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:  (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(EAX)
 26   89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
 27   b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy to EAX
 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   # initialize s into EDI
 57   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
 58   # initialize benchmark length n into EDX
 59   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
 60   8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
 61   # initialize benchmark data into ESI
 62   8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
 63   81          0/subop/add         3/mod/direct    6/rm32/ESI    .           .             .           .           .               4/imm32           # add to ESI
 64   # initialize loop counter i into ECX
 65   b9/copy                         .               .             .           .             .           .           .               0/imm32/exit      # copy to ECX
 66   # while (i/ECX < n/EDX)
 67 $kernel_string_loop:
 68   39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
 69   74/jump-if-equal  $kernel_string_break/disp8
 70     # c1/EAX, c2/EBX = *s, *benchmark
 71   b8/copy  0/imm32  # clear EAX
 72   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 73   bb/copy  0/imm32  # clear EBX
 74   8a/copy                         0/mod/indirect  6/rm32/ESI    .           .             .           3/r32/EBX   .               .                 # copy byte at *ESI to lower byte of EBX
 75     # if (c1 == 0) return false
 76   3d/compare-EAX  0/imm32
 77   74/jump-if-equal  $kernel_string_fail/disp8
 78     # if (c1 != c2) return false
 79   39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX with EBX
 80   75/jump-if-not-equal  $kernel_string_fail/disp8
 81     # ++s1, ++s2, ++i
 82   41/inc-ECX
 83   46/inc-ESI
 84   47/inc-EDI
 85   # end while
 86   eb/jump  $kernel_string_loop/disp8
 87 $kernel_string_break:
 88   # if (*s/EDI == 0) return true
 89   b8/copy  0/imm32  # clear EAX
 90   8a/copy                         0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/EAX   .               .                 # copy byte at *EDI to lower byte of EAX
 91   3d/compare-EAX  0/imm32
 92   75/jump-if-not-equal  $kernel_string_fail/disp8
 93   b8/copy                         .               .             .           .             .           .           .               1/imm32           # copy to EAX
 94   eb/jump  $kernel_string_end/disp8
 95   # return false
 96 $kernel_string_fail:
 97   b8/copy                         .               .             .           .             .           .           .               0/imm32           # copy to EAX
 98 
 99 $kernel_string_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 == data
254 
255 Null_kernel_string:
256   00/null
257 Abc_kernel_string:
258   41/A 62/b 63/c 00/null
259 
260 # vim:ft=subx:nowrap:tw&