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# Checking null-terminated ascii strings.
#
# By default we create strings as arrays of bytes, and all arrays have a 4-byte
# length prefix.
#
# However, we sometimes need to deal with null-prefixed strings when interacting
# with the Linux kernel. This layer implements a function for comparing a
# null-terminated 'kernel string' with a length-prefixed 'SubX string'.
#
# To run (from the subx directory):
# $ ./subx translate 05[0-2]*.subx -o /tmp/tmp52
# $ ./subx run /tmp/tmp52 # runs a series of tests
# ...... # all tests pass
#
# (We can't yet run the tests when given a "test" commandline argument,
# because checking for it would require the function being tested! Breakage
# would cause tests to not run, rather than to fail as we'd like.)
== 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
Entry: # run all tests
e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
# syscall(exit, Num-test-failures)
8b/copy 0/mod/indirect 5/rm32/.disp32 . . 3/r32/ebx Num-test-failures/disp32 # copy *Num-test-failures to ebx
b8/copy-to-eax 1/imm32/exit
cd/syscall 0x80/imm8
# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
kernel-string-equal?: # s : (address kernel-string), benchmark : (address array byte) -> eax : boolean
# pseudocode:
# n = benchmark->length
# s1 = s
# s2 = benchmark->data
# i = 0
# while (i < n)
# c1 = *s1
# c2 = *s2
# if (c1 == 0) return false
# if (c1 != c2) return false
# ++s1, ++s2, ++i
# return *s1 == 0
#
# registers:
# i: ecx
# n: edx
# s1: edi
# s2: esi
# c1: eax
# c2: ebx
#
# . prologue
55/push-ebp
89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
# . save registers
51/push-ecx
52/push-edx
53/push-ebx
56/push-esi
57/push-edi
# var s1/edi : (address byte) = s
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 7/r32/edi 8/disp8 . # copy *(ebp+8) to edi
# var n/edx : int = benchmark->length
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx
8b/copy 0/mod/indirect 2/rm32/edx . . . 2/r32/edx . . # copy *edx to edx
# var s2/esi : (address byte) = benchmark->data
8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 6/r32/esi 0xc/disp8 . # copy *(ebp+12) to esi
81 0/subop/add 3/mod/direct 6/rm32/esi . . . . . 4/imm32 # add to esi
# var i/ecx : int = 0
b9/copy-to-ecx 0/imm32/exit
# var c1/eax : byte = 0
b8/copy-to-eax 0/imm32
# var c2/ebx : byte = 0
bb/copy-to-ebx 0/imm32
$kernel-string-equal?:loop:
# if (i >= n) break
39/compare 3/mod/direct 1/rm32/ecx . . . 2/r32/edx . . # compare ecx with edx
7d/jump-if-greater-or-equal $kernel-string-equal?:break/disp8
# c1 = *s1
8a/copy-byte 0/mod/indirect 7/rm32/edi . . . 0/r32/AL . . # copy byte at *edi to AL
# c2 = *s2
8a/copy-byte 0/mod/indirect 6/rm32/esi . . . 3/r32/BL . . # copy byte at *esi to BL
# if (c1 == 0) return false
3d/compare-eax-and 0/imm32
74/jump-if-equal $kernel-string-equal?:false/disp8
# if (c1 != c2) return false
39/compare 3/mod/direct 0/rm32/eax . . . 3/r32/ebx . . # compare eax and ebx
75/jump-if-not-equal $kernel-string-equal?:false/disp8
# ++i
41/increment-ecx
# ++s1
47/increment-edi
# ++s2
46/increment-esi
eb/jump $kernel-string-equal?:loop/disp8
$kernel-string-equal?:break:
# return *s1 == 0
8a/copy-byte 0/mod/indirect 7/rm32/edi . . . 0/r32/AL . . # copy byte at *edi to AL
3d/compare-eax-and 0/imm32
75/jump-if-not-equal $kernel-string-equal?:false/disp8
$kernel-string-equal?:true:
b8/copy-to-eax 1/imm32
eb/jump $kernel-string-equal?:end/disp8
$kernel-string-equal?:false:
b8/copy-to-eax 0/imm32
$kernel-string-equal?:end:
# . restore registers
5f/pop-to-edi
5e/pop-to-esi
5b/pop-to-ebx
5a/pop-to-edx
59/pop-to-ecx
# . epilogue
89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
5d/pop-to-ebp
c3/return
# - tests
test-compare-null-kernel-string-with-empty-array:
# eax = kernel-string-equal?(Null-kernel-string, "")
# . . push args
68/push ""/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 1, msg)
# . . push args
68/push "F - test-compare-null-kernel-string-with-empty-array"/imm32
68/push 1/imm32/true
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-null-kernel-string-with-non-empty-array:
# eax = kernel-string-equal?(Null-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push Null-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 0, msg)
# . . push args
68/push "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
68/push 0/imm32/false
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-kernel-string-with-equal-array:
# eax = kernel-string-equal?(_test-Abc-kernel-string, "Abc")
# . . push args
68/push "Abc"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 1, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 1/imm32/true
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-kernel-string-with-inequal-array:
# eax = kernel-string-equal?(_test-Abc-kernel-string, "Adc")
# . . push args
68/push "Adc"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-kernel-string-with-empty-array:
# eax = kernel-string-equal?(_test-Abc-kernel-string, "")
# . . push args
68/push ""/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-equal-array"/imm32
68/push 0/imm32/false
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-kernel-string-with-shorter-array:
# eax = kernel-string-equal?(_test-Abc-kernel-string, "Ab")
# . . push args
68/push "Ab"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-shorter-array"/imm32
68/push 0/imm32/false
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
test-compare-kernel-string-with-longer-array:
# eax = kernel-string-equal?(_test-Abc-kernel-string, "Abcd")
# . . push args
68/push "Abcd"/imm32
68/push _test-Abc-kernel-string/imm32
# . . call
e8/call kernel-string-equal?/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
# check-ints-equal(eax, 0, msg)
# . . push args
68/push "F - test-compare-kernel-string-with-longer-array"/imm32
68/push 0/imm32/false
50/push-eax
# . . call
e8/call check-ints-equal/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 0xc/imm32 # add to esp
c3/return
== data
Null-kernel-string: # (address kernel-string)
00/null
_test-Abc-kernel-string: # (address kernel-string)
41/A 62/b 63/c 00/null
# . . vim:nowrap:textwidth=0
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