1 ## Checking null-terminated ascii strings.
  2 #
  3 # By default we create strings with a 4-byte length prefix rather than a null suffix.
  4 # However we still need null-prefixed strings when interacting with the Linux
  5 # kernel in a few places. This layer implements a function for comparing
  6 # a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
  7 #
  8 # To run (from the subx directory):
  9 #   $ subx translate 05[0-2]*.subx -o /tmp/tmp52
 10 #   $ subx run /tmp/tmp52  # runs a series of tests
 11 #   ......  # all tests pass
 12 #
 13 # (We can't yet run the tests when given a "test" commandline argument,
 14 # because checking for it would require the function being tested! Breakage
 15 # would cause tests to not run, rather than to fail as we'd like.)
 16 
 17 == code
 18 
 19 # instruction                     effective address                                                   operand     displacement    immediate
 20 # op          subop               mod             rm32          base        index         scale       r32
 21 # 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
 22 
 23 # main:  (manual test if this is the last file loaded)
 24   e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
 25   # exit(Num-test-failures)
 26   8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
 27   b8/copy-to-EAX  1/imm32
 28   cd/syscall  0x80/imm8
 29 
 30 # compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
 31 # reason for the name: the only place we should have null-terminated ascii strings is from commandline args
 32 kernel-string-equal:  # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
 33   # prolog
 34   55/push-EBP
 35   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 36   # save registers
 37   51/push-ECX
 38   52/push-EDX
 39   53/push-EBX
 40   56/push-ESI
 41   57/push-EDI
 42 
 43   # pseudocode:
 44   #   initialize n = b.length
 45   #   initialize s1 = s
 46   #   initialize s2 = b.data
 47   #   i = 0
 48   #   for (i = 0; i < n; ++n)
 49   #     c1 = *s1
 50   #     c2 = *s2
 51   #     if c1 == 0
 52   #       return false
 53   #     if c1 != c2
 54   #       return false
 55   #   return *s1 == 0
 56   #
 57   # initialize s into EDI
 58   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
 59   # initialize benchmark length n into EDX
 60   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
 61   8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
 62   # initialize benchmark data into ESI
 63   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
 64   81          0/subop/add         3/mod/direct    6/rm32/ESI    .           .             .           .           .               4/imm32           # add to ESI
 65   # initialize loop counter i into ECX
 66   b9/copy-to-ECX  0/imm32/exit
 67   # while (i/ECX < n/EDX)
 68 $kernel-string-equal:loop:
 69   39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
 70   74/jump-if-equal  $kernel-string-equal:break/disp8
 71     # c1/EAX, c2/EBX = *s, *benchmark
 72   b8/copy-to-EAX  0/imm32
 73   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 74   bb/copy-to-EBX  0/imm32
 75   8a/copy                         0/mod/indirect  6/rm32/ESI    .           .             .           3/r32/EBX   .               .                 # copy byte at *ESI to lower byte of EBX
 76     # if (c1 == 0) return false
 77   3d/compare-EAX  0/imm32
 78   74/jump-if-equal  $kernel-string-equal:false/disp8
 79     # if (c1 != c2) return false
 80   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX with EBX
 81   75/jump-if-not-equal  $kernel-string-equal:false/disp8
 82     # ++s1, ++s2, ++i
 83   41/inc-ECX
 84   46/inc-ESI
 85   47/inc-EDI
 86   # end while
 87   eb/jump  $kernel-string-equal:loop/disp8
 88 $kernel-string-equal:break:
 89   # if (*s/EDI == 0) return true
 90   b8/copy-to-EAX  0/imm32
 91   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 92   3d/compare-EAX  0/imm32
 93   75/jump-if-not-equal  $kernel-string-equal:false/disp8
 94 $kernel-string-equal:true:
 95   b8/copy-to-EAX  1/imm32
 96   eb/jump  $kernel-string-equal:end/disp8
 97   # return false
 98 $kernel-string-equal:false:
 99   b8/copy-to-EAX  0/imm32
100 
101 $kernel-string-equal:end:
102   # restore registers
103   5f/pop-to-EDI
104   5e/pop-to-ESI
105   5b/pop-to-EBX
106   5a/pop-to-EDX
107   59/pop-to-ECX
108   # epilog
109   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
110   5d/pop-to-EBP
111   c3/return
112 
113 ## tests
114 
115 test-compare-null-kernel-string-with-empty-array:
116   # EAX = kernel-string-equal(Null-kernel-string, "")
117     # push args
118   68/push  ""/imm32
