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Diffstat (limited to 'linux/110stop.subx.')
| -rw-r--r-- | linux/110stop.subx. | 231 |
1 files changed, 0 insertions, 231 deletions
diff --git a/linux/110stop.subx. b/linux/110stop.subx. deleted file mode 100644 index 965d7ae5..00000000 --- a/linux/110stop.subx. +++ /dev/null @@ -1,231 +0,0 @@ -# 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. The value will initially be empty. What should the -# target be? The simplest way to compute the target requires violating the -# usual pattern of function calls. -# -# Normally function calls follow this pattern: -# * push all args on the stack -# * call -# * increment ESP to pop all args off the stack -# -# When passing in a new exit descriptor, the process of calling looks like this: -# * allocate space for the exit descriptor on the stack -# * assign the exit descriptor to some register 'r' -# ... some time later ... -# * push all args on the stack, including register 'r' -# * save ESP to *r (the target of the exit descriptor) -# -# The value saved to the target isn't quite right, so stop needs to adjust its -# exit-descriptor argument 'ed': -# copy *ed to ESP -# decrement ESP by 4 (from last arg to return address) -# ret -# -# Performing the decrement inside 'stop' reduces our overhead for creating exit -# descriptors to a single 2-byte instruction. -# We only adjust the target if we ever actually call 'stop'. -# -# There's no good way to abstract away the target computation, since it depends -# on the number of args a function is called with. - -== 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, but so it goes. -tailor-exit-descriptor: # ed : (address exit-descriptor), nbytes : int -> <void> - # . prolog - 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 - # . epilog - 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 prolog; 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 prolog, 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) - 81 5/subop/subtract 3/mod/direct 4/rm32/ESP . . . . . 8/imm32 # subtract from ESP - 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 - # call/ed(ed) _test-stop-1(ed) - # w/exit(ed) _test-stop-1(ed) - # . _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 - # . epilog - # 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) - # . prolog - 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 - # . epilog - 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) - # . prolog - 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: - # . epilog - 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 |