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. Don't allocate variables between 43 # tailor-exit-descriptor and the call it's for. 44 tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int -> <void> 45 # . prolog 46 55/push-EBP 47 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 48 # . save registers 49 50/push-EAX 50 51/push-ECX 51 # EAX = nbytes 52 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 0/r32/EAX 0xc/disp8 . # copy *(EBP+12) to EAX 53 # Let X be the value of ESP in the caller, before the call to tailor-exit-descriptor. 54 # The return address for a call in the caller's body will be at: 55 # X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address) 56 # X-12 if the caller takes 8 bytes of args 57 # ..and so on 58 # That's the value we need to return: X-nbytes-4 59 # 60 # However, we also need to account for the perturbance to ESP caused by the 61 # call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4 62 # bytes for the return address and 4 bytes to push EBP above. 63 # So EBP at this point is X-16. 64 # 65 # So the return address for the next call in the caller is: 66 # EBP+8 if the caller takes 4 bytes of args 67 # EBP+4 if the caller takes 8 bytes of args 68 # EBP if the caller takes 12 bytes of args 69 # EBP-4 if the caller takes 16 bytes of args 70 # ..and so on 71 # That's EBP+12-nbytes. 72 # option 1: 6 + 3 bytes 73 #? 2d/subtract 3/mod/direct 0/rm32/EAX . . . . . 8/imm32 # subtract from EAX 74 #? 8d/copy-address 0/mod/indirect 4/rm32/sib 5/base/EBP 0/index/EAX . 0/r32/EAX . . # copy EBP+EAX to EAX 75 # option 2: 2 + 4 bytes 76 f7 3/subop/negate 3/mod/direct 0/rm32/EAX . . . . . . # negate EAX 77 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 78 # copy EAX to ed->target 79 8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX 80 89/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy EAX to *ECX 81 # initialize ed->value 82 c7 0/subop/copy 1/mod/*+disp8 1/rm32/ECX . . . . 4/disp8 0/imm32 # copy to *(ECX+4) 83 $tailor-exit-descriptor:end: 84 # . restore registers 85 59/pop-to-ECX 86 58/pop-to-EAX 87 # . epilog 88 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 89 5d/pop-to-EBP 90 c3/return 91 92 stop: # ed : (address exit-descriptor), value : int 93 # no prolog; one way or another, we're going to clobber registers 94 # EAX = ed 95 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX 4/disp8 . # copy *(ESP+4) to EAX 96 # if (ed->target == 0) really exit 97 81 7/subop/compare 0/mod/indirect 0/rm32/EAX . . . . . 0/imm32 # compare *EAX 98 75/jump-if-not-equal $stop:fake/disp8 99 # . syscall(exit, value) 100 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 3/r32/EBX 8/disp8 . # copy *(ESP+8) to EBX 101 b8/copy-to-EAX 1/imm32/exit 102 cd/syscall 0x80/imm8 103 $stop:fake: 104 # otherwise: 105 # ed->value = value+1 106 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX 107 41/increment-ECX 108 89/copy 1/mod/*+disp8 0/rm32/EAX . . . 1/r32/ECX 4/disp8 . # copy ECX to *(EAX+4) 109 # perform a non-local jump to ed->target 110 8b/copy 0/mod/indirect 0/rm32/EAX . . . 4/r32/ESP . . # copy *EAX to ESP 111 $stop:end: 112 c3/return # doesn't return to caller 113 114 test-stop-skips-returns-on-exit: 115 # This looks like the standard prolog, but is here for different reasons. 116 # A function calling 'stop' can't rely on EBP persisting past the call. 117 # 118 # Use EBP here as a stable base to refer to locals and arguments from in the 119 # presence of push/pop/call instructions. 120 # *Don't* use EBP as a way to restore ESP. 121 55/push-EBP 122 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 123 # Make room for an exit descriptor on the stack. That's almost always the 124 # right place for it, available only as long as it's legal to use. Once this 125 # containing function returns we'll need a new exit descriptor. 126 # var ed/EAX : (address exit-descriptor) 127 68/push 0/imm32 128 68/push 0/imm32 129 89/copy 3/mod/direct 0/rm32/EAX . . . 4/r32/ESP . . # copy ESP to EAX 130 # Size the exit-descriptor precisely for the next call below, to _test-stop-1. 131 # tailor-exit-descriptor(ed, 4) 132 # . . push args 133 68/push 4/imm32/nbytes-of-args-for-_test-stop-1 134 50/push-EAX 135 # . . call 136 e8/call tailor-exit-descriptor/disp32 137 # . . discard args 138 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 139 # . _test-stop-1(ed) 140 # . . push args 141 50/push-EAX 142 # . . call 143 e8/call _test-stop-1/disp32 144 # registers except ESP may be clobbered at this point 145 # restore args 146 58/pop-to-EAX 147 # check that _test-stop-1 tried to call exit(1) 148 # . check-ints-equal(ed->value, 2, msg) # i.e. stop was called with value 1 149 # . . push args 150 68/push "F - test-stop-skips-returns-on-exit"/imm32 151 68/push 2/imm32 152 # . . push ed->value 153 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 4/disp8 . # push *(EAX+4) 154 # . . call 155 e8/call check-ints-equal/disp32 156 # . . discard args 157 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 158 # . epilog 159 # don't restore ESP from EBP; manually reclaim locals 160 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 161 5d/pop-to-EBP 162 c3/return 163 164 _test-stop-1: # ed : (address exit-descriptor) 165 # . prolog 166 55/push-EBP 167 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 168 # _test-stop-2(ed) 169 # . . push args 170 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 171 # . . call 172 e8/call _test-stop-2/disp32 173 # should never get past this point 174 $_test-stop-1:dead-end: 175 # . . discard args 176 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP 177 # signal test failed: check-ints-equal(1, 0, msg) 178 # . . push args 179 68/push "F - test-stop-skips-returns-on-exit"/imm32 180 68/push 0/imm32 181 68/push 1/imm32 182 # . . call 183 e8/call check-ints-equal/disp32 184 # . . discard args 185 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 186 # . epilog 187 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 188 5d/pop-to-EBP 189 c3/return 190 191 _test-stop-2: # ed : (address exit-descriptor) 192 # . prolog 193 55/push-EBP 194 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 195 # . stop(ed, 1) 196 # . . push args 197 68/push 1/imm32 198 ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8) 199 # . . call 200 e8/call stop/disp32 201 # should never get past this point 202 $_test-stop-2:dead-end: 203 # . epilog 204 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 205 5d/pop-to-EBP 206 c3/return 207 208 # . . vim:nowrap:textwidth=0