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

Michael Clark via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 6 17:22:11 PDT 2017

From the top of ld64’s ld.cpp

// start temp HACK for cross builds
extern "C" double log2 ( double );
//#define __MATH__
// end temp HACK for cross builds

and a bit further down

	//fprintf(stderr, "FinalSection(%16s, %16s) _segmentOrder=%3d, _sectionOrder=0x%08X\n",
	//		this->segmentName(), this->sectionName(), _segmentOrder, _sectionOrder);

and a bit further down again ld64 uses qsort instead of std::sort

	//fprintf(stderr, "UNSORTED final sections:\n");
	//for (std::vector<ld::Internal::FinalSection*>::iterator it = sections.begin(); it != sections.end(); ++it) {
	//	fprintf(stderr, "final section %p %s/%s\n", (*it), (*it)->segmentName(), (*it)->sectionName());
	qsort(&sections[0], sections.size(), sizeof(FinalSection*), &InternalState::FinalSection::sectionComparer);
	//fprintf(stderr, "SORTED final sections:\n");
	//for (std::vector<ld::Internal::FinalSection*>::iterator it = sections.begin(); it != sections.end(); ++it) {
	//	fprintf(stderr, "final section %p %s/%s\n", (*it), (*it)->segmentName(), (*it)->sectionName());

I doubt that would pass the LLVM projects’ code review. I could also raise an issue or fix this ld64 bug if LLD mach-o was supported and it was in the LLVM bugzilla: “Invalid zero page virtual address when linking with -static -image_base 0x7ffe00000000”.

- https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99 <https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99>

I’m not using Objective-C so LLD may well fit my purposes. Now determined to try out LLD on macos :-D

> On 7 Jun 2017, at 11:30 AM, Rui Ueyama <ruiu at google.com> wrote:
> 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 <mailto: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 <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 <https://github.com/opensource-apple/xnu/blob/dc0628e187c3148723505cf1f1d35bb948d3195b/bsd/kern/kern_exec.c#L1072-L1111>
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