# 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