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

[Michael Clark via llvm-dev]

Hi Rui,

The motivation would be primarily that LLVM/Clang/LLD are community projects such that if I or someone in the community added support for e.g. symbol aliases, then it could be reviewed and potentially merged. ld64 on the other hand does not have a community process for patch submission and code review that I am aware of so its unlikely that if someone from the community came up with a patch to support aliases that it would be merged.

In that case I might check out the LLD code and try linking “x86_64-xnu-musl” with it. My requirements are likely simpler than Apple’s however I do need symbol aliases and these are not supported by ld64. The linker synthesised symbols are likely not too difficult to add if they are not present… now on my to do list…

Michael.

> 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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