[llvm-dev] Targeting old glibc

[Alexandre Bique via llvm-dev]

On Tue, Nov 10, 2020 at 7:01 PM Fāng-ruì Sòng <maskray at google.com> wrote:
> > How to find what is pulling libmvec?
>
> If you build a -DCMAKE_BUILD_TYPE=Debug lld, you can set a breakpoint
> on lld/ELF/MarkLive.cpp:114 (ss->getFile().isNeeded) and check which
> symbol makes
> libmvec.so needed.

I'm afraid this is going to far for me :)

> > On the other hand, I don't understand why I have to specify that I
> > want exp from glibc 2.2 or 2.29, I just want the exp symbol, and if
> > possible the latest version available on the system.
> > Because I don't have a high precision requirement so they would all be
> > fine. Is that possible?
>
> Symbol versioning is complex ;-) While the runtime behavior is
> documented by Linux Standard Base Core Specification, Generic Part,
> there is no good documentation on the linker behavior.
> LLD has evolved to the current state after contributors "observe GNU
> ld behaviors and patch LLD when needed" :(

I think there might be some discussions needed with
glibc/coreutils/gcc/llvm because it is way too complicated.
I believe that targeting an older glibc version should not be more
complicated than
 -DGLIBC_COMPAT_LEVEL=0x020205 -Wl,compat=GLIBC_2.2.5

On OSX there is a target platform command line option, and I believe
it is the right approach, yet I can't really comment on it.

> For an undefined symbol, there is only this form: sym at ver
> For a defined symbol, both sym at ver (non-default; sometimes called
> hidden) and sym@@ver (default) exist.
>
> For a defined symbol named sym at ver, it only resolves an undefined
> symbol sym at ver.
> For a defined symbol named sym@@ver, it resolves both undefined
> sym at ver and undefined sym.
>
> If you have glibc 2.29, there are exp at GLIBC_2.2.5 (non-default) and
> exp@@GLIBC_2.29 (default), your 'exp' undefined symbol
> will resolve to exp@@GLIBC_2.29 (default) and not work with older glibc.
>
> If you know that your translation unit does not define 'exp' (usual
> case), you can write
>
> .symver exp,exp at GLIBC_2.2.5
> call exp
> or
> .symver exp,exp@@@GLIBC_2.2.5
> call exp
>
> Then the undefined symbol 'exp' will be renamed to 'exp at GLIBC_2.2.5'
> and get bound to the old version at link time. It will thus work with
> old glibc.

I have one .cpp file with:

__asm__(".symver powf,powf at GLIBC_2.2.5");
__asm__(".symver expf,expf at GLIBC_2.2.5");
__asm__(".symver exp2f,exp2f at GLIBC_2.2.5");
__asm__(".symver log2f,log2f at GLIBC_2.2.5");
__asm__(".symver logf,logf at GLIBC_2.2.5");

__asm__(".symver log,log at GLIBC_2.2.5");
__asm__(".symver log2,log2 at GLIBC_2.2.5");
__asm__(".symver exp,exp at GLIBC_2.2.5");
__asm__(".symver exp2,exp2 at GLIBC_2.2.5");
__asm__(".symver pow,pow at GLIBC_2.2.5");

But it does not solve the issue. I've tried many variations.

Do you know if it is possible after the link, to edit the shared
library and replace the dependency on GLIBC 2.29 to GLIBC 2.2.5?

Regards,
Alexandre