1 # stop: dependency-injected wrapper around the exit() syscall 2 # 3 # We'd like to be able to write tests for functions that call exit(), and to 4 # make assertions about whether they exit() or not in a given situation. To 5 # achieve this we'll call exit() via a smarter wrapper called 'stop'. 6 # 7 # In the context of a test, calling a function X that calls 'stop' (directly 8 # or through further intervening calls) will unwind the stack until X returns, 9 # so that we can say check any further assertions after the execution of X. To 10 # achieve this end, we'll pass the return address of X as a 'target' argument 11 # into X, plumbing it through to 'stop'. When 'stop' gets a non-null target it 12 # unwinds the stack until the target. If it gets a null target it calls 13 # exit(). 14 # 15 # We'd also like to get the exit status out of 'stop', so we'll combine the 16 # input target with an output status parameter into a type called 'exit-descriptor'. 17 # 18 # So the exit-descriptor looks like this: 19 # target : address # return address for 'stop' to unwind to 20 # value : int # exit status stop was called with 21 # 22 # 'stop' thus takes two parameters: an exit-descriptor and the exit status. 23 # 24 # 'stop' won't bother cleaning up any other processor state besides the stack, 25 # such as registers. Only ESP will have a well-defined value after 'stop' 26 # returns. (This is a poor man's setjmp/longjmp, if you know what that is.) 27 # 28 # Before you can call any function that may call 'stop', you need to pass in an 29 # exit-descriptor to it. To create an exit-descriptor use 'tailor-exit-descriptor' 30 # below. It's not the most pleasant abstraction in the world. 31 # 32 # An exit-descriptor's target is its input, computed during 'tailor-exit-descriptor'. 33 # Its value is its output, computed during stop and available to the test. 34 35 == code 36 # instruction effective address register displacement immediate 37 # . op subop mod rm32 base index scale r32 38 # . 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 39 40 # main: 41 e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'. 42 #? e8/call test-stop-skips-returns-on-exit/disp32 43 # syscall(exit, Num-test-failures) 44 8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX 45 b8/copy-to-EAX 1/imm32/exit 46 cd/syscall 0x80/imm8 47 48 # Configure an exit-descriptor for a call pushing 'nbytes' bytes of args to 49 # the stack. 50 # Ugly that we need to know the size of args, but so it goes. 51 tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int -> <void> 52 # . prolog 53 55/push-EBP 54 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 55 # . save registers 56 50/push-EAX 57 51/push-ECX 58 # EAX = nbytes 59 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 0/r32/EAX 0xc/disp8 . # copy *(EBP+12) to EAX 60 # Let X be the value of ESP in the caller, before the call to tailor-exit-descriptor. 61 # The return address for a call in the caller's body will be at: 62 # X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address) 63 # X-12 if the caller takes 8 bytes of args 64 # ..and so on 65 # That's the value we need to return: X-nbytes-4 66 # 67 # However, we also need to account for the perturbance to ESP caused by the 68 # call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4 69 # bytes for the return address and 4 bytes to push EBP above. 70 # So EBP at this point is X-16. 71 # 72 # So the return address for the next call in the caller is: 73 # EBP+8 if the caller takes 4 bytes of args 74 # EBP+4 if the caller takes 8 bytes of args 75 # EBP if the caller takes 12 bytes of args 76 # EBP-4 if the caller takes 16 bytes of args 77 # ..and so on 78 # That's EBP+12-nbytes. 79 # option 1: 6 + 3 bytes 80 #? 2d/subtract 3/mod/direct 0/rm32/EAX . . . . . 8/imm32 # subtract from EAX 81 #? 8d/copy-address 0/mod/indirect 4/rm32/sib 5/base/EBP 0/index/EAX . 0/r32/EAX . . # copy EBP+EAX to EAX 82 # option 2: 2 + 4 bytes 83 f7 3/subop/negate 3/mod/direct 0/rm32/EAX . . . . . . # negate EAX 84 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 85 # copy EAX to ed->target 86 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX 87 89/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy EAX to *ECX 88 # initialize ed->value 89 c7 0/subop/copy 1/mod/*+disp8 1/rm32/ECX . . . . 