Diverse Double-Compiling, --with-c-toolchain and trusting trust

[zimoun]

Hi,

Reading the recent discussions about Reproducible Builds, see [1, 2, 3],
I was in the mood to use the recent option ’–with-c-toolchain’ to
demonstrate how Guix is cool!  But I have failed because I miss some UI,
I guess.

Well, my understanding of Diverse Double-Compiling can be summarized as:

Let’s consider that you have the source code of compiler (say ’tcc’
because it compiles fast) and 2 another compilers (say ’clang’ and
’gcc’).

| step | source | compiled with | produces |
|------+--------+---------------+----------|
| #1   | tcc    | clang         | tcc-A    |
| #2   | tcc    | tcc-A         | tcc-B    |
| #3   | tcc    | gcc           | tcc-C    |

Nothing ensures that tcc-{A,B,C} are bit-to-bit identical –– even they
should have different binary code –– but they are functionally
equivalent, or something is already wrong.

The next steps is to recompile:

| step | source | compiled with | produces |
|------+--------+---------------+----------|
| #4   | tcc    | tcc-B         | tcc-1    |
| #5   | tcc    | tcc-C         | tcc-2    |

And now, if everything is ok, then ’tcc-1’ and ’tcc-2’ must be
bit-identical.  Otherwise, the binaries ’clang’ *or* ’gcc’ are
compromised.  Assuming that the source code of ’tcc’ is audited and not
compromised. ;-)

If the source of the compilers used at step #1 and #2 are available,
then the same procedure can be applied to detect an attack.


Well, the idea is to implement the procedure with Guix: step #1,

  guix build tcc --with-c-toolchain=tcc=clang-toolchain

but then I do not know how to use the output to complete the step #2.
Is it possible to do it at the CLI level?  Or do I have to write some
Scheme?


Thank you in advance for any tips.

All the best,
simon

1: 
<https://lists.reproducible-builds.org/pipermail/rb-general/2020-October/002056.html>
2: 
<https://reproducible-builds.org/news/2019/12/21/reproducible-bootstrap-of-mes-c-compiler/>
3: <https://dwheeler.com/trusting-trust/#real-world>