IDA - ObjC - Meaning of _BYTE and dword_0?

Question

This is my first experience disassembling/reverse engineering, and am having a bit of difficulty keeping up. For reference, I'm trying to uncover the inner workings of a pretty good library in my industry. The following method takes in a string and returns some content represented by long long.

signed __int64 __cdecl +[Lib generateLongLongValue:](Lib_meta *self, SEL a2, id a3)
{
  void *v3; // x0
  void *v4; // x19
  char *v5; // x0
  char *v6; // x20
  unsigned __int64 v7; // x25
  signed __int64 v8; // x22
  signed __int64 v9; // x26
  int v10; // w0
  signed __int64 v11; // x8
  signed __int64 v12; // x23
  int v13; // w0
  __int16 v14; // w24
  __int16 i; // w23
  __int16 v16; // w0
  bool v17; // w9
  signed __int64 v18; // x20

  v3 = (void *)objc_retain(a3, a2);
  v4 = v3;
  v5 = (char *)objc_msgSend(v3, "length");
  v6 = v5;
  v7 = (unsigned __int64)(v5 - 1);
  if ( v5 == (_BYTE *)&dword_0 + 1 )
  {
LABEL_25:
    v18 = -1LL;
    goto LABEL_26;
  }
  ...
}

Converted into standard Objective-C, this would look like so (I think):

+ (NSInteger)generateLongLongValue:(NSString*)string
{
    NSInteger v18;

    if (string.length - 1 == (_BYTE *)&dword_0 + 1) {
        v18 = -1LL;
        goto LABEL_26;
    }

    ...
}

I'm really lost as to what (_BYTE *)&dword_0 + 1 is supposed to mean here. Could somebody please provide some sort of explanation, or provide resources that I can look into so I can understand for myself?

Cheers

Answer 1

I suspect you’ve got a false positive during analysis and a constant (0 or 1) got wrongly converted to an offset (probably because in your database 0 is a valid address). The decompiler uses info from disassembly so in cases like this it may be necessary to clean up the disassembly:

Press Tab on the line with &dword_0 and if there is an offset dword_0 or similar in the disassembly, press O to remove it, then Tab back to pseudocode and press F5 to refresh.