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
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
|
# Checking null-terminated ascii strings.
#
# By default we create strings with a 4-byte length prefix rather than a null suffix.
# However we still need null-prefixed strings when interacting with the Linux
# kernel in a few places. This layer implements a function for comparing
# a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
#
# To run (from the subx directory):
# $ ./subx translate 05[0-2]*.subx -o /tmp/tmp52
# $ ./subx run /tmp/tmp52 # runs a series of tests
# ...... # all tests pass
#
# (We can't yet run the tests when given a "test" commandline argument,
# because checking for it would require the function being tested! Breakage
# would cause tests to not run, rather than to fail as we'd like.)
== code
# instruction effective address register displacement immediate
# . op subop mod rm32 base index scale r32
# . 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes
# main:
e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
# syscall(exit, Num-test-failures)
8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX
b8/copy-to-EAX 1/imm32/exit
cd/syscall 0x80/imm8
# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
kernel-string-equal: # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
# . prolog
55/push-EBP
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
# . save registers
51/push-ECX
52/push-EDX
53/push-EBX
56/push-ESI
57/push-EDI
# pseudocode:
# initialize n = b->length
# initialize s1 = s
# initialize s2 = b->data
# i = 0
# for (i = 0; i < n; ++n)
# c1 = *s1
# c2 = *s2
# if c1 == 0
# return false
# if c1 != c2
# return false
# return *s1 == 0
#
# initialize s into EDI
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 7/r32/EDI 8/disp8 . # copy *(EBP+8) to EDI
# initialize benchmark length n into EDX
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 2/r32/EDX 0xc/disp8 . # copy *(EBP+12) to EDX
8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX
# initialize benchmark data into ESI
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 6/r32/ESI 0xc/disp8 . # copy *(EBP+12) to ESI
81 0/subop/add 3/mod/direct 6/rm32/ESI . . . . . 4/imm32 # add to ESI
# initialize loop counter i into ECX
b9/copy-to-ECX 0/imm32/exit
# while (i/ECX < n/EDX)
$kernel-string-equal:loop:
39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX
74/jump-if-equal $kernel-string-equal:break/disp8
# c1/EAX, c2/EBX = *s, *benchmark
b8/copy-to-EAX 0/imm32
8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
bb/copy-to-EBX 0/imm32
8a/copy 0/mod/indirect 6/rm32/ESI . . . 3/r32/EBX . . # copy byte at *ESI to lower byte of EBX
# if (c1 == 0) return false
3d/compare-EAX 0/imm32
74/jump-if-equal $kernel-string-equal:false/disp8
# if (c1 != c2) return false
39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX with EBX
75/jump-if-not-equal $kernel-string-equal:false/disp8
# ++s1, ++s2, ++i
41/inc-ECX
46/inc-ESI
47/inc-EDI
# end while
eb/jump $kernel-string-equal:loop/disp8
$kernel-string-equal:break:
# if (*s/EDI == 0) return true
b8/copy-to-EAX 0/imm32
8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX
3d/compare-EAX 0/imm32
75/jump-if-not-equal $kernel-string-equal:false/disp8
$kernel-string-equal:true:
b8/copy-to-EAX 1/imm32
eb/jump $kernel-string-equal:end/disp8
# return false
$kernel-string-equal:false:
b8/copy-to-EAX 0/imm32
$kernel-string-equal:end:
# . restore registers
5f/pop-to-EDI
5e/pop-to-ESI
5b/pop-to-EBX
5a/pop-to-EDX
59/pop-to-ECX
# . epilog
89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP
5d/pop-to-EBP
c3/return
# - tests
test-compare-null-kernel-string-with-empty-array:
# EAX = kernel-string-equal(Null-kernel-string, "")
# . . push args
68/push ""/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 1, msg)
# . . push args
68/push "F - test-compare-null-kernel-string-with-empty-array"/imm32
68/push 1/imm32/true
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-null-kernel-string-with-non-empty-array:
# EAX = kernel-string-equal(Null-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 0, msg)
# . . push args
68/push "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
68/push 0/imm32/false
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-kernel-string-with-equal-array:
# EAX = kernel-string-equal(Abc-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 1, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 1/imm32/true
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-kernel-string-with-inequal-array:
# EAX = kernel-string-equal(Abc-kernel-string, "Adc")
# . . push args
68/push "Adc"/imm32
68/push Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-kernel-string-with-empty-array:
# EAX = kernel-string-equal(Abc-kernel-string, "")
# . . push args
68/push ""/imm32
68/push Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-kernel-string-with-shorter-array:
# EAX = kernel-string-equal(Abc-kernel-string, "Ab")
# . . push args
68/push "Ab"/imm32
68/push Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-shorter-array"/imm32
68/push 0/imm32/false
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
test-compare-kernel-string-with-longer-array:
# EAX = kernel-string-equal(Abc-kernel-string, "Abcd")
# . . push args
68/push "Abcd"/imm32
68/push Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP
# check-ints-equal(EAX, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-longer-array"/imm32
68/push 0/imm32/false
50/push-EAX
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP
c3/return
== data
Null-kernel-string:
00/null
Abc-kernel-string:
41/A 62/b 63/c 00/null
# . . vim:nowrap:textwidth=0
|