llvm dev - Jun 2017 - LLD support for mach-o aliases (weak or otherwise)

> On Jun 6, 2017, at 4:08 PM, Michael Clark via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi Folks,
> 
> I’m working on a port of musl libc to macos (arch triple is
“x86_64-xnu-musl”) to solve some irreconcilable issues I’m having with
libSystem.dylib. I don’t want to use glibc for various reasons, mainly because I
want to static link. I have static PIE + ASLR working which is not actually
supported by the Apple toolchain (*1), but I managed to get it to work. I’m sure
Apple might say “Don’t do that”, but from looking at the history of the xnu
kernel ABI, it seems to be very stable between versions.
I am from Apple, and I will say “Don’t do that.” The kernel ABI for our
platforms is not stable, we only guarantee stability at the dynamic link
boundary (in this case public symbols exported from libSystem). While the kernel
syscall numbers have not changed (though the kernel team reserves the right to
do that), the parameter lists and argument marshaling for them certainly has
changed. We also do not support static executables on our system.

We even had bincompat issues related to this i rolled their during the last
major release (macOS 10.12 Sierra): Go implemented its own syscall support,
which caused all of their binaries that used gettimeofday since the internal
interface changed <https://github.com/golang/go/issues/16606
<https://github.com/golang/go/issues/16606>>. More broadly you can look
at a discussion of their issues here:
<https://github.com/golang/go/issues/16606
<https://github.com/golang/go/issues/16606>>. In their case they want
to avoid invoking an external linker like ld64 or lld as opposed to avoiding
libSystem, but the effect is the same, they shipped a tool that caused
unsuspecting developers to have surprise bincompat issues that were entirely
avoidable.

Ultimately if you are doing this on your own for your own for fun thats great,
but if this something you intend to ship to other people please reconsider. It
is more than a theoretical concern that it will break.

Louis
> In any case the musl libc source makes extensive use of weak aliases,
perhaps to allow easier interposition of C library routines, however aliases,
weak or otherwise are not currently supported by ld64.
> 
> It appears that the mach-o format supports aliases, but the functionality
has not been exposed via the linker (ld64/LLD).
> 
> - http://blog.omega-prime.co.uk/?p=121
<http://blog.omega-prime.co.uk/?p=121>
> 
> The musl code does the following which currently errors out saying aliases
are not currently supported:
> 
> #undef weak_alias
> #define weak_alias(old, new) \
>         extern __typeof(old) new __attribute__((weak, alias(#old)))
> 
> and the macro is used internally like this:
> 
> int __pthread_join(pthread_t t, void **res)
> {
>         // implementation here
> }
> 
> weak_alias(__pthread_join, pthread_join);
> 
> The problem is the actual export used by clients is an alias and I want to
maintain source compatibility.
> 
> I seem to have found a way to semi-emulate aliases (at least within one
module). My goal is to at least turn them into strong aliases somehow, so I can
at a minimum make the musl source compatible with clang on macos. The following
compiles but foo is not exported:
> 
> $ cat a.c
> #include <stdio.h>
> 
> void foo() __attribute__((weak_import)) __asm("_bar");
> 
> void bar()
> {
>         printf("bar\n");
> }
> 
> int main()
> {
>         foo();
> }
> 
> $ cc -c a.c -o a.o
> $ nm a.o
> 0000000000000000 T _bar
> 0000000000000020 T _main
>                  U _printf
> 
> Any ideas how I can get foo as an exported symbol? 
> 
> Is weak alias or plan alias support planned for mach-o in LLD?
> 
> The goal at a minimum is to make the weak_alias macro emit a strong alias
with clang/ld64 or clang/LLD? so I don’t need to diverge too much from the
upstream musl source (as the lack of alias support currently requires me to
rename function declarations in the source). Of course pthreads which I’m
working on now are going to be completely different… but musl has support for
architecture specific overrides in its build system.
> 
> BTW I now have some quite non-trivial programs compiling against musl-xnu +
libcxx + libcxxabi on macos.
> 
> There are a lot of libcxx changes like this:
> 
> -#ifdef __APPLE__
> +#if defined(__APPLE__) && !defined(_LIBCPP_HAS_MUSL_LIBC)
> 
> Michael.
> 
> [1] https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99
<https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99>_______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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> On Jun 14, 2017, at 2:47 PM, Michael Clark via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
>> 
>> On 15 Jun 2017, at 6:50 AM, Louis Gerbarg <lgerbarg at apple.com
<mailto:lgerbarg at apple.com>> wrote:
>> 
>>> 
>>> On 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’m working on a port of musl libc to macos (arch triple is
“x86_64-xnu-musl”) to solve some irreconcilable issues I’m having with
libSystem.dylib. I don’t want to use glibc for various reasons, mainly because I
want to static link. I have static PIE + ASLR working which is not actually
supported by the Apple toolchain (*1), but I managed to get it to work. I’m sure
Apple might say “Don’t do that”, but from looking at the history of the xnu
kernel ABI, it seems to be very stable between versions.
>> 
>> I am from Apple, and I will say “Don’t do that.” The kernel ABI for our
platforms is not stable, we only guarantee stability at the dynamic link
boundary (in this case public symbols exported from libSystem). While the kernel
syscall numbers have not changed (though the kernel team reserves the right to
do that), the parameter lists and argument marshaling for them certainly has
changed. We also do not support static executables on our system.
>> 
>> We even had bincompat issues related to this i rolled their during the
last major release (macOS 10.12 Sierra): Go implemented its own syscall support,
which caused all of their binaries that used gettimeofday since the internal
interface changed <https://github.com/golang/go/issues/16606
<https://github.com/golang/go/issues/16606>>. More broadly you can look
at a discussion of their issues here:
<https://github.com/golang/go/issues/16606
<https://github.com/golang/go/issues/16606>>. In their case they want
to avoid invoking an external linker like ld64 or lld as opposed to avoiding
libSystem, but the effect is the same, they shipped a tool that caused
unsuspecting developers to have surprise bincompat issues that were entirely
avoidable.
> 
> I’m aware that Go has had some issues with the XNU ABI boundary.
> 
> I’m working on a CPU simulator / binary translator and I need control of
the process address space layout. It seems I may ultimately need to use
Hypervisor.framework however that is a lot more work in the short term.
I actually thought about mentioning Hypervisor.framework, but I was not sure
about your use case. If you need full control of the address space (PAGE_ZERO
control, overriding the shared cache mappings, etc) that is really the only
supported mechanism.
> The issue I am having with libSystem.dylib is the lack of weak linkage
(versus weak_import) i.e. weak aliases. I don’t want to use a wrapper binary
with DYLD_INSERT_LIBRARIES. I want to interpose Libc symbols with some of the
symbols present in my binary (memory allocator, mmap). Interposition support is
somewhat lacking in the Mach-O toolchain and runtime linker despite the Mach-O
format technically supporting what I need (N_INDR and N_WEAK_DEF).
> 
> - https://developer.apple.com/documentation/kernel/nlist_64
<https://developer.apple.com/documentation/kernel/nlist_64>
Dyld does not generally use nlists at runtime except for things like dladdr(),
and has not for the last 10 years or so. Instead dyld uses a trie to publish
exports, and and a small byte code language to describe binding imports. We
still support using nlists for old binaries, but anything built with recent
tools also contains the newer trie and bind op codes which will be used if they
are available. I do not think our tries can express the sort of import semantics
you want.

