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

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 7 20:30:27 PDT 2017

On Wed, Jun 7, 2017 at 2:46 PM, Michael Clark via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 8 Jun 2017, at 4:53 AM, Rui Ueyama <ruiu at google.com> wrote:
> 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.
> That’s actually good news!
> If there is a Mach-O linker that is able to self host Clang builds on
> macOS, then this is a really good starting point.
> From reading a tiny bit about the history, and the LLVM pages on the
> design of the various linkers, it seems like there is a difference in
> opinion with respect to the Atom based design of the Mach-O LLD, and
> whether or not there was to be an abstract design that supports ELF,
> PE/COFF and Mach-O. It seems not. One would also assume that LTO and/or
> -ffunction-sections -fdata-sections would obviate the need for Atoms, and
> that it may in fact increase the complexity of the linker.

InputSection (ELF) and Chunk (COFF) are basically "atoms". The main
technical obstacle to using atoms for ELF and COFF is that the atom model
used in the original linker design assumes a 1:1 mapping of symbols to
"atoms" (and the symbol points to the start of the atom). In ELF and COFF
symbols and InputSection/Chunk's are decoupled because you can have
multiple symbols pointing anywhere in the InputSection/Chunk.

> From my cursory examination of the source it seems that lld/lib should
> perhaps be renamed lld/MachO and become the MachO linker besides the ELF
> and COFF directproes as the common code is not being used by the ELF and
> the PE/COFF linkers.

The original linker design used by MachO had greater ambitions than the
current ELF and COFF designs. It was more aiming for the sort of linker
model explained in Paul Bowen-Hugett's talk at the 2016 LLVM developer
meeting https://youtu.be/-pL94rqyQ6c?t=20m29s

One way to think about this is that ld64 is already a fast linker that is
controlled by the people that work on LLD at Apple. So there isn't much
incentive to do what ELF and COFF have done which is at this point getting
to a production quality linker program. The original hope for LLD was to go
beyond ld64's capabilities to enable new and interesting use cases (see the
talk I linked above for some examples). However, for ELF and COFF there
wasn't a linker controlled by the LLVM community for those platforms, and
so merely reimplementing existing linker programs (with some extra
attention to QoI and being modern) was an interesting enough goal in and of
itself to push their development. The LLVM community did not want to wait
to get ELF and COFF working pending the materialization of next generation
linker use cases; simply meeting the requirements of existing linker uses
cases was sufficient.

(Note: we still aren't aware of any concrete analysis or experiment
demonstrating real benefit to these "next generation linker use cases";
many of them seem quite interesting, but under closer inspection there are
a lot of issues that haven't been fully explored)

Anyway, that was a very long way of saying that the MachO linker is
actually a very different design and even source organization (it was
intended to be factored along certain library boundaries, but we haven't
seen any uses cases that would use that), so that moving it to lld/MachO
doesn't really make much sense.

> I just need to figure out how to build and invoke the Mach-O linker. There
> is no ‘ld’ in the llvm bin directory as one would be led to believe. I’ll
> dig into the CMakeLists.txt. I guess lld/lib//Driver/DarwinLdDriver.cpp
> is the entry point. lld//lib/Driver/CMakeLists.txt however only appears
> to define a library, versus an executable and there is no top level MachO
> directory like there is for the other 2 linkers.
> $ lld
> lld is a generic driver.
> Invoke ld.lld (Unix), ld (Mac) or lld-link (Windows) instead.
> $ ld.lld --version
> LLD 4.0.0
> $ lld-link --version
> ignoring unknown argument: --version
> error: no input files
> If I know which CMakeLists.txt defines the binary that hosts the main
> function and installs it, then I can take it from there.

You can see the logic that it uses in lld/tools/lld/lld.cpp

To access the MachO linker, you will want to either run `lld -flavor darwin
...` or invoke lld through a symlink such that argv[0] is `ld` (this is
only enabled when LLD is compiled to run on an Apple host machine (#if

I guess we could install an `ld64` symlink to access the MachO linker, but
the actual system linker on macOS is never actually invoked via the name
`ld64` (that's just a name for the linker itself; not the binary; the
binary is always `ld`).

-- Sean Silva

> 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_i
>>> nit_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
>>> _______________________________________________
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