119   68/push  Null-kernel-string/imm32
120     # call
121   e8/call  kernel-string-equal/disp32
122     # discard args
123   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
124   # call check-ints-equal(EAX, 1, msg)
125     # push args
126   68/push  "F - test-compare-null-kernel-string-with-empty-array"/imm32
127   68/push  1/imm32/true
128   50/push-EAX
129     # call
130   e8/call  check-ints-equal/disp32
131     # discard args
132   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
133   c3/return
134 
135 test-compare-null-kernel-string-with-non-empty-array:
136   # EAX = kernel-string-equal(Null-kernel-string, "Abc")
137     # push args
138   68/push  "Abc"/imm32
139   68/push  Null-kernel-string/imm32
140     # call
141   e8/call  kernel-string-equal/disp32
142     # discard args
143   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
144   # call check-ints-equal(EAX, 0, msg)
145     # push args
146   68/push  "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
147   68/push  0/imm32/false
148   50/push-EAX
149     # call
150   e8/call  check-ints-equal/disp32
151     # discard args
152   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
153   c3/return
154 
155 test-compare-kernel-string-with-equal-array:
156   # EAX = kernel-string-equal(Abc-kernel-string, "Abc")
157     # push args
158   68/push  "Abc"/imm32
159   68/push  Abc-kernel-string/imm32
160     # call
161   e8/call  kernel-string-equal/disp32
162     # discard args
163   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
164   # call check-ints-equal(EAX, 1, msg)
165     # push args
166   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
167   68/push  1/imm32/true
168   50/push-EAX
169     # call
170   e8/call  check-ints-equal/disp32
171     # discard args
172   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
173   c3/return
174 
175 test-compare-kernel-string-with-inequal-array:
176   # EAX = kernel-string-equal(Abc-kernel-string, "Adc")
177     # push args
178   68/push  "Adc"/imm32
179   68/push  Abc-kernel-string/imm32
180     # call
181   e8/call  kernel-string-equal/disp32
182     # discard args
183   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
184   # call check-ints-equal(EAX, 0, msg)
185     # push args
186   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
187   68/push  0/imm32/false
188   50/push-EAX
189     # call
190   e8/call  check-ints-equal/disp32
191     # discard args
192   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
193   c3/return
194 
195 test-compare-kernel-string-with-empty-array:
196   # EAX = kernel-string-equal(Abc-kernel-string, "")
197     # push args
198   68/push  ""/imm32
199   68/push  Abc-kernel-string/imm32
200     # call
201   e8/call  kernel-string-equal/disp32
202     # discard args
203   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
204   # call check-ints-equal(EAX, 0)
205     # push args
206   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
207   68/push  0/imm32/false
208   50/push-EAX
209     # call
210   e8/call  check-ints-equal/disp32
211     # discard args
212   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
213   c3/return
214 
215 test-compare-kernel-string-with-shorter-array:
216   # EAX = kernel-string-equal(Abc-kernel-string, "Ab")
217     # push args
218   68/push  "Ab"/imm32
219   68/push  Abc-kernel-string/imm32
220     # call
221   e8/call  kernel-string-equal/disp32
222     # discard args
223   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
224   # call check-ints-equal(EAX, 0)
225     # push args
226   68/push  "F - test-compare-kernel-string-with-shorter-array"/imm32
227   68/push  0/imm32/false
228   50/push-EAX
229     # call
230   e8/call  check-ints-equal/disp32
231     # discard args
232   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
233   c3/return
234 
235 test-compare-kernel-string-with-longer-array:
236   # EAX = kernel-string-equal(Abc-kernel-string, "Abcd")
237     # push args
238   68/push  "Abcd"/imm32
239   68/push  Abc-kernel-string/imm32
240     # call
241   e8/call  kernel-string-equal/disp32
242     # discard args
243   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
244   # call check-ints-equal(EAX, 0)
245     # push args
246   68/push  "F - test-compare-kernel-string-with-longer-array"/imm32
247   68/push  0/imm32/false
248   50/push-EAX
249     # call
250   e8/call  check-ints-equal/disp32
251     # discard args
252   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
253   c3/return
254 
255 == data
256 
257 Null-kernel-string:
258   00/null
259 Abc-kernel-string:
260   41/A 62/b 63/c 00/null
261 
262 # vim:nowrap:textwidth=0