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decompiler won't compile!

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decompiler won't compile!

 
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  #1
Nov 24th, 2006
I have some old programs compiled in the aout format which I would like to recompile with ELF for Linux.
I found this decompiler program (REC) which claims to be able to do the trick.
Unfortunatly, the way he has the the download setup is not very good for a direct Linux install, so I decided to use the sorce code. It comes up with a ton of warnings and an undefined reference error for the mneu variable used as a function arg.

I don't know anything about C. So I ask-Is this just bad code or it is salvagable with a few tweeks?

C Syntax (Toggle Plain Text) 
  1.  
  2. /*    dismips.c
  3.  *
  4.  *    Created by Giampiero Caprino
  5.  *    Backer Street Software
  6.  *
  7.  *    This software is part of REC, the reverse engineering compiler.
  8.  *    You are free to use, copy, modify and distribute this software.
  9.  *    If you fix bugs and or extend this software, I
  10.  *    will be happy to include your changes in the
  11.  *    most current version of the source.
  12.  *    just send mail to caprino@netcom.com
  13.  */
  14.  
  15. struct instr {
  16.     char    *name;
  17.     unsigned long    opcode;
  18.     unsigned long    opcode2;
  19.     int    (*optype)();
  20.     int    (*op1)();
  21.     int    (*op2)();
  22.     int    (*op3)();
  23. };
  24.  
  25. static    char    opcode[32];    /* opcode mnemonic of current instruction */
  26. static    char    ops[8][32];    /* operands of current instruction */
  27. static    char    *op[8];        /* pointers to operand */
  28. static    char    **opp;        /* pointer to current operand */
  29. static    long    cur_pc;        /* address of current instruction */
  30. extern    char    mneu[];        /* the output string from the disassembler */
  31.  
  32. static    int    op26(struct instr *ip, unsigned long val)
  33. {
  34.     if((val >> 26) == ip->opcode)
  35.         return(1);
  36.     return(0);
  37. }
  38.  
  39. static    int    op32(struct instr *ip, unsigned long val)
  40. {
  41.     if(val == ip->opcode)
  42.         return(1);
  43.     return(0);
  44. }
  45.  
  46. static    int    op26_16(struct instr *ip, unsigned long val)
  47. {
  48.     if((val >> 26) == ip->opcode && ((val >> 16) & 0x1F) == ip->opcode2)
  49.         return(1);
  50.     return(0);
  51. }
  52.  
  53. static    int    op26__16(struct instr *ip, unsigned long val)
  54. {
  55.     if((val >> 26) == ip->opcode && (val & 0xFFFF) == ip->opcode2)
  56.         return(1);
  57.     return(0);
  58. }
  59.  
  60. static    int    op21_6(struct instr *ip, unsigned long val)
  61. {
  62.     if((val >> 21) == ip->opcode && (val & 0x3F) == ip->opcode2)
  63.         return(1);
  64.     return(0);
  65. }
  66.  
  67. static    int    op5___11(struct instr *ip, unsigned long val)
  68. {
  69.     if((val >> 26) == ip->opcode && (val & 0x7FF) == ip->opcode2)
  70.         return(1);
  71.     return(0);
  72. }
  73.  
  74. static    int    op26_5(struct instr *ip, unsigned long val)
  75. {
  76.     if((val >> 26) == ip->opcode && (val & 0x3F) == ip->opcode2)
  77.         return(1);
  78.     return(0);
  79. }
  80.  
  81. static    int    op26_16_11(struct instr *ip, unsigned long val)
