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Re: [Tinycc-devel] enforced immutability - proposed research project

From: Michael Matz
Subject: Re: [Tinycc-devel] enforced immutability - proposed research project
Date: Mon, 18 Jan 2021 15:57:59 +0100 (CET)
User-agent: Alpine 2.21 (LSU 202 2017-01-01)


On Mon, 18 Jan 2021, Barath Aron via Tinycc-devel wrote:


On Mon, 18 Jan 2021 12:32:29 +0100 Bruno Haible <bruno@clisp.org> wrote:
        ((struct data *) p)->x = 9; // Violation of immutability,
crashes ...

If you do not allow such cast, you don't have to do anything at runtime.

Depends what happens with the original wp in Brunos example. To remind, the context was:

  wp = malloc(); wp->foo = ...;
  p = freeze(wp);

Now, if you only disallow to cast away immutability, then you still would be able to write to the object via *wp. So, you either need to do something at runtime, or encode the fact that wp becomes invalid after freezing it. That might sound easy, but you also need to invalidate all _copies_ of wp:

  wp = malloc(); init(wp);
  p = freeze(wp);
  globalp->foo = ...;  // should be disallowed

where init(wp) is something like:

   struct S *globalp;
   void init(struct S *x) { x->foo = 1; globalp = x; }

Doing things at runtime also isn't super-easy: you either waste full pages for each allocation, no matter how small (in order to write-protect them at freeze), or you need to copy contents around (to write-controlled areas) invalidating addresses already pointing to it, or you need to do checked-writes for each memory write. The latter is basically what current bounds-checking in tcc already does, just with even more meta-data.

I think the latter would actually be the quickest route to success here: reuse the bounds-checking code. You just need a way to register read-only regions (which is what freezing does), and then check that list of regions at each write.


Also, freestanding mode would work that way too.
But for that, you need a more complex type system.


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