CODEWITHSUNDEEP
cassemblymipsreverse-engineering
George0541
George0541

Reputation: 43

Converting MIPS assembly to C

I am working on converting MIPS to C, and I've been stuck on this for several days.

I've started with manually converting MIPS to C, and even though I've been working on it for several days, my resulting program in C still doesn't run as needed, and I feel like there is a small mistake that I overlooked, and I would really appreciate your help.

I started conversion with toplevel_fnc, and then with subroutine_fnc.

Here is a part of the MIPS code.

Subroutine:

build/program-mips:     file format elf32-tradbigmips


Disassembly of section my_text:

00404ed0 <subroutine_fnc>:
  404ed0:   27bdffd8    addiu   sp,sp,-40
  404ed4:   afbe0024    sw  s8,36(sp)
  404ed8:   03a0f025    move    s8,sp
  404edc:   afc40028    sw  a0,40(s8)
  404ee0:   8fc40028    lw  a0,40(s8)
  404ee4:   27c20018    addiu   v0,s8,24
  404ee8:   00402825    move    a1,v0
  404eec:   24060001    li  a2,1
  404ef0:   24020fa3    li  v0,4003
  404ef4:   0000000c    syscall
  404ef8:   afc70008    sw  a3,8(s8)
  404efc:   afc2000c    sw  v0,12(s8)
  404f00:   8fc20008    lw  v0,8(s8)
  404f04:   00000000    nop
  404f08:   14400004    bnez    v0,404f1c <subroutine_fnc+0x4c>
  404f0c:   00000000    nop
  404f10:   8fc2000c    lw  v0,12(s8)
  404f14:   10000002    b   404f20 <subroutine_fnc+0x50>
  404f18:   00000000    nop
  404f1c:   2402ffff    li  v0,-1
  404f20:   afc20010    sw  v0,16(s8)
  404f24:   8fc20010    lw  v0,16(s8)
  404f28:   00000000    nop
  404f2c:   04410006    bgez    v0,404f48 <subroutine_fnc+0x78>
  404f30:   00000000    nop
  404f34:   24040001    li  a0,1
  404f38:   24020fa1    li  v0,4001
  404f3c:   0000000c    syscall
  404f40:   afc70008    sw  a3,8(s8)
  404f44:   afc20014    sw  v0,20(s8)
  404f48:   8fc20010    lw  v0,16(s8)
  404f4c:   00000000    nop
  404f50:   14400004    bnez    v0,404f64 <subroutine_fnc+0x94>
  404f54:   00000000    nop
  404f58:   00001025    move    v0,zero
  404f5c:   10000002    b   404f68 <subroutine_fnc+0x98>
  404f60:   00000000    nop
  404f64:   83c20018    lb  v0,24(s8)
  404f68:   03c0e825    move    sp,s8
  404f6c:   8fbe0024    lw  s8,36(sp)
  404f70:   27bd0028    addiu   sp,sp,40
  404f74:   03e00008    jr  ra
  404f78:   00000000    nop

and toplevel:

