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

[Elijah Stone]

On Tue, 19 Jan 2021, Bruno Haible wrote:

> MAP_FIXED is a portability nightmare nowadays, where most OSes implement 
> address space randomization and put the shared libraries at arbitrary 
> and random locations in the address space. You better stay away from it.
>
> The implementation I pointed to therefore doesn't use MAP_FIXED, just 
> plain MAP_SHARED.

The problem is, without a fixed offset between the mutable and immutable 
spaces, certain desirable semantics become harder to implement.  For 
example, we want mutable and immutable references to the same object to 
compare equal.

One potential solution is to rely on overcommit.  Map a giant memory 
region, and then remap the second portion (using MAP_FIXED) to alias the 
first half.  (Using MAP_FIXED to overwrite existing address space mappings 
_is_ a supported usage.)

 -E