1 ## Null-terminated vs length-prefixed 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 examples/ex11.subx -o examples/ex11
 10 #   $ subx run examples/ex11  # 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 # instruction                     effective address                                                   operand     displacement    immediate
 19 # op          subop               mod             rm32          base        index         scale       r32
 20 # 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
 21 
 22 # main:
 23   e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
 24   # syscall(exit, EAX)
 25   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
 26   b8/copy-to-EAX  1/imm32
 27   cd/syscall  0x80/imm8
 28 
 29 # compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
 30 # reason for the name: the only place we should have null-terminated ascii strings is from commandline args
 31 kernel-string-equal:  # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
 32   # prolog
 33   55/push-EBP
 34   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
 35   # save registers
 36   51/push-ECX
 37   52/push-EDX
 38   53/push-EBX
 39   56/push-ESI
 40   57/push-EDI
 41 
 42   # pseudocode:
 43   #   initialize n = b.length
 44   #   initialize s1 = s
 45   #   initialize s2 = b.data
 46   #   i = 0
 47   #   for (i = 0; i < n; ++n)
 48   #     c1 = *s1
 49   #     c2 = *s2
 50   #     if c1 == 0
 51   #       return false
 52   #     if c1 != c2
 53   #       return false
 54   #   return *s1 == 0
 55   # initialize s into EDI
 56   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
 57   # initialize benchmark length n into EDX
 58   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
 59   8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
 60   # initialize benchmark data into ESI
 61   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
 62   81          0/subop/add         3/mod/direct    6/rm32/ESI    .           .             .           .           .               4/imm32           # add to ESI
 63   # initialize loop counter i into ECX
 64   b9/copy-to-ECX  0/imm32/exit
 65   # while (i/ECX < n/EDX)
 66 $kernel-string-equal:loop:
 67   39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
 68   74/jump-if-equal  $kernel-string-equal:break/disp8
 69     # c1/EAX, c2/EBX = *s, *benchmark
 70   b8/copy-to-EAX  0/imm32
 71   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 72   bb/copy-to-EBX  0/imm32
 73   8a/copy                         0/mod/indirect  6/rm32/ESI    .           .             .           3/r32/EBX   .               .                 # copy byte at *ESI to lower byte of EBX
 74     # if (c1 == 0) return false
 75   3d/compare-EAX  0/imm32
 76   74/jump-if-equal  $kernel-string-equal:false/disp8
 77     # if (c1 != c2) return false
 78   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX with EBX
 79   75/jump-if-not-equal  $kernel-string-equal:false/disp8
 80     # ++s1, ++s2, ++i
 81   41/inc-ECX
 82   46/inc-ESI
 83   47/inc-EDI
 84   # end while
 85   eb/jump  $kernel-string-equal:loop/disp8
 86 $kernel-string-equal:break:
 87   # if (*s/EDI == 0) return true
 88   b8/copy-to-EAX  0/imm32
 89   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 90   81          7/subop/compare     3/mod/direct    0/rm32/EAX    .           .             .           .           .               0/imm32           # compare EAX
 91   75/jump-if-not-equal  $kernel-string-equal:false/disp8
 92   b8/copy-to-EAX  1/imm32
 93 $kernel-string-equal:true:
 94   eb/jump  $kernel-string-equal:end/disp8
 95   # return false
 96 $kernel-string-equal:false:
 97   b8/copy-to-EAX  0/imm32
 98 
 99 $kernel-string-equal:end:
100   # restore registers
101   5f/pop-to-EDI
102   5e/pop-to-ESI
103   5b/pop-to-EBX
104   5a/pop-to-EDX
105   59/pop-to-ECX
106   # end
107   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
108   5d/pop-to-EBP
109   c3/return
110 
111 ## tests
112 
113 test-compare-null-kernel-string-with-empty-array:
114   # EAX = kernel-string-equal(Null-kernel-string, "")
115     # push args
116   68/push  ""/imm32
117   68/push  Null-kernel-string/imm32
118     # call
119   e8/call  kernel-string-equal/disp32
120     # discard args
121   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
122   # call check-ints-equal(EAX, 1, msg)
123     # push args
124   68/push  "F - test-compare-null-kernel-string-with-empty-array"/imm32
125   68/push  1/imm32/true
126   50/push-EAX
127     # call
128   e8/call  check-ints-equal/disp32
129     # discard args
130   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
131   c3/return
132 
133 test-compare-null-kernel-string-with-non-empty-array:
134   # EAX = kernel-string-equal(Null-kernel-string, "Abc")
135     # push args
136   68/push  "Abc"/imm32
137   68/push  Null-kernel-string/imm32
138     # call
139   e8/call  kernel-string-equal/disp32
140     # discard args
141   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
142   # call check-ints-equal(EAX, 0, msg)
143     # push args
144   68/push  "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
145   68/push  0/imm32/false
146   50/push-EAX
147     # call
148   e8/call  check-ints-equal/disp32
149     # discard args
150   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
151   c3/return
152 
153 test-compare-kernel-string-with-equal-array:
154   # EAX = kernel-string-equal(Abc-kernel-string, "Abc")
155     # push args
156   68/push  "Abc"/imm32
157   68/push  Abc-kernel-string/imm32
158     # call
159   e8/call  kernel-string-equal/disp32
160     # discard args
161   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
162   # call check-ints-equal(EAX, 1, msg)
163     # push args
164   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
165   68/push  1/imm32/true
166   50/push-EAX
167     # call
168   e8/call  check-ints-equal/disp32
169     # discard args
170   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
