[llvm-dev] Targeting old glibc

[Fāng-ruì Sòng via llvm-dev]

On Tue, Nov 10, 2020 at 12:27 AM Alexandre Bique
<bique.alexandre at gmail.com> wrote:
>
> Thank you very much for your help Fāng-ruì Sòng.
>
> I've tried various things like linking directly to libm-2.32 but it
> didn't work, it seems that libm-2.32 still pulls libmvec.
>
> I have the following linker flags:
>
> CLANG_LDFLAGS="-fuse-ld=lld -static-libstdc++ -static-libgcc
> -fvisibility=hidden -fdata-sections -ffunction-sections"
> CLANG_LDFLAGS="$CLANG_LDFLAGS -Wl,--gc-sections -Wl,-O1
> -Wl,--as-needed -Wl,--strip-all"
> CLANG_LDFLAGS="$CLANG_LDFLAGS -Wl,-y,exp,-y,exp at GLIBC_2.29,-y,exp at GLIBC_2.2.5"
>
> 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.

> 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" :(

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.

> Regards,
> Alexandre Bique
>
> On Mon, Nov 9, 2020 at 7:06 PM Fāng-ruì Sòng <maskray at google.com> wrote:
> >
> > On Mon, Nov 9, 2020 at 10:04 AM Fāng-ruì Sòng <maskray at google.com> wrote:
> > >
> > > No. A shared object is needed if it is linked in (1)--no-as-needed
> > > mode or (2) after --gc-sections, a symbol referenced by a live input
> > > section  is defined by the shared object.
> >
> > A shared object is needed if it is linked in (1) --no-as-needed mode
> > or (2) after --gc-sections, a symbol referenced by a live input
> > section  is defined by the shared object as non-STB_WEAK.
> >
> > > You'll need to check whether a symbol is defined by libmvec.so.1
> > >
> > > On Mon, Nov 9, 2020 at 9:57 AM Alexandre Bique
> > > <bique.alexandre at gmail.com> wrote:
> > > >
> > > > On Mon, Nov 9, 2020 at 6:52 PM Fāng-ruì Sòng <maskray at google.com> wrote:
> > > > > Recent libm.a and libm.so are actually linker scripts
> > > > >
> > > > > % cat /usr/lib/x86_64-linux-gnu/libm.a
> > > > > /* GNU ld script
> > > > > */
> > > > > OUTPUT_FORMAT(elf64-x86-64)
> > > > > GROUP ( /usr/lib/x86_64-linux-gnu/libm-2.31.a
> > > > > /usr/lib/x86_64-linux-gnu/libmvec.a )
> > > > > % cat /usr/lib/x86_64-linux-gnu/libm.so
> > > > > /* GNU ld script
> > > > > */
> > > > > OUTPUT_FORMAT(elf64-x86-64)
> > > > > GROUP ( /lib/x86_64-linux-gnu/libm.so.6  AS_NEEDED (
> > > > > /lib/x86_64-linux-gnu/libmvec.so.1 ) )
> > > >
> > > > In this script the libmvec is mentioned as "AS_NEEDED", and it is not
> > > > needed in my case, yet lld pulls its dependency toward exp at GLIBC_2.29.
> > > >
> > > > Is it a bug in lld?
> > > >
> > > > Regards,
> > > > Alexandre
> > >
> > >
> > >
> > > --
> > > 宋方睿
> >
> >
> >
> > --
> > 宋方睿



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