# Checking null-terminated ascii strings. # # By default we create strings with a 4-byte length prefix rather than a null suffix. # However we still need null-prefixed strings when interacting with the Linux # kernel in a few places. 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 # main: 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 : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean # . prolog 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 # pseudocode: # initialize n = b->length # initialize s1 = s # initialize s2 = b->data # i = 0 # for (i = 0; i < n; ++n) # c1 = *s1 # c2 = *s2 # if c1 == 0 # return false # if c1 != c2 # return false # return *s1 == 0 # # initialize s into EDI 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 7/r32/EDI 8/disp8 . # copy *(EBP+8) to EDI # initialize benchmark length n into EDX 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 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 # initialize benchmark data into ESI 8b/copy 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . 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 # initialize loop counter i into ECX b9/copy-to-ECX 0/imm32/exit # while (i/ECX < n/EDX) $kernel-string-equal:loop: 39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX 74/jump-if-equal $kernel-string-equal:break/disp8 # c1/EAX, c2/EBX = *s, *benchmark b8/copy-to-EAX 0/imm32 8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX bb/copy-to-EBX 0/imm32 8a/copy 0/mod/indirect 6/rm32/ESI . . . 3/r32/EBX . . # copy byte at *ESI to lower byte of EBX # if (c1 == 0) return false 3d/compare-EAX 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 with EBX 75/jump-if-not-equal $kernel-string-equal:false/disp8 # ++s1, ++s2, ++i 41/inc-ECX 46/inc-ESI 47/inc-EDI # end while eb/jump $kernel-string-equal:loop/disp8 $kernel-string-equal:break: # if (*s/EDI == 0) return true b8/copy-to-EAX 0/imm32 8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX 3d/compare-EAX 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 # return false $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 # . epilog 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(Abc-kernel-string, "Abc") # . . push args 68/push "Abc"/imm32 68/push 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(Abc-kernel-string, "Adc") # . . push args 68/push "Adc"/imm32 68/push 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(Abc-kernel-string, "") # . . push args 68/push ""/imm32 68/push 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(Abc-kernel-string, "Ab") # . . push args 68/push "Ab"/imm32 68/push 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(Abc-kernel-string, "Abcd") # . . push args 68/push "Abcd"/imm32 68/push 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: 00/null Abc-kernel-string: 41/A 62/b 63/c 00/null # . . vim:nowrap:textwidth=0