I see two ways of potentially doing it (short of hypervisor.framework). Both of
them are a bit gross and have some bincompat risks, but given you are an open
source project and can rev if need be that may not be an enormous issue:

You could specify a custom segment in your executable with zero file size and a
vm size the blocks out the address range you need and then unmap it. There may
be practical limits on it that prevent you from achieving what you want.

You could use implicit interposing. This is a feature add so that ASAN binaries
can avoid the the whole re-exec with DYLD_INSERT_LIBRARIES issue. It is not
guaranteed to be stable, but in practice it is probably the most stable option
short of using a hypervisor. The way it works:

Define all the symbols in a dylib along with an interpose section (as though you
were going to load it with DYLD_INSERT_LIBRARIES). Directly link your executable
to binary. Dyld will discover the interpose during dependency analysis (before
libSystem initializers are run) and apply the interpose. This has only been
tested in the case of our sanitizer runtimes, but it *SHOULD* work.
> 
> If I could use N_INDR and N_WEAK_DEF to have early bound (runtime link
time) interposition with symbols in my binary replacing the C library allocator
and mmap, and have libSystem use my implementations then I would be happy.
libSystem itself would need to use weak aliases. This is possible with C
libraries on other platforms.
> 
> I’ve tried relentlessly to intercept the malloc_zone implementation.
malloc_zone_register is not sufficient as some of the internal zones are tied to
the internals of Libc and I am getting heap collisions with Libc allocated
objects and my guest address space. On Linux I have enough control to do what I
need and can interpose my symbols to implement versions of libc functions that I
wish to override. The problem on darwin is that I am not able to interpose the
malloc implementation until main starts, and at that point it is tool late as
the C library already has created its internal zones. I’m also unable to
interpose mmap. I have already looked at the interpose symbol tricks but they
don’t meet my purposes  (not wanting to re-exec with DYLD_INSERT_LIBRARIES).
Weak aliases from libSystem to the allocator implementation and various public
symbols along with N_INDR and N_WEAK_DEF would be required for me to achieve
what I need to achieve (somewhat similarly to the elegant internal
implementation of musl libc).
> 
> With my current solution (musl on xnu) I have successfully reserved 0x1000
– 0x7fff_0000_0000. Essentially the low 128TiB minus 4GiB at the top of the
address space where I place my translator and translator stack. This is
satisfactory for my user mode simulator to emulate Linux processes on macOS.
> 
> I think Hypervisor.framework is probably the correct interface to be using
if I want to avoid the kernel ABI, however that is a lot more work that making
syscall wrappers and I would need to implement communication from VM process to
the host process.
I think that this is probably your best choice from a binary compatibility
standpoint in the long run. It is a lot of work, though I am not sure if it is
really that much more work than trying to port a new libc or maintain a custom
toolchain.

