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 # Configure an exit-descriptor for a call pushing 'nbytes' bytes of args to 41 # the stack. 42 # Ugly that we need to know the size of args, but so it goes. 43 tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int -> <void> 44 # . prolog 45 55/push-EBP 46 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 47 # . save registers 48 50/push-EAX 49 51/push-ECX 50 # EAX = nbytes 51 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 0/r32/EAX 0xc/disp8 . # copy *(EBP+12) to EAX 52 # Let X be the value of ESP in the caller, before the call to tailor-exit-descriptor. 53 # The return address for a call in the caller's body will be at: 54 # X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address) 55 # X-12 if the caller takes 8 bytes of args 56 # ..and so on 57 # That's the value we need to return: X-nbytes-4 58 # 59 # However, we also need to account for the perturbance to ESP caused by the 60 # call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4 61 # bytes for the return address and 4 bytes to push EBP above. 62 # So EBP at this point is X-16. 63 # 64 # So the return address for the next call in the caller is: 65 # EBP+8 if the caller takes 4 bytes of args 66 # EBP+4 if the caller takes 8 bytes of args 67 # EBP if the caller takes 12 bytes of args 68 # EBP-4 if the caller takes 16 bytes of args 69 # ..and so on 70 # That's EBP+12-nbytes. 71 # option 1: 6 + 3 bytes 72 #? 2d/subtract 3/mod/direct 0/rm32/EAX . . . . . 8/imm32 # subtract from EAX 73 #? 8d/copy-address 0/mod/indirect 4/rm32/sib 5/base/EBP 0/index/EAX . 0/r32/EAX . . # copy EBP+EAX to EAX 74 # option 2: 2 + 4 bytes 75 f7 3/subop/negate 3/mod/direct 0/rm32/EAX . . . . . . # negate EAX 76 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 77 # copy EAX to ed->target 78 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX 79 89/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy EAX to *ECX 80 # initialize ed->value 81 c7 0/subop/copy 1/mod/*+disp8 1/rm32/ECX . . . . 4/disp8 0/imm32 # copy to *(ECX+4) 82 $tailor-exit-descriptor:end: 83 # . restore registers 84 59/pop-to-ECX 85 58/pop-to-EAX 86 # . epilog 87 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 88 5d/pop-to-EBP 89 c3/return 90 91 stop: # ed : (address exit-descriptor), value : int 92 # no prolog; one way or another, we're going to clobber registers 93 # EAX = ed 94 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX 4/disp8 . # copy *(ESP+4) to EAX 95 # if (ed->target == 0) really exit 96 81 7/subop/compare 0/mod/indirect 0/rm32/EAX . . . . . 0/imm32 # compare *EAX 97 75/jump-if-not-equal $stop:fake/disp8 98 # . syscall(exit, value) 99 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 3/r32/EBX 8/disp8 . # copy *(ESP+8) to EBX 100 b8/copy-to-EAX 1/imm32/exit 101 cd/syscall 0x80/imm8 102 $stop:fake: 103 # otherwise: 104 # ed->value = value+1 105 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX 106 41/increment-ECX 107 89/copy 1/mod/*+disp8 0/rm32/EAX . . . 1/r32/ECX 4/disp8 . # copy ECX to *(EAX+4) 108 # perform a non-local jump to ed->target 109 8b/copy 0/mod/indirect 0/rm32/EAX . . . 4/r32/ESP . . # copy *EAX to ESP 110 $stop:end: 111 c3/return # doesn't return to caller 112 113 test-stop-skips-returns-on-exit: 114 # This looks like the standard prolog, but is here for different reasons. 115 # A function calling 'stop' can't rely on EBP persisting past the call. 116 # 117 # Use EBP here as a stable base to refer to locals and arguments from in the 118 # presence of push/pop/call instructions. 119 # *Don't* use EBP as a way to restore ESP. 120 55/push-EBP 121 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 122 # Make room for an exit descriptor on the stack. That's almost always the 123 # right place for it, available only as long as it's legal to use. Once this 124 # containing function returns we'll need a new exit descriptor. 125 # var ed/EAX : (address exit-descriptor) 126 81 5/subop/subtract 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # subtract from ESP 127 89/copy 3/mod/direct 0/rm32/EAX . . . 4/r32/ESP . . # copy ESP to EAX 128 # Size the exit-descriptor precisely for the next call below, to _test-stop-1. 129 # tailor-exit-descriptor(ed, 4) 130 # . . push args 131 68/push 4/imm32/nbytes-of-args-for-_test-stop-1 132 50/push-EAX 133 # . . call 134 e8/call tailor-exit-descriptor/disp32 135 # . . discard args 136 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 137 # . _test-stop-1(ed) 138 # . . push args 139 50/push-EAX 140 # . . call 141 e8/call _test-stop-1/disp32 142 # registers except ESP may be clobbered at this point 143 # restore args 144 58/pop-to-EAX 145 # check that _test-stop-1 tried to call exit(1) 146 # check-ints-equal(ed->value, 2, msg) # i.e. stop was called with value 1 147 # . . push args 148 68/push "F - test-stop-skips-returns-on-exit"/imm32 149 68/push 2/imm32 150 # . . push ed->value 151 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 4/disp8 . # push *(EAX+4) 152 # . . call 153 e8/call check-ints-equal/disp32 154 # . . discard args 155 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 156 # . epilog 157 # don't restore ESP from EBP; manually reclaim locals 158 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 159 5d/pop-to-EBP 160 c3/return 161 162 _test-stop-1: # ed : (address exit-descriptor) 163 # . prolog 164 55/push-EBP 165 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 166 # _test-stop-2(ed) 167 # . . push args 168 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 169 # . . call 170 e8/call _test-stop-2/disp32 171 # should never get past this point 172 $_test-stop-1:dead-end: 173 # . . discard args 174 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP 175 # signal test failed: check-ints-equal(1, 0, msg) 176 # . . push args 177 68/push "F - test-stop-skips-returns-on-exit"/imm32 178 68/push 0/imm32 179 68/push 1/imm32 180 # . . call 181 e8/call check-ints-equal/disp32 182 # . . discard args 183 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 184 # . epilog 185 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 186 5d/pop-to-EBP 187 c3/return 188 189 _test-stop-2: # ed : (address exit-descriptor) 190 # . prolog 191 55/push-EBP 192 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 193 # . stop(ed, 1) 194 # . . push args 195 68/push 1/imm32 196 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 197 # . . call 198 e8/call stop/disp32 199 # should never get past this point 200 $_test-stop-2:dead-end: 201 # . epilog 202 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 203 5d/pop-to-EBP 204 c3/return 205 206 # . . vim:nowrap:textwidth=0