171   c3/return
172 
173 test-compare-kernel-string-with-inequal-array:
174   # EAX = kernel-string-equal(Abc-kernel-string, "Adc")
175     # push args
176   68/push  "Adc"/imm32
177   68/push  Abc-kernel-string/imm32
178     # call
179   e8/call  kernel-string-equal/disp32
180     # discard args
181   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
182   # call check-ints-equal(EAX, 0, msg)
183     # push args
184   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
185   68/push  0/imm32/false
186   50/push-EAX
187     # call
188   e8/call  check-ints-equal/disp32
189     # discard args
190   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
191   c3/return
192 
193 test-compare-kernel-string-with-empty-array:
194   # EAX = kernel-string-equal(Abc-kernel-string, "")
195     # push args
196   68/push  ""/imm32
197   68/push  Abc-kernel-string/imm32
198     # call
199   e8/call  kernel-string-equal/disp32
200     # discard args
201   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
202   # call check-ints-equal(EAX, 0)
203     # push args
204   68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
205   68/push  0/imm32/false
206   50/push-EAX
207     # call
208   e8/call  check-ints-equal/disp32
209     # discard args
210   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
211   c3/return
212 
213 test-compare-kernel-string-with-shorter-array:
214   # EAX = kernel-string-equal(Abc-kernel-string, "Ab")
215     # push args
216   68/push  "Ab"/imm32
217   68/push  Abc-kernel-string/imm32
218     # call
219   e8/call  kernel-string-equal/disp32
220     # discard args
221   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
222   # call check-ints-equal(EAX, 0)
223     # push args
224   68/push  "F - test-compare-kernel-string-with-shorter-array"/imm32
225   68/push  0/imm32/false
226   50/push-EAX
227     # call
228   e8/call  check-ints-equal/disp32
229     # discard args
230   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
231   c3/return
232 
233 test-compare-kernel-string-with-longer-array:
234   # EAX = kernel-string-equal(Abc-kernel-string, "Abcd")
235     # push args
236   68/push  "Abcd"/imm32
237   68/push  Abc-kernel-string/imm32
238     # call
239   e8/call  kernel-string-equal/disp32
240     # discard args
241   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
242   # call check-ints-equal(EAX, 0)
243     # push args
244   68/push  "F - test-compare-kernel-string-with-longer-array"/imm32
245   68/push  0/imm32/false
246   50/push-EAX
247     # call
248   e8/call  check-ints-equal/disp32
249     # discard args
250   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
251   c3/return
252 
253 ## helpers
254 
255 # print msg to stderr if a != b, otherwise print "."
256 check-ints-equal:  # (a : int, b : int, msg : (address array byte)) -> boolean
257   # prolog
258   55/push-EBP
259   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
260   # save registers
261   51/push-ECX
262   53/push-EBX
263   # load args into EAX, EBX and ECX
264   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           0/r32/EAX   0x8/disp8       .                 # copy *(EBP+8) to EAX
265   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           3/r32/EBX   0xc/disp8       .                 # copy *(EBP+12) to EBX
266   # if EAX == b/EBX
267   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX and EBX
268   75/jump-if-unequal  $check-ints-equal:else/disp8
269     # print('.')
270       # push args
271   68/push  "."/imm32
272       # call
273   e8/call  write-stderr/disp32
274       # discard args
275   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
276     # return
277   eb/jump  $check-ints-equal:end/disp8
278   # else:
279 $check-ints-equal:else:
280   # copy msg into ECX
281   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           1/r32/ECX   0x10/disp8       .                # copy *(EBP+16) to ECX
282     # print(ECX)
283       # push args
284   51/push-ECX
285       # call
286   e8/call  write-stderr/disp32
287       # discard args
288   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
289     # print newline
290       # push args
291   68/push  Newline/imm32
292       # call
293   e8/call  write-stderr/disp32
294       # discard args
295   81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
296 $check-ints-equal:end:
297   # restore registers
298   5b/pop-to-EBX
299   59/pop-to-ECX
300   # end
301   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
302   5d/pop-to-EBP
303   c3/return
304 
305 write-stderr:  # s : (address array byte) -> <void>
306   # prolog
307   55/push-EBP
308   89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
309   # save registers
310   50/push-EAX
311   51/push-ECX
312   52/push-EDX
313   53/push-EBX
314   # syscall(write, 2/stderr, (data) s+4, (size) *s)
315     # fd = 2 (stderr)
316   bb/copy-to-EBX  2/imm32
317     # x = s+4
318   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           1/r32/ECX   8/disp8         .                 # copy *(EBP+8) to ECX
319   81          0/subop/add         3/mod/direct    1/rm32/ECX    .           .             .           .           .               4/imm32           # add to ECX
320     # size = *s
321   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           2/r32/EDX   8/disp8         .                 # copy *(EBP+8) to EDX
322   8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
323     # syscall
324   b8/copy-to-EAX  4/imm32/write
325   cd/syscall  0x80/imm8
326   # restore registers
327   5b/pop-to-EBX
328   5a/pop-to-EDX
329   59/pop-to-ECX
330   58/pop-to-EAX
331   # end
332   89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
333   5d/pop-to-EBP
334   c3/return
335 
336 == data
337 
338 Newline:
339   # size
340   01 00 00 00
341   # data
342   0a/newline
343 
344 # for kernel-string-equal tests
345 Null-kernel-string:
346   00/null
347 Abc-kernel-string:
348   41/A 62/b 63/c 00/null
349 
350 # vim:nowrap:textwidth=0