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
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
|
# read: analogously to write, support reading from in-memory streams in
# addition to file descriptors.
#
# We can pass it either a file descriptor or an address to a stream. If a
# file descriptor is passed in, we _read from it using the right syscall. If a
# stream is passed in (a fake file descriptor), we read from it instead. This
# lets us initialize input for tests.
#
# A little counter-intuitively, the output of 'read' ends up in.. a stream. So
# tests end up doing a redundant copy. Why? Well, consider the alternatives:
#
# a) Reading into a string, and returning a pointer to the end of the read
# region, or a count of bytes written. Now this count or end pointer must be
# managed separately by the caller, which can be error-prone.
#
# b) Having 'read' return a buffer that it allocates. But there's no way to
# know in advance how large to make the buffer. If you read less than the
# size of the buffer you again end up needing to manage initialized vs
# uninitialized memory.
#
# c) Creating more helpful variants like 'read-byte' or 'read-until' which
# also can take a file descriptor or stream, just like 'write'. But such
# primitives don't exist in the Linux kernel, so we'd be implementing them
# somehow, either with more internal buffering or by making multiple
# syscalls.
#
# Reading into a stream avoids these problems. The buffer is externally
# provided and the caller has control over where it's allocated, its lifetime,
# and so on. The buffer's read and write pointers are internal to it so it's
# easier to keep in a consistent state. And it can now be passed directly to
# helpers like 'read-byte' or 'read-until' that only need to support streams,
# never file descriptors.
#
# Like with 'write', we assume our data segment will never begin at an address
# shorter than 0x08000000, so any smaller arguments are assumed to be real
# file descriptors.
#
# As a reminder, a stream looks like this:
# write: int # index at which to write to next
# read: int # index at which to read next
# data: (array byte) # prefixed by length as usual
== 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
read: # f : fd or (address stream), s : (address stream) -> num-bytes-read/eax
# . prolog
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# if (f < 0x08000000) return _read(f, s) # f can't be a user-mode address, so treat it as a kernel file descriptor
81 7/subop/compare 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 0x08000000/imm32 # compare *(ebp+8)
73/jump-if-greater-unsigned-or-equal $read:fake/disp8
# . . push args
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
# . . call
e8/call _read/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# return
eb/jump $read:end/disp8
$read:fake:
# otherwise, treat 'f' as a stream to scan from
# . save registers
56/push-esi
57/push-edi
# esi = f
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 8/disp8 . # copy *(ebp+8) to esi
# edi = s
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 0xc/disp8 . # copy *(ebp+12) to esi
# eax = _buffer-4(out = &s->data[s->write], outend = &s->data[s->length],
# in = &f->data[f->read], inend = &f->data[f->write])
# . . push &f->data[f->write]
8b/copy 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # copy *esi to eax
8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy esi+eax+12 to eax
50/push-eax
# . . push &f->data[f->read]
8b/copy 1/mod/*+disp8 6/rm32/esi . . . 0/r32/eax 4/disp8 . # copy *(esi+4) to eax
8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy esi+eax+12 to eax
50/push-eax
# . . push &s->data[s->length]
8b/copy 1/mod/*+disp8 7/rm32/edi . . . 0/r32/eax 8/disp8 . # copy *(edi+8) to eax
8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/edi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy edi+eax+12 to eax
50/push-eax
# . . push &s->data[s->write]
8b/copy 0/mod/indirect 7/rm32/edi . . . 0/r32/eax . . # copy *edi to eax
8d/copy-address 1/mod/*+disp8 4/rm32/sib 7/base/edi 0/index/eax . 0/r32/eax 0xc/disp8 . # copy edi+eax+12 to eax
50/push-eax
# . . call
e8/call _buffer-4/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0x10/imm32 # add to esp
# s->write += eax
01/add 0/mod/indirect 7/rm32/edi . . . 0/r32/eax . . # add eax to *edi
# f->read += eax
01/add 1/mod/*+disp8 6/rm32/esi . . . 0/r32/eax 4/disp8 . # add eax to *(esi+4)
# . restore registers
5f/pop-to-edi
5e/pop-to-esi
$read:end:
# . epilog
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# - helpers
# '_buffer' is like '_append', but silently stops instead of aborting when it runs out of space
# 3-argument variant of _buffer
_buffer-3: # out : address, outend : address, s : (array byte) -> num_bytes_buffered/eax
# . prolog
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# . save registers
51/push-ecx
# eax = _buffer-4(out, outend, &s->data[0], &s->data[s->length])
# . . push &s->data[s->length]
8b/copy 1/mod/*+disp8 5/rm32/ebp . . 0/r32/eax 0x10/disp8 . # copy *(ebp+16) to eax
8b/copy 0/mod/indirect 0/rm32/eax . . . 1/r32/ecx . . # copy *eax to ecx
8d/copy-address 1/mod/*+disp8 4/rm32/sib 0/base/eax 1/index/ecx . 1/r32/ecx 4/disp8 . # copy eax+ecx+4 to ecx
51/push-ecx
# . . push &s->data[0]
8d/copy-address 1/mod/*+disp8 0/rm32/eax . . . 1/r32/ecx 4/disp8 . # copy eax+4 to ecx
51/push-ecx
# . . push outend
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
# . . push out
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
# . . call
e8/call _buffer-4/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0x10/imm32 # add to esp
$_buffer-3:end:
# . restore registers
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
# 4-argument variant of _buffer
_buffer-4: # out : address, outend : address, in : address, inend : address -> num_bytes_buffered/eax
# . 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
# eax/num_bytes_buffered = 0
b8/copy-to-eax 0/imm32
# edi = out
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 8/disp8 . # copy *(ebp+8) to edi
# edx = outend
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx
# esi = in
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0x10/disp8 . # copy *(ebp+16) to esi
# ecx = inend
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0x14/disp8 . # copy *(ebp+20) to ecx
$_buffer-4:loop:
# if (in >= inend) break
39/compare 3/mod/direct 6/rm32/esi . . . 1/r32/ecx . . # compare esi with ecx
73/jump-if-greater-or-equal-unsigned $_buffer-4:end/disp8
# if (out >= outend) break # for now silently ignore filled up buffer
39/compare 3/mod/direct 7/rm32/edi . . . 2/r32/edx . . # compare edi with edx
73/jump-if-greater-or-equal-unsigned $_buffer-4:end/disp8
# *out = *in
8a/copy-byte 0/mod/indirect 6/rm32/esi . . . 3/r32/BL . . # copy byte at *esi to BL
88/copy-byte 0/mod/indirect 7/rm32/edi . . . 3/r32/BL . . # copy byte at BL to *edi
