Add mod_init_funcs to mac link section

[mitsuhiko]

Relates to the broken fix from #1. On my machine this makes the test pass again at least.

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=1466674

Reading that issue it sounds like the correct fix would be section __DATA,__mod_init_func,mod_init_funcs, but a symbol starting with I.

[BertalanD]

Hello @mitsuhiko !

LLD developer here that has been working on fixing that Chromium issue.

The __DATA_CONST segment isn't supposed to be used directly in object files; it's the linker's job to move certain sections there when targeting an OS version that supports it (macOS 10.15, iOS 13.0 and later, see -data_const in ld's manpage). The correct link_section value is therefore "__DATA,__mod_init_func".

gory details

It would actually work with __DATA_CONST too if mod_init_funcs was specified in the section directive, as it sets the S_MOD_INIT_FUNC_POINTERS flag. That is what LLD and Apple's linkers use to determine whether a section contains init pointers and needs to be processed accordingly. For __DATA,__mod_init_func, MC automatically recognizes that it needs to be set:

https://github.com/llvm/llvm-project/blob/02c7be5094fa3ffb3f118d9e2d69af435b7c8ef6/llvm/lib/MC/MCParser/DarwinAsmParser.cpp#L309-L312

When targeting macOS 13.0, iOS 13.4 or later, runtime relocations are emitted in the fixup chains format to allow page-in linking. As part of this the pointers (which need relocation) in __mod_init_func are transformed by the linker to __init_offsets, which contain relative offsets to the initializers (no relocs needed).

https://github.com/llvm/llvm-project/blob/02c7be5094fa3ffb3f118d9e2d69af435b7c8ef6/lld/MachO/InputSection.cpp#L57-L60

If the flag is not set, the section doesn't get transformed, so ends up as __mod_init_func in the binary; which is ignored by dyld if __init_offsets exists too.

Additionally, we fixed the issue where non-local symbols (i.e. whose assembly names that don't start with uppercase/lowercase L) in __mod_init_func caused a crash during linking: llvm/llvm-project#97156. So no changes are necessary in that front.