Louis
> 
> I’m actually implementing Linux syscall emulation in a user simulator so
the kernel ABI is probably the technically correct layer. The full system
emulator ultimately needs to use Hypervisor.framework if I am to use hardware
paging instead of soft MMU. I have two simulators, a user-mode sim that emulates
the Linux ABI and a full system emulator: https://rv8.io/
<https://rv8.io/> and I really want to support RISC-V Linux on macOS in
the user mode simulator.
> 
> Proper Linux ABI emulation on macOS would ultimately require kernel
support, at minimum something like binfmt misc, but ideally a kext that
implements another ABI personality (much like Linux ABI emulation on Windows) in
addition to the BSD personality. In fact the FreeBSD linux compat could be used
if the FreeBSD portion of XNU is synced up with current, and we’d get bug fixes
for long standing issues like the macOS TCP_NOPUSH bug that has long since been
fixed in FreeBSD.
> 
>> Ultimately if you are doing this on your own for your own for fun thats
great, but if this something you intend to ship to other people please
reconsider. It is more than a theoretical concern that it will break.
>> 
>> Louis
>> 
>>> In any case the musl libc source makes extensive use of weak
aliases, perhaps to allow easier interposition of C library routines, however
aliases, weak or otherwise are not currently supported by ld64.
>>> 
>>> It appears that the mach-o format supports aliases, but the
functionality has not been exposed via the linker (ld64/LLD).
>>> 
>>> - http://blog.omega-prime.co.uk/?p=121
<http://blog.omega-prime.co.uk/?p=121>
>>> 
>>> The musl code does the following which currently errors out saying
aliases are not currently supported:
>>> 
>>> #undef weak_alias
>>> #define weak_alias(old, new) \
>>>         extern __typeof(old) new __attribute__((weak, alias(#old)))
>>> 
>>> and the macro is used internally like this:
>>> 
>>> int __pthread_join(pthread_t t, void **res)
>>> {
>>>         // implementation here
>>> }
>>> 
>>> weak_alias(__pthread_join, pthread_join);
>>> 
>>> The problem is the actual export used by clients is an alias and I
want to maintain source compatibility.
>>> 
>>> I seem to have found a way to semi-emulate aliases (at least within
one module). My goal is to at least turn them into strong aliases somehow, so I
can at a minimum make the musl source compatible with clang on macos. The
following compiles but foo is not exported:
>>> 
>>> $ cat a.c
>>> #include <stdio.h>
>>> 
>>> void foo() __attribute__((weak_import)) __asm("_bar");
>>> 
>>> void bar()
>>> {
>>>         printf("bar\n");
>>> }
>>> 
>>> int main()
>>> {
>>>         foo();
>>> }
>>> 
>>> $ cc -c a.c -o a.o
>>> $ nm a.o
>>> 0000000000000000 T _bar
>>> 0000000000000020 T _main
>>>                  U _printf
>>> 
>>> Any ideas how I can get foo as an exported symbol? 
>>> 
>>> Is weak alias or plan alias support planned for mach-o in LLD?
>>> 
>>> The goal at a minimum is to make the weak_alias macro emit a strong
alias with clang/ld64 or clang/LLD? so I don’t need to diverge too much from the
upstream musl source (as the lack of alias support currently requires me to
rename function declarations in the source). Of course pthreads which I’m
working on now are going to be completely different… but musl has support for
architecture specific overrides in its build system.
>>> 
>>> BTW I now have some quite non-trivial programs compiling against
musl-xnu + libcxx + libcxxabi on macos.
>>> 
>>> There are a lot of libcxx changes like this:
>>> 
>>> -#ifdef __APPLE__
>>> +#if defined(__APPLE__) && !defined(_LIBCPP_HAS_MUSL_LIBC)
>>> 
>>> Michael.
>>> 
>>> [1]
https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99
<https://gist.github.com/michaeljclark/0a805652ec4be987a782afb902f06a99>_______________________________________________
>>> 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>
> _______________________________________________
> LLVM Developers mailing list
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