[llvm-dev] Targeting old glibc

[Alexandre Bique via llvm-dev]

I'm following up with libc-help:
https://sourceware.org/pipermail/libc-help/2020-November/005555.html
Alexandre Bique

On Thu, Nov 12, 2020 at 11:01 AM Alexandre Bique
<bique.alexandre at gmail.com> wrote:
>
> On Wed, Nov 11, 2020 at 10:37 PM Fāng-ruì Sòng <maskray at google.com> wrote:
> > If you want to drop symbol versioning with llvm-objcopy:
> >
> > * llvm-objcopy -R .gnu.version -R .gnu.version_r in.so out.so
> > * However, llvm-objcopy zeroes out the section content so at runtime glibc ld.so will error
> >    "unsupported version 0 of Verneed record".
> >    Thus we need to drop the DT_VERNEED tag as well.
> >    To achieve this, we can rewrite the DT_VERNEED tag to DT_NULL (0), terminating
> >    the dynamic table, effectively deleting all DT_VER* tags. DT_RELACOUNT is also
> >    deleted. Fortunately its absence does not affect correctness.
> >    This requires binary patching
> >
> >    ...
> >    0x000000006ffffffb (FLAGS_1)            Flags: NOW
> > - 0x000000006ffffffe (VERNEED)            0x8ef0
> > - 0x000000006fffffff (VERNEEDNUM)         5
> > - 0x000000006ffffff0 (VERSYM)             0x89c0
> > - 0x000000006ffffff9 (RELACOUNT)          1536
> >    0x0000000000000000 (NULL)               0x0
> >
> >
> > In the end, the solution is:
> >
> > r2 -wqc '/x feffff6f00000000 @ section..dynamic; w0 16 @ hit0_0' a.so; llvm-objcopy -R .gnu.version -R .gnu.version_r a.so
>
> Thank you very much for your help.
>
> If I run:
>   r2 -wqc '/x feffff6f00000000 @ section..dynamic; w0 16 @ hit0_0' a.so
> It results in a plugin that does not load anymore because: undefined
> symbol __at_exit, version
>
> Then if I continue with llvm-objcopy -R .gnu.version -R .gnu.version_r
> a.so; then the plugins can be dlopen() but when used something goes
> wrong and the plugin aborts.
>
> I'm not certain about continuing in this direction because it is
> getting very involved and if anything goes wrong difficult to support.
>
> Thank you so much Fāng-ruì Sòng for all your help and patience.
>
> Best regards,
> Alexandre BIQUE