  82. {
  83.     if((val >> 26) == ip->opcode && (val & 0x001F07FF) == ip->opcode2)
  84.         return(1);
  85.     return(0);
  86. }
  87.  
  88. static    int    op26_21(struct instr *ip, unsigned long val)
  89. {
  90.     if((val >> 26) == ip->opcode && (val & 0x001FFFFF) == ip->opcode2)
  91.         return(1);
  92.     return(0);
  93. }
  94.  
  95. static    int    op_r21(struct instr *ip, unsigned long val)
  96. {
  97.     sprintf(*opp, "r%d", (val >> 21) & 0x1F);
  98.     return(1);
  99. }
  100.  
  101. static    int    op_r16(struct instr *ip, unsigned long val)
  102. {
  103.     sprintf(*opp, "r%d", (val >> 16) & 0x1F);
  104.     return(1);
  105. }
  106.  
  107. static    int    op_r11(struct instr *ip, unsigned long val)
  108. {
  109.     sprintf(*opp, "r%d", (val >> 11) & 0x1F);
  110.     return(1);
  111. }
  112.  
  113. static    int    op_i16(struct instr *ip, unsigned long val)
  114. {
  115.     long    lv;
  116.  
  117.     lv = (long)val;
  118.     lv <<= 16;
  119.     lv >>= 16;
  120.     sprintf(*opp, "#0x%lx", lv);
  121.     return(1);
  122. }
  123.  
  124. static    int    op_u16(struct instr *ip, unsigned long val)
  125. {
  126.     sprintf(*opp, "#0x%lx", val & 0xffff);
  127.     return(1);
  128. }
  129.  
  130. static    int    op_i6(struct instr *ip, unsigned long val)
  131. {
  132.     sprintf(*opp, "#%ld", (val >> 6) & 0x1f);
  133.     return(1);
  134. }
  135.  
  136. static    int    op_h16(struct instr *ip, unsigned long val)
  137. {
  138.     sprintf(*opp, "#0x%lx", val << 16);
  139.     return(1);
  140. }
  141.  
  142. static    int    op_b21(struct instr *ip, unsigned long val)
  143. {
  144.     long    lv;
  145.  
  146.     lv = (long)val;
  147.     lv <<= 16;
  148.     lv >>= 16;
  149.     sprintf(*opp, "%ld(r%d)", lv, (val >> 21) & 0x1F);
  150.     return(1);
  151. }
  152.  
  153. static    int    op_p16(struct instr *ip, unsigned long val)
  154. {
  155.     long    lv;
  156.  
  157.     lv = (long)val;
  158.     lv <<= 16;
  159.     lv >>= 14;
  160.     lv += cur_pc + 4;
  161.     sprintf(*opp, "0x%lx", lv);
  162.     return(1);
  163. }
  164.  
  165. static    int    op_p26(struct instr *ip, unsigned long val)
  166. {
  167.     long    lv;
  168.  
  169.     lv = (long)val & 0x03FFFFFF;
  170.     lv <<= 2;
  171.     lv |= ((cur_pc + 4) & 0xF0000000);
  172.     sprintf(*opp, "0x%lx", lv);
  173.     return(1);
  174. }
  175.  
  176. static    int    op_null(struct instr *ip, unsigned long val)
  177. {
  178.     return(0);
  179. }
  180.  
  181. struct    instr    itab[] = {
  182.     { "add",    0, 0x20,    op5___11, op_r11, op_r21, op_r16 },
  183.     { "addi",    8, 0x00,    op26, op_r16, op_r21, op_i16 },
  184.     { "addiu",    9, 0x00,    op26, op_r16, op_r21, op_i16 },
  185.     { "addu",    0, 0x21,    op5___11, op_r11, op_r21, op_r16 },
  186.     { "and",    0, 0x24,    op5___11, op_r11, op_r21, op_r16 },
  187.     { "andi",    0xc, 0,     op26, op_r16, op_r21, op_u16 },
  188.     { "beq",    0x4, 0,     op26, op_r21, op_r16, op_p16 },
  189.     { "beql",    0x14, 0,     op26, op_r21, op_r16, op_p16 },
  190.     { "bgez",    0x1, 0x1,     op26_16, op_r21, op_p16, op_null },
  191.     { "bgezal",    0x1, 0x11,     op26_16, op_r21, op_p16, op_null },
  192.     { "bgezall",    0x1, 0x13,     op26_16, op_r21, op_p16, op_null },
