# 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, 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
# . _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