00404f7c <toplevel_fnc>:
  404f7c:   27bdffc0    addiu   sp,sp,-64
  404f80:   afbf003c    sw  ra,60(sp)
  404f84:   afbe0038    sw  s8,56(sp)
  404f88:   03a0f025    move    s8,sp
  404f8c:   afc00018    sw  zero,24(s8)
  404f90:   10000040    b   405094 <toplevel_fnc+0x118>
  404f94:   00000000    nop
  404f98:   83c30034    lb  v1,52(s8)
  404f9c:   2402005c    li  v0,92
  404fa0:   14620022    bne v1,v0,40502c <toplevel_fnc+0xb0>
  404fa4:   00000000    nop
  404fa8:   8fc20018    lw  v0,24(s8)
  404fac:   00000000    nop
  404fb0:   24420001    addiu   v0,v0,1
  404fb4:   afc20018    sw  v0,24(s8)
  404fb8:   24040001    li  a0,1
  404fbc:   3c020041    lui v0,0x41
  404fc0:   244260e0    addiu   v0,v0,24800
  404fc4:   00402825    move    a1,v0
  404fc8:   24060002    li  a2,2
  404fcc:   24020fa4    li  v0,4004
  404fd0:   0000000c    syscall
  404fd4:   afc7001c    sw  a3,28(s8)
  404fd8:   afc20020    sw  v0,32(s8)
  404fdc:   8fc2001c    lw  v0,28(s8)
  404fe0:   00000000    nop
  404fe4:   14400004    bnez    v0,404ff8 <toplevel_fnc+0x7c>
  404fe8:   00000000    nop
  404fec:   8fc20020    lw  v0,32(s8)
  404ff0:   10000002    b   404ffc <toplevel_fnc+0x80>
  404ff4:   00000000    nop
  404ff8:   2402ffff    li  v0,-1
  404ffc:   afc20024    sw  v0,36(s8)
  405000:   8fc20024    lw  v0,36(s8)
  405004:   00000000    nop
  405008:   04410022    bgez    v0,405094 <toplevel_fnc+0x118>
  40500c:   00000000    nop
  405010:   24040001    li  a0,1
  405014:   24020fa1    li  v0,4001
  405018:   0000000c    syscall
  40501c:   afc7001c    sw  a3,28(s8)
  405020:   afc20028    sw  v0,40(s8)
  405024:   1000001b    b   405094 <toplevel_fnc+0x118>
  405028:   00000000    nop
  40502c:   24040001    li  a0,1
  405030:   27c20034    addiu   v0,s8,52
  405034:   00402825    move    a1,v0
  405038:   24060001    li  a2,1
  40503c:   24020fa4    li  v0,4004
  405040:   0000000c    syscall
  405044:   afc7001c    sw  a3,28(s8)
  405048:   afc2002c    sw  v0,44(s8)
  40504c:   8fc2001c    lw  v0,28(s8)
  405050:   00000000    nop
  405054:   14400004    bnez    v0,405068 <toplevel_fnc+0xec>
  405058:   00000000    nop
  40505c:   8fc2002c    lw  v0,44(s8)
  405060:   10000002    b   40506c <toplevel_fnc+0xf0>
  405064:   00000000    nop
  405068:   2402ffff    li  v0,-1
  40506c:   afc20024    sw  v0,36(s8)
  405070:   8fc20024    lw  v0,36(s8)
  405074:   00000000    nop
  405078:   04410006    bgez    v0,405094 <toplevel_fnc+0x118>
  40507c:   00000000    nop
  405080:   24040001    li  a0,1
  405084:   24020fa1    li  v0,4001
  405088:   0000000c    syscall
  40508c:   afc7001c    sw  a3,28(s8)
  405090:   afc20030    sw  v0,48(s8)
  405094:   00002025    move    a0,zero
  405098:   0c1013b4    jal 404ed0 <subroutine_fnc>
  40509c:   00000000    nop
  4050a0:   00021600    sll v0,v0,0x18
  4050a4:   00021603    sra v0,v0,0x18
  4050a8:   a3c20034    sb  v0,52(s8)
  4050ac:   83c20034    lb  v0,52(s8)
  4050b0:   00000000    nop
  4050b4:   1c40ffb8    bgtz    v0,404f98 <toplevel_fnc+0x1c>
  4050b8:   00000000    nop
  4050bc:   8fc20018    lw  v0,24(s8)
  4050c0:   03c0e825    move    sp,s8
  4050c4:   8fbf003c    lw  ra,60(sp)
  4050c8:   8fbe0038    lw  s8,56(sp)
  4050cc:   27bd0040    addiu   sp,sp,64
  4050d0:   03e00008    jr  ra
  4050d4:   00000000    nop

program data


build/program-mips:     file format elf32-tradbigmips

Contents of section my_data:
 4160e0 5c5c0000                             \\..

and I wrote it in C as:

int toplevel_fnc(void) {
  int a = 0;
 
  while(1) {
    char c = subroutine_fnc(0);
 
    if (c <= 0) {
      return a;
    }
 
    if (c == 92) {
      a++;
      int d = write(1, "\\", 2);
      if (d < 0) {
        exit(1);
      }
    } else {
      int e = write(1, &c, 1);
      if (e < 0) {
        exit(1);
      }
    }
  }
  return a;
}

and

int subroutine_fnc(int a) {
  char c;
   
  int b = read(a, &c, 1);
  if (b < 0) {
    exit(1);
  } else if (b == 0) {
    return 0;
  }
  return c;
}

I am feeling lost, and also kind of stupid for asking here, but I feel like there is a small mistake that I overlooked. I've been working on this for the past three days, and I can't find the solution on how to correctly convert MIPS to C.

Please, I would be very glad if anyone could help me out on what is wrong in my code.

Thank you.

Upvotes: 2

Views: 2467

Answers (1)

Armali
Armali

Reputation: 19375

  • The assembly program writes two backslashes, whereas the C write(1, "\\", 2) writes one backslash and one NUL, because \\ in a string literal denotes only one backslash; you meant to write(1, "\\\\", 2).
  • The char is unsigned - this makes another deviation, since at a place in the assembly program a byte is sign extended and later tested with bgtz, while in the C code 
        char c = subroutine_fnc(0);

    if (c <= 0)
    the if condition will be true only for c = 0. Use signed char c … instead.

Upvotes: 1

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