Re: Self compiling compiler

On 2016-03-31, Nils M Holm <nmh@t3x.org> wrote:
> ERSHC <ershc123@nyc.rr.com> wrote:
>> I'd like to teach an undergraduate class based on the Thompson hack
>> (see Ken Thompson's Turing Award lecture). I don't want to write a
>> "whole" compiler, so need a compiler that can compile itself,
>> preferably one that runs under Linux and is reasonably small. Can
>> anyone point me to something more appropriate than gcc (which is far
>> from small)?
>
> For something really small, check out Lindberg's Foogol. Here's a version
> I converted to C89: http://www.t3x.org/files/foogol5.shar.Z
>
> For something more complete, but still much simpler than GCC, have a look
> at SubC: http://www.t3x.org/subc/index.html There's also a book describing
> its internals: http://www.t3x.org/reload/index.html (Yes, I'm the author!)
>
> There are various other small compilers, from very simple ones to
> multi-platform optimizing compilers on my home page. Feel free to
> look around!


What matters here is not the compiler size but:


- The compiler has to be self-bootstrapping.
    The Thompson trick can't be perpetuated on something which
    strictly requires another compiler in order to build, and is then
    not re-compiled with itself. The end goal of the trick is not
    to have any traces of that trick in any piece of source code;
    the result is a rogue compiler binary which replicates the rogue
    code when it is used to compile itself, and which alters
    some additional program, like a security-sensitive utility
    (su, login, ..)


- how easy is it, within the compiler's framework, to do this:
  - insert a piece of code which recognizes a static source
      code structure (perhaps as an abstract-syntax-tree-level pattern
      match) so that it can say "hey, I'm compiling myself: that parse is a
      piece of *me*!" or "hey, I'm compiling the login program!".
  - obtain the above AST-recognizing code not as C source, but as as a
      piece of AST material which the compiler itself can insert into a
      parse.
  - obtain, as an AST, some rogue code which the malicious compiler
      inserts into the parse of the target program such as login,
      as well as the rogue code which perpetuates all of this.


If the compiler is small, but its data structures and internal API's are
a dog's breakfast, it will be tedious to do.


(Mind you GCC is big *and* a dog's breakfast.).

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