llvm dev - Jun 2017 - LLD support for ld64 mach-o linker synthesised symbols

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>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
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OK. I see that the Mach-O linker is not even built when LLD is enabled in
Release_40, only the PE/COFF and ELF linkers are built.

From looking at reviews it appears that Clang was able to be linked with LLD on
Darwin about 2 years ago, so Mach-O support seems to have regressed.

Curious as to pointers to primordial branches with whatever needs to be
resurrected. I couldn’t find any Mach-O cmake flags to enable its build. A
pointer to a branch or tag that might have a working Mach-O LLD would be a
start.
> On 7 Jun 2017, at 11:38 AM, Michael Clark <michaeljclark at mac.com>
wrote:
> 
> 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
<mailto: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>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> 
>> 
> 
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On Tue, Jun 6, 2017 at 11:14 PM, Michael Clark via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> OK. I see that the Mach-O linker is not even built when LLD is enabled in
> Release_40, only the PE/COFF and ELF linkers are built.
>
> From looking at reviews it appears that Clang was able to be linked with
> LLD on Darwin about 2 years ago, so Mach-O support seems to have regressed.
>
Only a few changes have been made to the Mach-O port in the last two years,
so I'd doubt if it has regressed. It could be the case that clang's
output
has changed in such a way that the linker is not able to handle it.

> Curious as to pointers to primordial branches with whatever needs to be
> resurrected. I couldn’t find any Mach-O cmake flags to enable its build. A
> pointer to a branch or tag that might have a working Mach-O LLD would be a
> start.
>
>
> On 7 Jun 2017, at 11:38 AM, Michael Clark <michaeljclark at mac.com>
wrote:
>
> 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> 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/dc0628e187c
>> 3148723505cf1f1d35bb948d3195b/bsd/kern/kern_exec.c#L1072-L1111
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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