Mu - linux/110stop.subx

From 3350c34a74844e21ea69077e01efff3bae64bdcd Mon Sep 17 00:00:00 2001 From: Kartik Agaram Date: Tue, 23 Mar 2021 17:31:08 -0700 Subject: . --- html/linux/110stop.subx.html | 279 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 279 insertions(+) create mode 100644 html/linux/110stop.subx.html (limited to 'html/linux/110stop.subx.html') diff --git a/html/linux/110stop.subx.html b/html/linux/110stop.subx.html new file mode 100644 index 00000000..2b1dd4c5 --- /dev/null +++ b/html/linux/110stop.subx.html @@ -0,0 +1,279 @@ + + + + +Mu - linux/110stop.subx + + + + + + + + + + +https://github.com/akkartik/mu/blob/main/linux/110stop.subx +
+  1 # stop: dependency-injected wrapper around the exit() syscall
+  2 #
+  3 # We'd like to be able to write tests for functions that call exit(), and to
+  4 # make assertions about whether they exit() or not in a given situation. To
+  5 # achieve this we'll call exit() via a smarter wrapper called 'stop'.
+  6 #
+  7 # In the context of a test, calling a function X that calls 'stop' (directly
+  8 # or through further intervening calls) will unwind the stack until X returns,
+  9 # so that we can say check any further assertions after the execution of X. To
+ 10 # achieve this end, we'll pass the return address of X as a 'target' argument
+ 11 # into X, plumbing it through to 'stop'. When 'stop' gets a non-null target it
+ 12 # unwinds the stack until the target. If it gets a null target it calls
+ 13 # exit().
+ 14 #
+ 15 # We'd also like to get the exit status out of 'stop', so we'll combine the
+ 16 # input target with an output status parameter into a type called 'exit-descriptor'.
+ 17 #
+ 18 # So the exit-descriptor looks like this:
+ 19 #   target: address  # return address for 'stop' to unwind to
+ 20 #   value: int  # exit status stop was called with
+ 21 #
+ 22 # 'stop' thus takes two parameters: an exit-descriptor and the exit status.
+ 23 #
+ 24 # 'stop' won't bother cleaning up any other processor state besides the stack,
+ 25 # such as registers. Only esp will have a well-defined value after 'stop'
+ 26 # returns. (This is a poor man's setjmp/longjmp, if you know what that is.)
+ 27 #
+ 28 # Before you can call any function that may call 'stop', you need to pass in an
+ 29 # exit-descriptor to it. To create an exit-descriptor use 'tailor-exit-descriptor'
+ 30 # below. It's not the most pleasant abstraction in the world.
+ 31 #
+ 32 # An exit-descriptor's target is its input, computed during 'tailor-exit-descriptor'.
+ 33 # Its value is its output, computed during stop and available to the test.
+ 34 
+ 35 == code
+ 36 #   instruction                     effective address                                                   register    displacement    immediate
+ 37 # . op          subop               mod             rm32          base        index         scale       r32
+ 38 # . 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
+ 39 
+ 40 # Configure an exit-descriptor for a call pushing 'nbytes' bytes of args to
+ 41 # the stack.
+ 42 # Ugly that we need to know the size of args. Don't allocate variables between
+ 43 # tailor-exit-descriptor and the call it's for.
+ 44 tailor-exit-descriptor:  # ed: (addr exit-descriptor), nbytes: int
+ 45     # . prologue
+ 46     55/push-ebp
+ 47     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+ 48     # . save registers
+ 49     50/push-eax
+ 50     51/push-ecx
+ 51     # eax = nbytes
+ 52     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
+ 53     # Let X be the value of esp in the caller, before the call to tailor-exit-descriptor.
+ 54     # The return address for a call in the caller's body will be at:
+ 55     #   X-8 if the caller takes 4 bytes of args for the exit-descriptor (add 4 bytes for the return address)
+ 56     #   X-12 if the caller takes 8 bytes of args
+ 57     #   ..and so on
+ 58     # That's the value we need to return: X-nbytes-4
+ 59     #
+ 60     # However, we also need to account for the perturbance to esp caused by the
+ 61     # call to tailor-exit-descriptor. It pushes 8 bytes of args followed by 4
+ 62     # bytes for the return address and 4 bytes to push ebp above.
+ 63     # So ebp at this point is X-16.
+ 64     #
+ 65     # So the return address for the next call in the caller is:
+ 66     #   ebp+8 if the caller takes 4 bytes of args
+ 67     #   ebp+4 if the caller takes 8 bytes of args
+ 68     #   ebp if the caller takes 12 bytes of args
+ 69     #   ebp-4 if the caller takes 16 bytes of args
+ 70     #   ..and so on
+ 71     # That's ebp+12-nbytes.
