Assignment 7: Some Assembly Required
EXTENDED Due Monday, November 7th, before 5pm
The goals for this assignment are:
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Understand basic x86_64 assembly instructions, especially memory addressing
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Visualize the execution of x86_64 assembly instructions
This is a written assignment, not a coding assignment!
1. Operand practice
Suppose memory has the following values:
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And suppose our registers have the following values:
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Fill in the following table with the corresponding form, translation, and value for each of the given operands.
Operand |
Form |
Translation |
Value |
%rax |
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0x4(%rax) |
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||
0x4(%rax, %r8, 4) |
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||
0xf00(,%rsi,2) |
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2. Hackers
In class, we looked at how we can cheat at a guessing game by overriding the
return address of a function to jump to the endGame function. In this question, we
will analyze the binary executable using GDB to extract the secret codes explicitly.
1) What GDB command can we use to set a breakpoint right after the function getSecretCode returns?
2) What register contains the return value from getSecretCode?
3) What GDB command can you use to inspect the value stored in the register containing the return value?
4) What is the secret number?
5) Place a breakpoint in calculateValue. How many times is calculateValue called by the program?
6) What is calculateValue doing? From the given source, we can see it takes a string. What is the return value?
7) What is the secret string?
3. The answer is 42
Recall the following example from class and
Dive into
Systems. This mysterious program reliably produces the output 42. In this
question, you will trace the assembly to understand why.
#include <stdio.h>
int assign(void) {
int y = 40;
return y;
}
int adder(void) {
int a;
return a + 2;
}
int main(void) {
int x;
assign();
x = adder();
printf("x is: %d\n", x);
return 0;
}Suppose that compiling the above program results in the following assembly instructions (x86_64)
0000000000001149 <assign>:
114d: 55 push %rbp
114e: 48 89 e5 mov %rsp,%rbp
1151: c7 45 fc 28 00 00 00 movl $0x28,-0x4(%rbp)
1158: 8b 45 fc mov -0x4(%rbp),%eax
115b: 5d pop %rbp
115c: c3 retq
000000000000115d <adder>:
1161: 55 push %rbp
1162: 48 89 e5 mov %rsp,%rbp
1165: 8b 45 fc mov -0x4(%rbp),%eax
1168: 83 c0 02 add $0x2,%eax
116b: 5d pop %rbp
116c: c3 retq
000000000000116d <main>:
1171: 55 push %rbp
1172: 48 89 e5 mov %rsp,%rbp
1175: 48 83 ec 10 sub $0x10,%rsp
1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
118b: 48 8d 3d 72 0e 00 00 lea 0xe72(%rip),%rdi
1192: b8 00 00 00 00 mov $0x0,%eax
1197: e8 b4 fe ff ff callq 1050 <printf@plt>
119c: b8 00 00 00 00 mov $0x0,%eax
11a1: c9 leaveq
11a2: c3 retq| In The Hitchhiker’s Guide to the Galaxy by Douglas Adams, the "Answer to the Ultimate Question of Life, the Universe, and Everything," calculated by an enormous supercomputer named Deep Thought over a period of 7.5 million years. |
1) Suppose this is the state of the stack immediately prior to executing main
Please show the before and after state (cross out old values).
000000000000116d <main>:
--> 1171: 55 push %rbp
1172: 48 89 e5 mov %rsp,%rbp
1175: 48 83 ec 10 sub $0x10,%rsp
1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
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"Stack top"
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2) What are the contents of registers and the stack after executing mov %rsp, %rbp?
Please show the before and after state (cross out old values).
000000000000116d <main>:
1171: 55 push %rbp
--> 1172: 48 89 e5 mov %rsp,%rbp
1175: 48 83 ec 10 sub $0x10,%rsp
1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
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"Stack top"
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3) What is the state of registers and the stack after executing sub $0x10, %rsp?
Please show the before and after state (cross out old values).
000000000000116d <main>:
1171: 55 push %rbp
1172: 48 89 e5 mov %rsp,%rbp
--> 1175: 48 83 ec 10 sub $0x10,%rsp
1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
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"Stack top"
|
4) What is the state of registers and the stack after executing callq 0x1149 <assign>?
Please show the before and after state (cross out old values).
000000000000116d <main>:
1171: 55 push %rbp
1172: 48 89 e5 mov %rsp,%rbp
1175: 48 83 ec 10 sub $0x10,%rsp
--> 1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
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"Stack top"
|
5) Skipping ahead, what is the state of registers and the stack when the program executes pop %rbp?
What two changes occur during pop?
Please show the before and after state (cross out old values).
0000000000001149 <assign>:
114d: 55 push %rbp
114e: 48 89 e5 mov %rsp,%rbp
1151: c7 45 fc 28 00 00 00 movl $0x28,-0x4(%rbp)
1158: 8b 45 fc mov -0x4(%rbp),%eax
--> 115b: 5d pop %rbp
115c: c3 retq
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"Stack top"
|
6) What is the state of registers and the stack after executing retq?
Please show the before and after state (cross out old values).
0000000000001149 <assign>:
114d: 55 push %rbp
114e: 48 89 e5 mov %rsp,%rbp
1151: c7 45 fc 28 00 00 00 movl $0x28,-0x4(%rbp)
1158: 8b 45 fc mov -0x4(%rbp),%eax
115b: 5d pop %rbp
--> 115c: c3 retq
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"Stack top"
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7) Skipping ahead, what are the state of registers and the stack when executing retq in adder?
Please show the before and after state (cross out old values).
000000000000115d <adder>:
1161: 55 push %rbp
1162: 48 89 e5 mov %rsp,%rbp
1165: 8b 45 fc mov -0x4(%rbp),%eax
1168: 83 c0 02 add $0x2,%eax
116b: 5d pop %rbp
--> 116c: c3 retq
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"Stack top"
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8) Consider the call the printf, callq 1050 <printf@plt>. The previous lines place the first
argument to printf into %rdi. Assume this contains "x is %d\n". The second argument will be
placed in %esi. What value will %esi containin?
000000000000116d <main>:
1171: 55 push %rbp
1172: 48 89 e5 mov %rsp,%rbp
1175: 48 83 ec 10 sub $0x10,%rsp
1179: e8 cb ff ff ff callq 1149 <assign>
117e: e8 da ff ff ff callq 115d <adder>
1183: 89 45 fc mov %eax,-0x4(%rbp)
1186: 8b 45 fc mov -0x4(%rbp),%eax
1189: 89 c6 mov %eax,%esi
118b: 48 8d 3d 72 0e 00 00 lea 0xe72(%rip),%rdi
1192: b8 00 00 00 00 mov $0x0,%eax
1197: e8 b4 fe ff ff callq 1050 <printf@plt>
119c: b8 00 00 00 00 mov $0x0,%eax
11a1: c9 leaveq
11a2: c3 retq4. Submit your work
This is a written assignment. Please submit a hard-copy in either Lecture or Lab, or submit to either Nina Fichera’s office (Park 348) or Aline Normoyle’s office (Park 200B).