Writing a compiler in its own language

When you write your first compiler for C, you write it in some other language. Now, you have a compiler for C in, say, assembler. Eventually, you will come to the place where you have to parse strings, specifically escape sequences. You will write code to convert \n to the character with the decimal code 10 (and \r to 13, etc).

After that compiler is ready, you will start to reimplement it in C. This process is called "bootstrapping".

The string parsing code will become:

...
if (c == 92) { // backslash
    c = getc();
    if (c == 110) { // n
        return 10;
    } else if (c == 92) { // another backslash
        return 92;
    } else {
        ...
    }
}
...

When this compiles, you have a binary which understands '\n'. This means you can change the source code:

...
if (c == '\\') {
    c = getc();
    if (c == 'n') {
        return '\n';
    } else if (c == '\\') {
        return '\\';
    } else {
        ...
    }
}
...

So where is the information that '\n' is the code for 13? It's in the binary! It's like DNA: Compiling C source code with this binary will inherit this information. If the compiler compiles itself, it will pass this knowledge on to its offspring. From this point on, there is no way to see from the source alone what the compiler will do.

If you want to hide a virus in the source of some program, you can do it like this: Get the source of a compiler, find the function which compiles functions and replace it with this one:

void compileFunction(char * name, char * filename, char * code) {
    if (strcmp("compileFunction", name) == 0 && strcmp("compile.c", filename) == 0) {
        code = A;
    } else if (strcmp("xxx", name) == 0 && strcmp("yyy.c", filename) == 0) {
        code = B;
    }

    ... code to compile the function body from the string in "code" ...
}

The interesting parts are A and B. A is the source code for compileFunction including the virus, probably encrypted in some way so it's not obvious from searching the resulting binary. This makes sure that compiling to compiler with itself will preserve the virus injection code.

B is the same for the function we want to replace with our virus. For example, it could be the function "login" in the source file "login.c" which is probably from the Linux kernel. We could replace it with a version that will accept the password "joshua" for the root account in addition to the normal password.

If you compile that and spread it as a binary, there will be no way to find the virus by looking at the source.

The original source of the idea: https://web.archive.org/web/20070714062657/http://www.acm.org/classics/sep95/

This is called "bootstrapping". You must first build a compiler (or interpreter) for your language in some other language (usually Java or C). Once that is done, you can write a new version of the compiler in language Foo. You use the first bootstrap compiler to compile the compiler, and then use this compiled compiler to compile everything else (including future versions of itself).

Most languages are indeed created in this fashion, partially because language designers like to use the language they are creating, and also because a non-trivial compiler often serves as a useful benchmark for how "complete" the language may be.

An example of this would be Scala. Its first compiler was created in Pizza, an experimental language by Martin Odersky. As of version 2.0, the compiler was completely re-written in Scala. From that point on, the old Pizza compiler could be completely discarded, due to the fact that the new Scala compiler could be used to compile itself for future iterations.

I recall listening to a Software Engineering Radio podcast wherein Dick Gabriel spoke about bootstrapping the original LISP interpreter by writing a bare-bones version in LISP on paper and hand assembling it into machine code. From then on, the rest of the LISP features were both written in and interpreted with LISP.