4/disp8 0/imm32 # copy to *(ECX+4) 90 $tailor-exit-descriptor:end: 91 # . restore registers 92 59/pop-to-ECX 93 58/pop-to-EAX 94 # . epilog 95 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 96 5d/pop-to-EBP 97 c3/return 98 99 stop: # ed : (address exit-descriptor), value : int 100 # no prolog; one way or another, we're going to clobber registers 101 # EAX = ed 102 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX 4/disp8 . # copy *(ESP+4) to EAX 103 # exit(value) if ed->target == 0 104 81 7/subop/compare 0/mod/indirect 0/rm32/EAX . . . . . 0/imm32 # compare *EAX 105 75/jump-if-not-equal $stop:fake/disp8 106 # syscall(exit, value) 107 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 3/r32/EBX 8/disp8 . # copy *(ESP+8) to EBX 108 b8/copy-to-EAX 1/imm32/exit 109 cd/syscall 0x80/imm8 110 $stop:fake: 111 # ed->value = value+1 112 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX 113 41/inc-ECX 114 89/copy 1/mod/*+disp8 0/rm32/EAX . . . 1/r32/ECX 4/disp8 . # copy ECX to *(EAX+4) 115 # non-local jump to ed->target 116 8b/copy 0/mod/indirect 0/rm32/EAX . . . 4/r32/ESP . . # copy *EAX to ESP 117 $stop:end: 118 c3/return # doesn't return to caller 119 120 test-stop-skips-returns-on-exit: 121 # This looks like the standard prolog, but is here for different reasons. 122 # A function calling 'stop' can't rely on EBP persisting past the call. 123 # 124 # Use EBP here as a stable base to refer to locals and arguments from in the 125 # presence of push/pop/call instructions. 126 # *Don't* use EBP as a way to restore ESP. 127 55/push-EBP 128 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 129 # Make room for an exit descriptor on the stack. That's almost always the 130 # right place for it, available only as long as it's legal to use. Once this 131 # containing function returns we'll need a new exit descriptor. 132 # var ed/EAX : (address exit-descriptor) 133 81 5/subop/subtract 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # subtract from ESP 134 89/copy 3/mod/direct 0/rm32/EAX . . . 4/r32/ESP . . # copy ESP to EAX 135 # Size the exit-descriptor precisely for the next call below, to _test-stop-1. 136 # tailor-exit-descriptor(ed, 4) 137 # . . push args 138 68/push 4/imm32/nbytes-of-args-for-_test-stop-1 139 50/push-EAX 140 # . . call 141 e8/call tailor-exit-descriptor/disp32 142 # . . discard args 143 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 144 # . _test-stop-1(ed) 145 # . . push args 146 50/push-EAX 147 # . . call 148 e8/call _test-stop-1/disp32 149 # registers except ESP may be clobbered at this point 150 # restore args 151 58/pop-to-EAX 152 # check that _test-stop-1 tried to call exit(1) 153 # check-ints-equal(ed->value, 2, msg) # i.e. stop was called with value 1 154 # . . push args 155 68/push "F - test-stop-skips-returns-on-exit"/imm32 156 68/push 2/imm32 157 # . . push ed->value 158 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 4/disp8 . # push *(EAX+4) 159 # . . call 160 e8/call check-ints-equal/disp32 161 # . . discard args 162 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 163 # . epilog 164 # don't restore ESP from EBP; manually reclaim locals 165 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 166 5d/pop-to-EBP 167 c3/return 168 169 _test-stop-1: # ed : (address exit-descriptor) 170 # . prolog 171 55/push-EBP 172 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 173 # _test-stop-2(ed) 174 # . . push args 175 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 176 # . . call 177 e8/call _test-stop-2/disp32 178 # should never get past this point 179 $_test-stop-1:dead-end: 180 # . . discard args 181 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP 182 # signal test failed: check-ints-equal(1, 0, msg) 183 # . . push args 184 68/push "F - test-stop-skips-returns-on-exit"/imm32 185 68/push 0/imm32 186 68/push 1/imm32 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 # . epilog 192 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 193 5d/pop-to-EBP 194 c3/return 195 196 _test-stop-2: # ed : (address exit-descriptor) 197 # . prolog 198 55/push-EBP 199 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 200 # . stop(ed, 1) 201 # . . push args 202 68/push 1/imm32 203 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 204 # . . call 205 e8/call stop/disp32 206 # should never get past this point 207 $_test-stop-2:dead-end: 208 # . epilog 209 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 210 5d/pop-to-EBP 211 c3/return 212 213 # . . vim:nowrap:textwidth=0