# Checking null-terminated strings. # # By default we create strings as arrays of bytes, and all arrays have a 4-byte # size prefix. # # However, we sometimes need to deal with null-terminated strings when # interacting with the Linux kernel. This layer implements a function for # comparing a null-terminated 'kernel string' with a size-prefixed 'SubX # string'. # # To run (from the subx directory): # $ ./bootstrap translate 05[0-2]*.subx -o /tmp/tmp52 # $ ./bootstrap 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 kernel-string-equal?: # s: (addr kernel-string), benchmark: (addr array byte) -> eax: boolean # pseudocode: # n = benchmark->size # 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: (addr 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->size 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: (addr 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->= $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/null 74/jump-if-= $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-!= $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/null 75/jump-if-!= $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-empt