1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
|
On Arch Linux, install:
arm-none-eabi-gcc
arm-none-eabi-gdb
qemu-arch-extra
To build the `present` binary, run:
make present
To start debugging it, run:
make run-present
# Example debugging session
This section shows how to debug using GDB.
exercises$ make run-present
qemu-arm -g 1337 present &
arm-none-eabi-gdb -q -ex 'tar rem localhost:1337' present
Reading symbols from present...done.
Remote debugging using localhost:1337
_start () at present.S:58
58 ldr r0, =input
Step a single instruction:
(gdb) stepi
59 ldr r1, =sbox
Press "Enter" to repeat the last command (stepi):
(gdb)
60 bl sub_block
Examine 16 bytes of memory from the address in register `$r0` (our "input"):
(gdb) x/16b $r0
0x2000: 5 1 2 3 4 5 6 7
0x2008: 8 9 10 11 12 13 14 15
Examine 16 bytes of memory as hex from the address in register `$r1` (our "sbox"):
(gdb) x/16x $r1
0x2010: 0x0c 0x05 0x06 0x0b 0x09 0x00 0x0a 0x0d
0x2018: 0x03 0x0e 0x0f 0x08 0x04 0x07 0x01 0x02
(gdb) stepi
sub_block () at present.S:30
30 eor r3, r3
Set a breakpoint on line 38, continue until that breakpoint:
(gdb) break 38
Breakpoint 1 at 0x1014: file present.S, line 38.
(gdb) c
Continuing.
Breakpoint 1, sub_block_loop () at present.S:38
38 and r4, #15
List the source code around the current line:
(gdb) l
33 bge sub_block_loop_end
34
35 // c = input[i] (r4 is low, r5 is high)
36 ldrb r4, [r0, r3]
37 mov r5, r4
38 and r4, #15
39 lsr r5, #4
40
41 // sbox[c_lo]
42 ldrb r4, [r1, r4]
Print the contents of some register (normally in decimal, using the `/x`
specifier it becomes hexadecimal):
(gdb) p $r0
$1 = 8192
(gdb) p/x $r0
$2 = 0x2000
(gdb) p/x $r3
$3 = 0x0
(gdb) p/x $r4
$4 = 0x5
(gdb) x/b $r0+$r3
0x2000: 0x05
A quick overview of all registers:
(gdb) info registers
r0 0x2000 8192
r1 0x2010 8208
r2 0x0 0
r3 0x0 0
r4 0x5 5
r5 0x5 5
r6 0x0 0
r7 0x0 0
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
sp 0xf6fff5e0 0xf6fff5e0
lr 0x1044 4164
pc 0x1014 0x1014 <sub_block_loop+16>
cpsr 0x80000010 -2147483632
Exiting the debugger:
(gdb) quit
A debugging session is active.
Inferior 1 [Remote target] will be killed.
Quit anyway? (y or n) y
QEMU: Terminated via GDBstub
|