  193.     { "bgezl",    0x1, 0x03,     op26_16, op_r21, op_p16, op_null },
  194.     { "bgtz",    0x7, 0x0,     op26_16, op_r21, op_p16, op_null },
  195.     { "bgtzl",    0x17, 0x0,     op26_16, op_r21, op_p16, op_null },
  196.     { "blez",    0x6, 0x0,     op26_16, op_r21, op_p16, op_null },
  197.     { "blezl",    0x16, 0x0,     op26_16, op_r21, op_p16, op_null },
  198.     { "bltz",    0x1, 0x0,     op26_16, op_r21, op_p16, op_null },
  199.     { "bltzal",    0x1, 0x10,     op26_16, op_r21, op_p16, op_null },
  200.     { "bltzall",    0x1, 0x12,     op26_16, op_r21, op_p16, op_null },
  201.     { "bne",    0x5, 0x0,     op26, op_r21, op_r16, op_p16 },
  202.     { "bnel",    0x15, 0x0,     op26, op_r21, op_r16, op_p16 },
  203.     /* { "break",    0x15, 0x0,     op26_16, op_r21, op_r16, op_p16 }, */
  204.     /* { "cache",    0x15, 0x0,     op26_16, op_r21, op_r16, op_p16 }, */
  205.     { "div",    0, 0x1a,    op26__16, op_r21, op_r16, op_null },
  206.     { "divu",    0, 0x1b,    op26__16, op_r21, op_r16, op_null },
  207.     { "j",        2, 0,        op26, op_p26, op_null, op_null },
  208.     { "jal",    3, 0,        op26, op_p26, op_null, op_null },
  209.     { "jalr",    0, 0x9,        op26_16_11, op_r11, op_r21, op_null },
  210.     { "jr",        0, 0x8,        op26_21, op_r21, op_null, op_null },
  211.     { "lb",        0x20, 0,    op26, op_r16, op_b21, op_null },
  212.     { "lbu",    0x24, 0,    op26, op_r16, op_b21, op_null },
  213.     { "lh",        0x21, 0,    op26, op_r16, op_b21, op_null },
  214.     { "lhu",    0x25, 0,    op26, op_r16, op_b21, op_null },
  215.     { "ll",        0x30, 0,    op26, op_r16, op_b21, op_null },
  216.     { "lui",    0x0F, 0,    op26, op_r16, op_h16, op_null },
  217.     { "lw",        0x23, 0,    op26, op_r16, op_b21, op_null },
  218.     { "lwu",    0x2F, 0,    op26, op_r16, op_b21, op_null },
  219.     { "lwl",    0x22, 0,    op26, op_r16, op_b21, op_null },
  220.     { "lwr",    0x26, 0,    op26, op_r16, op_b21, op_null },
  221.     { "mult",    0, 0x18,    op26__16, op_r21, op_r16, op_null },
  222.     { "multu",    0, 0x19,    op26__16, op_r21, op_r16, op_null },
  223.     { "nor",    0, 0x27,    op5___11, op_r11, op_r21, op_r16 },
  224.     { "or",        0, 0x25,    op5___11, op_r11, op_r21, op_r16 },
  225.     { "ori",    0xd, 0,        op26, op_r16, op_r21, op_u16 },
  226.     { "sb",        0x28, 0,    op26, op_r16, op_b21, op_null },
  227.     { "sc",        0x38, 0,     op26, op_r16, op_b21, op_null },
  228.     { "sh",        0x29, 0,    op26, op_r16, op_b21, op_null },
  229.     { "sll",    0, 0,        op26_5, op_r11, op_r16, op_i6 },
  230.     { "sllv",    0, 4,        op5___11, op_r11, op_r16, op_r21 },
  231.     { "slt",    0, 0x2a,    op5___11, op_r11, op_r21, op_r16 },
  232.     { "slti",    0xa, 0,        op26, op_r16, op_r21, op_i16 },
  233.     { "sltiu",    0xb, 0,        op26, op_r16, op_r21, op_i16 },
  234.     { "sltu",    0, 0x2b,    op5___11, op_r11, op_r21, op_r16 },
  235.     { "sra",    0, 3,        op21_6, op_r11, op_r16, op_i6 },
  236.     { "srav",    0, 7,        op5___11, op_r11, op_r16, op_r21 },
  237.     { "srl",    0, 2,        op21_6, op_r11, op_r16, op_i6 },
  238.     { "srlv",    0, 6,        op5___11, op_r11, op_r16, op_r21 },
  239.     { "sub",    0, 0x22,    op5___11, op_r11, op_r21, op_r16 },
  240.     { "subu",    0, 0x23,    op5___11, op_r11, op_r21, op_r16 },
