < n; i++) {
rr[i] = factor;
do_extra_work ();
123456
A ROP (Return-Oriented Programming) attack can be used to execute arbitrary
instructions by chaining together small pieces of code called "gadgets." Your goal is
to create a stack layout for a ROP attack that calls a function located at '0x4018bd3'.
Below is the assembly code for the function 'getbuf', which allocates 8 bytes of stack
space for a 'char' array. This array is then passed to the 'gets' function. Additionally,
you are provided with five useful gadgets and their addresses. Use these gadgets to
construct the stack layout.
Assembly for getbuf
1 getbuf:
sub
mov
$8, %rsp
%rsp, %rdi
call
gets
add
$8, %rsp
ret
#Allocate 8 bytes for buffer
#Load buffer address into %rdi
#Call gets with buffer
#Restore the stack pointer
#Return to caller
Stack
Layout
each
8-byte
(fill in
section)
Address
Value (8 bytes)
0x7fffffffdfc0
0x7fffffffdfb8
0x7fffffffdfb0
0x7fffffffdfa8
0x7fffffffdfa0
0x7fffffffdf98
0x7fffffffdf90
0x7fffffffdf88
Gadgets
Address Gadget
Ox4006a7 pop %rdi; ret
Ox4006a9…
Knowledge Booster
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