+ 72     # option 1: 6 + 3 bytes
+ 73 #?     2d/subtract                     3/mod/direct    0/rm32/eax    .           .             .           .           .               8/imm32           # subtract from eax
+ 74 #?     8d/copy-address                 0/mod/indirect  4/rm32/sib    5/base/ebp  0/index/eax   .           0/r32/eax   .               .                 # copy ebp+eax to eax
+ 75     # option 2: 2 + 4 bytes
+ 76     f7          3/subop/negate      3/mod/direct    0/rm32/eax    .           .             .           .           .               .                 # negate eax
+ 77     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
+ 78     # copy eax to ed->target
+ 79     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   8/disp8         .                 # copy *(ebp+8) to ecx
+ 80     89/copy                         0/mod/indirect  1/rm32/ecx    .           .             .           0/r32/eax   .               .                 # copy eax to *ecx
+ 81     # initialize ed->value
+ 82     c7          0/subop/copy        1/mod/*+disp8   1/rm32/ecx    .           .             .           .           4/disp8         0/imm32           # copy to *(ecx+4)
+ 83 $tailor-exit-descriptor:end:
+ 84     # . restore registers
+ 85     59/pop-to-ecx
+ 86     58/pop-to-eax
+ 87     # . epilogue
+ 88     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+ 89     5d/pop-to-ebp
+ 90     c3/return
+ 91 
+ 92 stop:  # ed: (addr exit-descriptor), value: int
+ 93     # no prologue; one way or another, we're going to clobber registers
+ 94     # eax = ed
+ 95     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           0/r32/eax   4/disp8         .                 # copy *(esp+4) to eax
+ 96     # if (ed == 0) really exit
+ 97     3d/compare-eax-and 0/imm32
+ 98     74/jump-if-=  $stop:real/disp8
+ 99     # if (ed->target == 0) really exit
+100     81          7/subop/compare     0/mod/indirect  0/rm32/eax    .           .             .           .           .               0/imm32           # compare *eax
+101     74/jump-if-=  $stop:real/disp8
+102 $stop:fake:
+103     # ed->value = value+1
+104     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           1/r32/ecx   8/disp8         .                 # copy *(esp+8) to ecx
+105     41/increment-ecx
+106     89/copy                         1/mod/*+disp8   0/rm32/eax    .           .             .           1/r32/ecx   4/disp8         .                 # copy ecx to *(eax+4)
+107     # perform a non-local jump to ed->target
+108     8b/copy                         0/mod/indirect  0/rm32/eax    .           .             .           4/r32/esp   .               .                 # copy *eax to esp
+109 $stop:end1:
+110     # never gets here
+111     c3/return  # doesn't return to caller
+112 $stop:real:
+113     # . syscall(exit, value)
+114     8b/copy                         1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none  .           3/r32/ebx   8/disp8         .                 # copy *(esp+8) to ebx
+115     e8/call  syscall_exit/disp32
+116 $stop:end2:
+117     # never gets here
+118     c3/return  # doesn't return to caller
+119 
+120 test-stop-skips-returns-on-exit:
+121     # This looks like the standard prologue, but is here for different reasons.
+122     # A function calling 'stop' can't rely on ebp persisting past the call.
+123     #
+124     # Use ebp here as a stable base to refer to locals and arguments from in the
+125     # presence of push/pop/call instructions.
+126     # *Don't* use ebp as a way to restore esp.
+127     55/push-ebp
+128     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+129     # Make room for an exit descriptor on the stack. That's almost always the
+130     # right place for it, available only as long as it's legal to use. Once this
+131     # containing function returns we'll need a new exit descriptor.
+132     # var ed/eax: exit-descriptor
+133     68/push  0/imm32
+134     68/push  0/imm32
+135     89/copy                         3/mod/direct    0/rm32/eax    .           .             .           4/r32/esp   .               .                 # copy esp to eax
+136     # Size the exit-descriptor precisely for the next call below, to _test-stop-1.
+137     # tailor-exit-descriptor(ed, 4)
+138     # . . push args
+139     68/push  4/imm32/nbytes-of-args-for-_test-stop-1
+140     50/push-eax
+141     # . . call
+142     e8/call  tailor-exit-descriptor/disp32
+143     # . . discard args
+144     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+145     # . _test-stop-1(ed)
+146     # . . push args
+147     50/push-eax
+148     # . . call
+149     e8/call  _test-stop-1/disp32
+150     # registers except esp may be clobbered at this point
+151     # restore args
+152     58/pop-to-eax
+153     # check that _test-stop-1 tried to call exit(1)
+154     # . check-ints-equal(ed->value, 2, msg)  # i.e. stop was called with value 1
+155     # . . push args
+156     68/push  "F - test-stop-skips-returns-on-exit"/imm32
+157     68/push  2/imm32
+158     # . . push ed->value
+159     ff          6/subop/push        1/mod/*+disp8   0/rm32/eax    .           .             .           .           4/disp8         .                 # push *(eax+4)
+160     # . . call
+161     e8/call  check-ints-equal/disp32
+162     # . . discard args
+163     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+164     # . epilogue
+165     # don't restore esp from ebp; manually reclaim locals
+166     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
+167     5d/pop-to-ebp
+168     c3/return
+169 
+170 _test-stop-1:  # ed: (addr exit-descriptor)
+171     # . prologue
+172     55/push-ebp
+173     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+174     # _test-stop-2(ed)
+175     # . . push args
+176     ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+177     # . . call
+178     e8/call  _test-stop-2/disp32
+179     # should never get past this point
+180 $_test-stop-1:dead-end:
+181     # . . discard args
+182     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
+183     # signal test failed: check-ints-equal(1, 0, msg)
+184     # . . push args
+185     68/push  "F - test-stop-skips-returns-on-exit"/imm32
+186     68/push  0/imm32
+187     68/push  1/imm32
+188     # . . call
+189     e8/call  check-ints-equal/disp32
+190     # . . discard args
+191     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
+192     # . epilogue
+193     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+194     5d/pop-to-ebp
+195     c3/return
+196 
+197 _test-stop-2:  # ed: (addr exit-descriptor)
+198     # . prologue
+199     55/push-ebp
+200     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
+201     # . stop(ed, 1)
+202     # . . push args
+203     68/push  1/imm32
+204     ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
+205     # . . call
+206     e8/call  stop/disp32
+207     # should never get past this point
+208 $_test-stop-2:dead-end:
+209     # . epilogue
+210     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
+211     5d/pop-to-ebp
+212     c3/return
+213 
+214 # . . vim:nowrap:textwidth=0
+
+ + + -- cgit 1.4.1-2-gfad0