  241.     { "sw",        0x2b, 0,    op26, op_r16, op_b21, op_null },
  242.     { "swl",    0x2a, 0,    op26, op_r16, op_b21, op_null },
  243.     { "swr",    0x2e, 0,    op26, op_r16, op_b21, op_null },
  244.     { "sync",    0xf, 0,        op32, op_null, op_null, op_null },
  245.     { "syscall",    0xc, 0,        op32, op_null, op_null, op_null },
  246.     { "xor",    0, 0x26,    op5___11, op_r11, op_r21, op_r16 },
  247.     { "xori",    0x0e, 0,    op26, op_r16, op_r21, op_u16 },
  248.     { 0 }
  249. };
  250.  
  251. char    *mips_disass(unsigned long addr, unsigned long val)
  252. {
  253.     struct    instr    *ip;
  254.     int    i;
  255.     char    *sep;
  256.  
  257.     for(i = 0; i < 8; ++i) {
  258.         ops[i][0] = 0;
  259.         op[i] = ops[i];
  260.     }
  261.     opp = op;
  262.     cur_pc = addr;
  263.     for(ip = itab; ip->name; ++ip) {
  264.         if(ip->optype && ip->optype(ip, val)) {
  265.         strcpy(opcode, ip->name);
  266.         if(ip->op1(ip, val))
  267.             ++opp;
  268.         if(ip->op2(ip, val))
  269.             ++opp;
  270.         if(ip->op3(ip, val))
  271.             ++opp;
  272.         break;
  273.         }
  274.     }
  275.     *op = 0;        /* end of arguments */
  276.     if(!ip->name) {
  277.         sprintf(opcode, "Unknown opcode");
  278.         sprintf(ops[0], "0x%08lx", val);
  279.         ops[1][0] = 0;
  280.     }
  281.     strcpy(mneu, opcode);
  282.     sep = " ";
  283.     for(i = 0; ops[i][0]; ++i) {
  284.         strcat(mneu, sep);
  285.         strcat(mneu, ops[i]);
  286.         sep = ",";
  287.     }
  288.     return(mneu);
  289. }

Also, this looks like it can only generate the machine code, rather than C code?
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Re: decompiler won't compile!

 
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  #2
Nov 24th, 2006
its nearly impossible to take a compiled executable and reconstruct the c program. how is a decompiler even supposed to know the original source was written in c, or c++, or basic, or fortran, or ... ? Also it is not possible for it to reconstruct the structures that may have been used. So I think you are wasting your time if you expect the decompiler to generate c code for you. Just not going to happen.
Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.
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Re: decompiler won't compile!

 
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  #3
Nov 24th, 2006
It would be nearly imposible to reconstruct C code as all compilers will compile it slightly different. Also, the same compiler might do it differently twice.
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Re: decompiler won't compile!

 
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  #4
Nov 24th, 2006
> Also, this looks like it can only generate the machine code, rather than C code?
True, and it's also specific to the MIPS processor as well, which means that at best, it will only turn a MIPS a.out file back into MIPS assembly, which might give you a chance to reassemble it to MIPS elf format.

> It comes up with a ton of warnings and an undefined reference error for the mneu variable
This file is most likely just one of MANY files in the whole project. It also lacks a main() for example.
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