[llvm-dev] LLD support for mach-o aliases (weak or otherwise)

[Michael Clark via llvm-dev]

I believe I have solved the problem at least such that I can maintain source compatibility with musl’s use of a weak_alias macro and calls through the aliases. Thanks to Jameson who gave me some off-list advice. I would like to post this to the mailing list so that others who are trying to get weak aliases working on Mach-O might find this in Google versus what I found which was error messages and no apparent working solutions.

From the nm output, it seems the approach below is generating a strong direct alias, however in my case that is satisfactory as I don’t need interposition support, rather my goal is source compatibility. It seems weak_import has no effect when the symbol is defined in the same module. i.e. it does not create weak linkage, and if I don’t define the symbol with .set I get unresolved symbol on link.

It seems we need N_INDR support as per the blog post and in this commit, and some syntax to coax the assembler into making a weak indirect alias:

- http://blog.omega-prime.co.uk/?p=121 <http://blog.omega-prime.co.uk/?p=121>
- http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20140804/230120.html <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20140804/230120.html>

Here is my strong direct alias, versus ideally what should be a weak indirect alias. It works for my use case (source compat).


$ cat alias.c 
#include <stdio.h>

#if HAVE_ALIASES

#define weak_alias(old, new) \
        extern __typeof(old) new __attribute__((weak, alias(#old)))

#else

#define weak_alias(old, new) \
	__asm__(".globl _" #new); \
	__asm__(".set _" #new ", _" #old); \
	extern __typeof(old) new __attribute__((weak_import))

#endif

void __foo()
{
	printf("foo\n");
}

weak_alias(__foo, foo);

int main()
{
	foo();
}
$ cc -DHAVE_ALIASES=1 -c alias.c -o alias.o
alias.c:22:1: error: only weak aliases are supported on darwin
weak_alias(__foo, foo);
^
alias.c:6:55: note: expanded from macro 'weak_alias'
        extern __typeof(old) new __attribute__((weak, alias(#old)))
                                                      ^
1 error generated.
$ cc -c alias.c -o alias.o
$ nm -x alias.o 
0000000000000000 0f 01 0000 00000001 ___foo
0000000000000000 0f 01 0000 00000003 _foo
0000000000000020 0f 01 0000 00000008 _main
0000000000000000 01 00 0000 0000000e _printf
$ cc alias.o
$ ./a.out 
foo


> On 7 Jun 2017, at 11:08 AM, Michael Clark <michaeljclark at mac.com> 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.
> 
> 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>
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