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Q1 Alignment Ailment

9 Points

struct S {

char a;

short b;

union {

struct {

struct S* c;

struct S* d;

} x;

struct {

long e;

int* f;

} y;

} z;

char g[20];

} star[2];

Q1.1

2 Points

If %rdi holds the starting address of star and you want %rax to hold the value of star[0].z.y.e after the

instruction, which instruction should be used, and what should the value of N be in the chosen instruction?

Which instruction?

mov N(%rdi), %rax

mov (%rdi, N), (%rax)

lea N(%rdi), %rax

lea (%rdi, N), %rax

mov N(%rdi), (%rax)

lea (%rdi, N), (%rax)

lea N(%rdi), (%rax)

mov (%rdi, N), %rax

Q1.2

2 Points

Value of N for the chosen instruction (please answer in decimal)

Q1.3

2 Points

What is the alignment size of the deﬁned structure type S? Please write in decimal.

Q1.4

3 Points

How many bytes in total does it take to store star in memory? Please write in decimal.

Q2 Switch Sanity

9 Points

Here is the assembly code for the function int foo(int x, int y);

000000000040052d :

40052d: 8d 47 ea lea -0x16(%rdi),%eax

400530: 83 f8 06 cmp $0x6,%eax

400533: 77 2d ja 400562

400535: 89 c0 mov %eax,%eax

400537: ff 24 c5 30 06 40 00 jmpq *0x400630(,%rax,8)

40053e: 89 f8 mov %edi,%eax

400540: 83 e0 0f and $0xf,%eax

400543: eb 25 jmp 40056a

400545: 83 f7 fe xor $0xfffffffe,%edi

400548: 8d 04 3f lea (%rdi,%rdi,1),%eax

40054b: eb 1d jmp 40056a

40054d: f7 d6 not %esi

40054f: 8d 04 3e lea (%rsi,%rdi,1),%eax

400552: eb 16 jmp 40056a

400554: 85 ff test %edi,%edi

400556: 0f 95 c2 setne %dl

400559: c1 ee 1f shr $0x1f,%esi

40055c: 89 f0 mov %esi,%eax

40055e: 21 d0 and %edx,%eax

400560: eb 08 jmp 40056a

400562: 39 f7 cmp %esi,%edi

400564: 0f 94 c0 sete %al

400567: 0fb6 c0 movzbl %al,%eax

40056a: 83 c0 01 add $0x1,%eax

40056d: c3 retq

Here is the C source code of the function foo with some parts (i.e., the most important parts) missing.

int foo(int x, int y) {

switch(x) {

/* a few case statements are missing here! */

case 28:

x = /* Uh oh, what the expression here? */

break;

default:

x = /* Oh no, the expression here is missing as well! */

}

return x + 1;

}

With GDB, you dumped out the 64 bytes of content starting from address 0x400630. The leftmost column shows the starting address of the row, and the 8 columns to the right show the 8 bytes stored contiguously starting from that address.

(gdb) x/64xb 0x400630

0x400630: 0x3e 0x05 0x40 0x00 0x00 0x00 0x00 0x00

0x400638:	0x62	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400640:	0x45	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400648:	0x48	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400650:	0x3e	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400658:	0x4d	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400660:	0x54	0x05	0x40	0x00	0x00	0x00	0x00	0x00
0x400668:	0x25	0x64	0x0a	0x00	0x01	0x1b	0x03	0x3b

Q2.1

2 Points

Fill in the missing expression in the default case .

x = (x == y)

Q2.2

2 Points

Fill in the missing expression in the case 28.

x = ((x != 0) && (y < 0))

Q2.3

2 Points

What is the return value of foo(28, -8) ? Please answer in decimal. If the value cannot be known from the information given in this question, please write X .

Q2.4

3 Points

What is the return value of foo(24, 24) ? Please answer in decimal. If the value cannot be known from the information given in this question, please write X .

Q3 Barred Bars

9 Points

You are using GDB to debug a program on a x86-64 machine. The program has a struct Bar and a variable bars deﬁned as follow:

struct Bar { short a; int b; };

struct Bar* bars[8][8];

From your interactions with GDB, you ﬁnd that the address of bars[0][0] is 0x6012a0 . Here is the memory dump of 1024 bytes of content starting from address 0x6012a0 . The leftmost column shows the starting address of the row, and the 8 columns to the right show the 8 bytes stored contiguously starting from that address.

(gdb) x/1024bx 0x6012a0

0x6012a0: 0xa8

0x6012a8: 0xb0

0x6012b0: 0xa0

0x6012b8: 0xa8

0x6012c0: 0x60

0x6012c8: 0x48

0x6012d0: 0x70

0x6012d8: 0x80

0x6012e0: 0x88

0x6012e8: 0x90

0x6012f0: 0x18

0x6012f8: 0xe8

0x601300: 0x28

0x601308: 0xd8

0x601310: 0xe8

0x601318: 0x60

0x601320: 0x88

0x601328: 0xa0

0x14 0x15 0x15 0x15 0x15 0x16 0x16 0x16 0x15 0x16 0x15 0x15 0x16 0x14 0x14 0x15 0x15 0x15

0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60 0x60

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00

0x601330: 0x30

0x601338: 0x00

0x601340: 0x58

0x601348: 0xe8

0x601350: 0xe0

0x601358: 0xe8

0x601360: 0xb0

0x601368: 0x60

0x601370: 0x10

0x601378: 0x18

0x601380: 0x80

0x601388: 0x58

0x601390: 0xe0

0x601398: 0x98

0x6013a0: 0xa0

0x6013a8: 0x30

0x6013b0: 0x58

0x6013b8: 0x20

0x6013c0: 0x60

0x6013c8: 0x38

0x6013d0: 0xe8

0x6013d8: 0xa0

0x6013e0: 0x38

0x6013e8: 0x90

0x6013f0: 0x18

0x6013f8: 0xf0

0x601400: 0x68

0x601408: 0x08

0x601410: 0x40

0x601418: 0xa8

0x601420: 0xf8

0x601428: 0xb0

0x601430: 0x30

0x601438: 0x70

0x601440: 0x88

0x16

0x15

0x14

0x15

0x14

0x16

0x15

0x16

0x15

0x16

0x14

0x16

0x15

0x16

0x15

0x14

0x15

0x16

0x15

0x60

0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00