[llvm-dev] LLD support for ld64 mach-o linker synthesised symbols

[Rui Ueyama via llvm-dev]

Hi Michael,

The Mach-O version of LLD is not being developed actively, and if some
feature is missing, it is likely that it's just not implemented. What is
your motivation to use LLD instead of ld64?

On Tue, Jun 6, 2017 at 4:08 PM, Michael Clark via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Folks,
>
> I have a question regarding LLD support for ld64 mach-o linker synthesised
> symbols. I did a quick search of the LLD source and I can not find support
> for them so before I start trying to use lld I thought I would ask.
>
> I have found a couple of cases where they are essential. i.e. where there
> is no other way to get the required information, such as getting the
> address of the mach-o headers of the current process, with ASLR enabled, if
> the process is not dyld as exec on macOS only provides the mach header
> address to dyld (*1). They are used inside of dyld and I am now using them
> in “x86_64-xnu-musl”.
>
> It’s possible to resolve a mach-o segment offset or a mach-o section
> offset using these special ld64 linker synthesised symbols. See
> resolveUndefines:
>
> - https://opensource.apple.com/source/ld64/ld64-274.2/
> src/ld/Resolver.cpp.auto.html
>
> There are 4 special symbol prefixes for the mach-o linker synthesised
> symbols:
>
> - segment$start$__SEGMENT
> - segment$end$__SEGMENT
> - section$start$__SEGMENT$__section
> - section$end$__SEGMENT$__section
>
> In asm:
>
> /* get imagebase and slide for static PIE and ASLR support in
> x86_64-xnu-musl */
>
> .align 3
> __image_base:
> .quad segment$start$__TEXT
> __start_static:
> .quad start
> .text
> .align 3
> .global start
> start:
>        xor %rbp,%rbp
>        mov %rsp,%rdi
>        andq $-16,%rsp
>        movq __image_base(%rip), %rsi
>        leaq start(%rip), %rdx
>        subq __start_static(%rip), %rdx
>        call __start_c
>
>
> In C:
>
> /* run C++ constructors in __libc_start_main for x86_64-xnu-musl */
>
> typedef void (*__init_fn)(int, char **, char **, char **);
> extern __init_fn  __init_start  __asm("section$start$__DATA$__
> mod_init_func");
> extern __init_fn  __init_end    __asm("section$end$__DATA$__
> mod_init_func”);
>
> static void __init_mod(int argc, char **argv, char **envp, char **applep)
> {
>         for (__init_fn *p = &__init_start; p < &__init_end; ++p) {
>                 (*p)(argc, argv, envp, applep);
>         }
> }
>
>
> Michael.
>
> [1] https://github.com/opensource-apple/xnu/blob/
> dc0628e187c3148723505cf1f1d35bb948d3195b/bsd/kern/kern_exec.c#L1072-L1111
>
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>
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