linkage_checker: replace Fiddle.dlopen with libSystem call

[carlocab]

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dlopening a library executes potentially untrusted code (e.g. if the
library has initialisers). We can avoid the dlopen call by asking
libSystem directly about whether a library can be found in the shared
cache.

Of course, the dlopen happens after a ENOENT, so the attack surface here
is relatively small. But relying on this still exposes us to a potential
TOCTOU¹ bug. Let's avoid it entirely by skipping dlopen altogether.

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brew typecheck currently returns an error. Looks like the RBI for Fiddle will need updating.

Footnotes

1. https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use ↩

[MikeMcQuaid]

Good idea!

[Bo98]

Thanks! Been meaning to look into this for a while.

[carlocab]

Added an RBI patterned after upstream's to handle with the typecheck errors. Not sure if that's the best way forward.

[Bo98]

Usually we add it to upstream.rbi along with a comment pointing to an upstream PR that updates fiddle.rbi to match brew ruby -e 'puts Fiddle.constants.sort.filter { |c| c.match?(/^(TYPE|SIZEOF)_/) }'

Seems like it's missing quite a few.