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 # input return address for 'stop' to unwind to 20 # value : int # output exit status stop was called with 21 # 22 # 'stop' thus takes two parameters: an exit-descriptor and the exit status. 23 # 24 # We 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 == code 29 30 # instruction effective address operand displacement immediate 31 # op subop mod rm32 base index scale r32 32 # 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 33 34 # main: (manual test if this is the last file loaded) 35 #? e8/call test-stop-skips-returns-on-exit/disp32 36 e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'. 37 # syscall(exit, Num-test-failures) 38 8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/EBX Num-test-failures/disp32 # copy *Num-test-failures to EBX 39 b8/copy-to-EAX 1/imm32 40 cd/syscall 0x80/imm8 41 42 # Configure an exit-descriptor for a call pushing 'nbytes' bytes of args to 43 # the stack. 44 # Ugly that we need to know the size of args, but so it goes. 45 tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int -> () 46 # prolog 47 55/push-EBP 48 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 49 # save registers 50 50/push-EAX 51 51/push-ECX 52 # EAX = nbytes 53 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 0/r32/EAX 0xc/disp8 . # copy *(EBP+12) to EAX 54 # Let X be the value of ESP in the caller, before the call to tailor-exit-descriptor. 55 # The return address for a call in the caller's body will be at: 56 # X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address) 57 # X-12 if the caller takes 8 bytes of args 58 # ..and so on 59 # That's the value we need to return: X-nbytes-4 60 # 61 # However, we also need to account for the perturbance to ESP caused by the 62 # call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4 63 # bytes for the return address and 4 bytes to push EBP above. 64 # So EBP at this point is X-16. 65 # 66 # So the return address for the next call in the caller is: 67 # EBP+8 if the caller takes 4 bytes of args 68 # EBP+4 if the caller takes 8 bytes of args 69 # EBP if the caller takes 12 bytes of args 70 # EBP-4 if the caller takes 16 bytes of args 71 # ..and so on 72 # That's EBP+12-nbytes. 73 # option 1: 6 + 3 bytes 74 #? 2d/subtract 3/mod/direct 0/rm32/EAX . . . . . 8/imm32 # subtract from EAX 75 #? 8d/copy-address 0/mod/indirect 4/rm32/sib 5/base/EBP 0/index/EAX . 0/r32/EAX . . # copy EBP+EAX to EAX 76 # option 2: 2 + 4 bytes 77 f7 3/subop/negate 3/mod/direct 0/rm32/EAX . . . . . . # negate EAX 78 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 79 # copy EAX to ed->target 80 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(EBP+8) to ECX 81 89/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy EAX to *ECX 82 # initialize ed->value 83 c7/copy 1/mod/*+disp8 1/rm32/ECX . . . . 4/disp8 0/imm32 # copy to *(ECX+4) 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 # exit(value) if ed->target == 0 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 102 cd/syscall 0x80/imm8 103 $stop:fake: 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/inc-ECX 107 89/copy 1/mod/*+disp8 0/rm32/EAX . . . 1/r32/ECX 4/disp8 . # copy ECX to *(EAX+4) 108 # non-local jump to ed->target 109 8b/copy 0/mod/indirect 0/rm32/EAX . . . 4/r32/ESP . . # copy *EAX to ESP 110 c3/return # doesn't return to caller 111 112 test-stop-skips-returns-on-exit: 113 # This looks like the standard prolog, but is here for different reasons. 114 # A function calling 'stop' can't rely on EBP persisting past the call. 115 # 116 # Use EBP here as a stable base to refer to locals and arguments from in the 117 # presence of push/pop/call instructions. 118 # *Don't* use EBP as a way to restore ESP. 119 55/push-EBP 120 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 121 # Make room for an exit descriptor on the stack. That's almost always the 122 # right place for it, available only as long as it's legal to use. Once this 123 # containing function returns we'll need a new exit descriptor. 124 # var ed/EAX : (address exit-descriptor) 125 81 5/subop/subtract 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # subtract from ESP 126 8d/copy-address 0/mod/indirect 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX . . # copy ESP to EAX 127 # Size the exit-descriptor precisely for the next call below, to _test-stop-1. 128 # tailor-exit-descriptor(ed, 4) 129 # push args 130 68/push 4/imm32/nbytes-of-args-for-_test-stop-1 131 50/push-EAX 132 # call 133 e8/call tailor-exit-descriptor/disp32 134 # discard args 135 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # add to ESP 136 # call _test-stop-1(ed) 137 # push args 138 50/push-EAX 139 # call 140 e8/call _test-stop-1/disp32 141 ## registers except ESP may be clobbered at this point 142 # restore args 143 58/pop-to-EAX 144 # check that _test-stop-1 tried to call exit(1) 145 # check-ints-equal(ed->value, 2, msg) # i.e. stop was called with value 1 146 # push args 147 68/push "F - test-stop-skips-returns-on-exit"/imm32 148 68/push 2/imm32 149 # push ed->value 150 ff 6/subop/push 1/mod/*+disp8 0/rm32/EAX . . . . 4/disp8 . # push *(EAX+4) 151 # call 152 e8/call check-ints-equal/disp32 153 # discard args 154 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 155 # epilog 156 5d/pop-to-EBP 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 c3/return 160 161 _test-stop-1: # ed : (address exit-descriptor) 162 # prolog 163 55/push-EBP 164 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 165 # _test-stop-2(ed) 166 # push args 167 ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 . # push *(EBP+8) 168 # call 169 e8/call _test-stop-2/disp32 170 ## should never get past this point 171 # discard args 172 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP 173 # signal test failed: check-ints-equal(1, 0, msg) 174 # push args 175 68/push "F - test-stop-skips-returns-on-exit"/imm32 176 68/push 0/imm32 177 68/push 1/imm32 178 # call 179 e8/call check-ints-equal/disp32 180 # discard args 181 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 0xc/imm32 # add to ESP 182 # epilog 183 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 184 5d/pop-to-EBP 185 c3/return 186 187 _test-stop-2: # ed : (address exit-descriptor) 188 # prolog 189 55/push-EBP 190 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 191 # call stop(ed, 1) 192 # push args 193 68/push 1/imm32 194 ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 8/disp8 . # push *(EBP+8) 195 # call 196 e8/call stop/disp32 197 ## should never get past this point 198 # epilog 199 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP 200 5d/pop-to-EBP 201 c3/return