[llvm-dev] Targeting old glibc

[Alexandre Bique via llvm-dev]

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