[gnutls-help] Bug#964284: guile-gnutls: update to use guile 3.0

[Ludovic Courtès]

Hi Daiki,

Daiki Ueno <ueno at gnu.org> skribis:

> Ludovic Courtès <ludo at gnu.org> writes:
>
>> This is because Guile >= 3.0.1 and >= 2.2.7 changes the GMP allocation
>> functions such that they go through libgc¹.  As a result, libgc may
>> reuse that memory when it becomes unreachable from its point of view; in
>> this case, since GnuTLS structures are not scanned by libgc, libgc
>> doesn’t “see” pointers to those bignums and thus considers they are no
>> longer reachable.
>
> That's interesting, though I might not follow completely.
>
>>   • In Guile-GnuTLS, arrange so that GnuTLS allocations are made through
>>     libgc.  Unfortunately, ‘gnutls_global_set_mem_functions’ was
>>     deprecated in GnuTLS 3.3.0 so this doesn’t look like an option.
>
> GnuTLS doesn't call mp_set_memory_functions, so even if it is possible,
> I doubt that it would affect the current behavior.

What happens currently is that data structures allocated by GnuTLS are
not scanned by the GC; thus, when those data structures refer to an
‘mpz_t’, the GC doesn’t “see” those references.

If we could arrange so that GnuTLS allocates via libgc, those ‘mpz_t’
references would become visible to the GC, which would solve this issue.

Would it be an option?

> On the other hand, if GnuTLS (or Nettle) internally allocates an
> mpz_t, it should be done using the libgc-backed allocator set by Guile
> and the pointers should be reachable until it is no longer, if I
> understand correctly.  Therefore, I suspect that there might be some
> code that confuses libgc to track the pointers; one thing that comes
> to my mind is a manual copy of mpz_t values:
> https://gitlab.com/gnutls/gnutls/-/blob/master/lib/nettle/pk.c#L141
>

> If you replace memcpy with mpz_init_set, does it work?

I tried the patch below but it doesn’t help.

Thanks for taking a look!

Ludo’.

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