# ++num_bytes_buffered
40/increment-eax
# ++in
46/increment-esi
# ++out
47/increment-edi
eb/jump $_buffer-4:loop/disp8
$_buffer-4: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
# idea: a clear-if-empty method on streams that clears only if f->read == f->write
# Unclear how I'd use it, though. Callers seem to need the check anyway.
# Maybe a better helper would be 'empty-stream?'
_read: # fd : int, s : (address stream) -> num-bytes-read/eax
# . 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
# esi = s
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0xc/disp8 . # copy *(ebp+12) to esi
# eax = s->write
8b/copy 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # copy *esi to eax
# edx = s->length
8b/copy 1/mod/*+disp8 6/rm32/esi . . . 2/r32/edx 8/disp8 . # copy *(esi+8) to edx
# syscall(read, fd, &s->data[s->write], s->length - s->write)
# . . fd : ebx
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 3/r32/ebx 8/disp8 . # copy *(ebp+8) to ebx
# . . data : ecx = &s->data[s->write]
8d/copy-address 1/mod/*+disp8 4/rm32/sib 6/base/esi 0/index/eax . 1/r32/ecx 0xc/disp8 . # copy esi+eax+12 to ecx
# . . size : edx = s->length - s->write
29/subtract 3/mod/direct 2/rm32/edx . . . 0/r32/eax . . # subtract eax from edx
# . . syscall
b8/copy-to-eax 3/imm32/read
cd/syscall 0x80/imm8
# add the result eax to s->write
01/add 0/mod/indirect 6/rm32/esi . . . 0/r32/eax . . # add eax to *esi
$_read:end:
# . restore registers
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
# Two options:
# 1 (what we have above):
# ecx = s
# eax = s->write
# edx = s->length
# # syscall
# ecx = lea ecx+eax+12
# edx = sub edx eax
#
# 2:
# ecx = s
# edx = s->length
# ecx = &s->data
# # syscall
# ecx = add ecx, s->write
# edx = sub edx, s->write
#
# Not much to choose between the two? Option 2 performs a duplicate load to
# use one less register, but doesn't increase the amount of spilling (ecx
# and edx must be used, and eax must be clobbered anyway).
# - tests
test-read-single:
# - write a single character into _test-stream, then read from it
# clear-stream(_test-stream)
# . . push args
68/push _test-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# clear-stream(_test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# write(_test-stream, "Ab")
# . . push args
68/push "Ab"/imm32
68/push _test-stream/imm32
# . . call
e8/call write/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# eax = read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
68/push _test-stream/imm32
# . . call
e8/call read/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 2, msg)
# . . push args
68/push "F - test-read-single: return value"/imm32
68/push 2/imm32
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
# check-stream-equal(_test-tmp-stream, "Ab", msg)
# . . push args
68/push "F - test-read-single"/imm32
68/push "Ab"/imm32
68/push _test-tmp-stream/imm32
# . . call
e8/call check-stream-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
# end
c3/return
test-read-is-stateful:
# - make two consecutive reads, check that their results are appended
# clear-stream(_test-stream)
# . . push args
68/push _test-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# clear-stream(_test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# write(_test-stream, "C")
# . . push args
68/push "C"/imm32
68/push _test-stream/imm32
# . . call
e8/call write/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
68/push _test-stream/imm32
# . . call
e8/call read/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# write(_test-stream, "D")
# . . push args
68/push "D"/imm32
68/push _test-stream/imm32
# . . call
e8/call write/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
68/push _test-stream/imm32
# . . call
e8/call read/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-stream-equal(_test-tmp-stream, "CD", msg)
# . . push args
68/push "F - test-read-is-stateful"/imm32
68/push "CD"/imm32
68/push _test-tmp-stream/imm32
# . . call
e8/call check-stream-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
# end
c3/return
test-read-returns-0-on-end-of-file:
# - read after hitting end-of-file, check that result is 0
# setup
# . clear-stream(_test-stream)
# . . push args
68/push _test-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# . clear-stream(_test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
# . . call
e8/call clear-stream/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# . write(_test-stream, "Ab")
# . . push args
68/push "Ab"/imm32
68/push _test-stream/imm32
# . . call
e8/call write/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# first read gets to end-of-file
# . read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
68/push _test-stream/imm32
# . . call
e8/call read/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# second read
# . read(_test-stream, _test-tmp-stream)
# . . push args
68/push _test-tmp-stream/imm32
68/push _test-stream/imm32
# . . call
e8/call read/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-read-returns-0-on-end-of-file"/imm32
68/push 0/imm32
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
# end
c3/return
== data
_test-tmp-stream:
# current write index
0/imm32
# current read index
0/imm32
# length
8/imm32
# data
00 00 00 00 00 00 00 00 # 8 bytes
# . . vim:nowrap:textwidth=0
|