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Diffstat (limited to 'linux/110stop.subx.')
| -rw-r--r-- | linux/110stop.subx. | 231 |
1 files changed, 231 insertions, 0 deletions
diff --git a/linux/110stop.subx. b/linux/110stop.subx. new file mode 100644 index 00000000..965d7ae5 --- /dev/null +++ b/linux/110stop.subx. @@ -0,0 +1,231 @@ +# 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 |