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
|
# stop: dependency-injected wrapper around the exit() syscall
#
# We'd like to be able to write tests for functions that call exit(), and to
# make assertions about whether they exit() or not in a given situation. To
# achieve this we'll call exit() via a smarter wrapper called 'stop'.
#
# In the context of a test, calling a function X that calls 'stop' (directly
# or through further intervening calls) will unwind the stack until X returns,
# so that we can say check any further assertions after the execution of X. To
# achieve this end, we'll pass the return address of X as a 'target' argument
# into X, plumbing it through to 'stop'. When 'stop' gets a non-null target it
# unwinds the stack until the target. If it gets a null target it calls
# exit().
#
# We'd also like to get the exit status out of 'stop', so we'll combine the
# input target with an output status parameter into a type called 'exit-descriptor'.
#
# So the exit-descriptor looks like this:
# target : address # return address for 'stop' to unwind to
# value : int # exit status stop was called with
#
# 'stop' thus takes two parameters: an exit-descriptor and the exit status.
#
# 'stop' won't bother cleaning up any other processor state besides the stack,
# such as registers. Only esp will have a well-defined value after 'stop'
# returns. (This is a poor man's setjmp/longjmp, if you know what that is.)
#
# Before you can call any function that may call 'stop', you need to pass in an
# exit-descriptor to it. To create an exit-descriptor use 'tailor-exit-descriptor'
# below. It's not the most pleasant abstraction in the world.
#
# An exit-descriptor's target is its input, computed during 'tailor-exit-descriptor'.
# Its value is its output, computed during stop and available to the test.
== 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
# Configure an exit-descriptor for a call pushing 'nbytes' bytes of args to
# the stack.
# Ugly that we need to know the size of args. Don't allocate variables between
# tailor-exit-descriptor and the call it's for.
tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int
# . prologue
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# . save registers
50/push-eax
51/push-ecx
# eax = nbytes
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 0/r32/eax 0xc/disp8 . # copy *(ebp+12) to eax
# Let X be the value of esp in the caller, before the call to tailor-exit-descriptor.
# The return address for a call in the caller's body will be at:
# X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address)
# X-12 if the caller takes 8 bytes of args
# ..and so on
# That's the value we need to return: X-nbytes-4
#
# However, we also need to account for the perturbance to esp caused by the
# call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4
# bytes for the return address and 4 bytes to push ebp above.
# So ebp at this point is X-16.
#
# So the return address for the next call in the caller is:
# ebp+8 if the caller takes 4 bytes of args
# ebp+4 if the caller takes 8 bytes of args
# ebp if the caller takes 12 bytes of args
# ebp-4 if the caller takes 16 bytes of args
# ..and so on
# That's ebp+12-nbytes.
# option 1: 6 + 3 bytes
#? 2d/subtract 3/mod/direct 0/rm32/eax . . . . . 8/imm32 # subtract from eax
#? 8d/copy-address 0/mod/indirect 4/rm32/sib 5/base/ebp 0/index/eax . 0/r32/eax . . # copy ebp+eax to eax
# option 2: 2 + 4 bytes
f7 3/subop/negate 3/mod/direct 0/rm32/eax . . . . . . # negate eax
8d/copy-address 1/mod/*+disp8 4/rm32/sib 5/base/ebp 0/index/eax . 0/r32/eax 0xc/disp8 . # copy ebp+eax+12 to eax
# copy eax to ed->target
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 8/disp8 . # copy *(ebp+8) to ecx
89/copy 0/mod/indirect 1/rm32/ecx . . . 0/r32/eax . . # copy eax to *ecx
# initialize ed->value
c7 0/subop/copy 1/mod/*+disp8 1/rm32/ecx . . . . 4/disp8 0/imm32 # copy to *(ecx+4)
$tailor-exit-descriptor:end:
# . restore registers
59/pop-to-ecx
58/pop-to-eax
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
stop: # ed : (address exit-descriptor), value : int
# no prologue; one way or another, we're going to clobber registers
# eax = ed
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none . 0/r32/eax 4/disp8 . # copy *(esp+4) to eax
# if (ed->target == 0) really exit
81 7/subop/compare 0/mod/indirect 0/rm32/eax . . . . . 0/imm32 # compare *eax
75/jump-if-not-equal $stop:fake/disp8
# . syscall(exit, value)
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none . 3/r32/ebx 8/disp8 . # copy *(esp+8) to ebx
b8/copy-to-eax 1/imm32/exit
cd/syscall 0x80/imm8
$stop:fake:
# otherwise:
# ed->value = value+1
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none . 1/r32/ecx 8/disp8 . # copy *(esp+8) to ecx
41/increment-ecx
89/copy 1/mod/*+disp8 0/rm32/eax . . . 1/r32/ecx 4/disp8 . # copy ecx to *(eax+4)
# perform a non-local jump to ed->target
8b/copy 0/mod/indirect 0/rm32/eax . . . 4/r32/esp . . # copy *eax to esp
$stop:end:
c3/return # doesn't return to caller
test-stop-skips-returns-on-exit:
# This looks like the standard prologue, but is here for different reasons.
# A function calling 'stop' can't rely on ebp persisting past the call.
#
# Use ebp here as a stable base to refer to locals and arguments from in the
# presence of push/pop/call instructions.
# *Don't* use ebp as a way to restore esp.
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# Make room for an exit descriptor on the stack. That's almost always the
# right place for it, available only as long as it's legal to use. Once this
# containing function returns we'll need a new exit descriptor.
# var ed/eax : (address exit-descriptor)
68/push 0/imm32
68/push 0/imm32
89/copy 3/mod/direct 0/rm32/eax . . . 4/r32/esp . . # copy esp to eax
# Size the exit-descriptor precisely for the next call below, to _test-stop-1.
# tailor-exit-descriptor(ed, 4)
# . . push args
68/push 4/imm32/nbytes-of-args-for-_test-stop-1
50/push-eax
# . . call
e8/call tailor-exit-descriptor/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# . _test-stop-1(ed)
# . . push args
50/push-eax
# . . call
e8/call _test-stop-1/disp32
# registers except esp may be clobbered at this point
# restore args
58/pop-to-eax
# check that _test-stop-1 tried to call exit(1)
# . check-ints-equal(ed->value, 2, msg) # i.e. stop was called with value 1
# . . push args
68/push "F - test-stop-skips-returns-on-exit"/imm32
68/push 2/imm32
# . . push ed->value
ff 6/subop/push 1/mod/*+disp8 0/rm32/eax . . . . 4/disp8 . # push *(eax+4)
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
# . epilogue
# don't restore esp from ebp; manually reclaim locals
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
5d/pop-to-ebp
c3/return
_test-stop-1: # ed : (address exit-descriptor)
# . prologue
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# _test-stop-2(ed)
# . . push args
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
# . . call
e8/call _test-stop-2/disp32
# should never get past this point
$_test-stop-1:dead-end:
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 4/imm32 # add to esp
# signal test failed: check-ints-equal(1, 0, msg)
# . . push args
68/push "F - test-stop-skips-returns-on-exit"/imm32
68/push 0/imm32
68/push 1/imm32
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
_test-stop-2: # ed : (address exit-descriptor)
# . prologue
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# . stop(ed, 1)
# . . push args
68/push 1/imm32
ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
# . . call
e8/call stop/disp32
# should never get past this point
$_test-stop-2:dead-end:
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# . . vim:nowrap:textwidth=0
|