path: root/059stop.subx

# 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

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