## 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 operand 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: (if this is the last file loaded) e8/call run_tests/disp32 # 'run_tests' is a function created automatically by SubX. It calls all functions that start with 'test_'. # exit(EAX) 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX b8/copy . . . . . . . 1/imm32 # copy 1 to EAX 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 4 to ESI # initialize loop counter i into ECX b9/copy . . . . . . . 0/imm32/exit # copy 1 to ECX # while (i/ECX < n/EDX) $kernel_string_loop: 39/compare 3/mod/direct 1/rm32/ECX . . . 2/r32/EDX . . # compare ECX with EDX 74/jump-if-equal $kernel_string_break/disp8 # c1/EAX, c2/EBX = *s, *benchmark b8/copy 0/imm32 # clear EAX 8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX bb/copy 0/imm32 # clear EBX 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 . . . . . . . 0/imm32 # compare EAX with 0 74/jump-if-equal $kernel_string_fail/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_fail/disp8 # ++s1, ++s2, ++i 41/inc-ECX 46/inc-ESI 47/inc-EDI # end while eb/jump $kernel_string_loop/disp8 $kernel_string_break: # if (*s/EDI == 0) return true b8/copy 0/imm32 # clear EAX 8a/copy 0/mod/indirect 7/rm32/EDI . . . 0/r32/EAX . . # copy byte at *EDI to lower byte of EAX 81 7/subop/compare 3/mod/direct 0/rm32/EAX . . . . . 0/imm32 # compare EAX with 0 75/jump-if-not-equal $kernel_string_fail/disp8 b8/copy . . . . . . . 1/imm32 # copy 1 to EAX eb/jump $kernel_string_end/disp8 # return false $kernel_string_fail: b8/copy . . . . . . . 0/imm32 # copy 0 to EAX $kernel_string_end: # restore registers 5f/pop-to-EDI 5e/pop-to-ESI 5b/pop-to-EBX 5a/pop-to-EDX 59/pop-to-ECX # end 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 8 to ESP # call 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 12 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 8 to ESP # call 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 12 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 8 to ESP # call 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 12 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 8 to ESP # call 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 12 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 8 to ESP # call check_ints_equal(EAX, 0) # 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 12 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 8 to ESP # call check_ints_equal(EAX, 0) # 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 12 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 8 to ESP # call check_ints_equal(EAX, 0) # 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 12 to ESP c3/return == data Null_kernel_string: 00/null Abc_kernel_string: 41/A 62/b 63/c 00/null # vim:ft=subx:nowrap:tw&