[lld] [lld][MachO] Support for -interposable (PR #131813)

[Shoaib Meenai via llvm-commits]

smeenai wrote:

If you're checking for lazy symbol bindings, targeting newer OS versions uses [chained fixups](https://github.com/qyang-nj/llios/blob/main/dynamic_linking/chained_fixups.md) by default, which gets rid of lazy bindings. The difference between interposable and non-interposable calls is whether they go through a stub, i.e.

```
$ cat interposable_test.c
void foo() {}
void bar() { foo(); }

$ clang -c interposable_test.c
$ ld -dylib -o normal.dylib interposable_test.o
$ ld -dylib -interposable -o interposable.dylib interposable_test.o

$ objdump -d normal.dylib

normal.dylib:	file format mach-o arm64

Disassembly of section __TEXT,__text:

0000000000003f88 <_foo>:
    3f88: d65f03c0     	ret

0000000000003f8c <_bar>:
    3f8c: a9bf7bfd     	stp	x29, x30, [sp, #-0x10]!
    3f90: 910003fd     	mov	x29, sp
    3f94: 97fffffd     	bl	0x3f88 <_foo>
    3f98: a8c17bfd     	ldp	x29, x30, [sp], #0x10
    3f9c: d65f03c0     	ret

$ objdump -d interposable.dylib

interposable.dylib:	file format mach-o arm64

Disassembly of section __TEXT,__text:

0000000000003f7c <_foo>:
    3f7c: d65f03c0     	ret

0000000000003f80 <_bar>:
    3f80: a9bf7bfd     	stp	x29, x30, [sp, #-0x10]!
    3f84: 910003fd     	mov	x29, sp
    3f88: 94000003     	bl	0x3f94
    3f8c: a8c17bfd     	ldp	x29, x30, [sp], #0x10
    3f90: d65f03c0     	ret

Disassembly of section __TEXT,__stubs:

0000000000003f94 <__stubs>:
    3f94: b0000010     	adrp	x16, 0x4000
    3f98: f9400210     	ldr	x16, [x16]
    3f9c: d61f0200     	br	x16
```

The stub is what enables the interposing to work, and the binding for the stub can be lazy or non-lazy.

You can repeat the same experiment with an executable and confirm that `-interposable` applies to it, as you said :) Could you just change the test to check that the call is going through a stub instead of checking for a lazy binding? Then this looks good to me.

https://github.com/llvm/llvm-project/pull/131813