[PATCH] libio: Properly link in libio functions in static binaries

[Florian Weimer]

* H. J. Lu:

> On Thu, Aug 21, 2025 at 11:49 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * H. J. Lu:
>>
>> > commit 3020f72618e4f1d7338cd42b8bc7b2813e961b5a
>> > Author: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
>> > Date:   Tue Dec 27 18:11:43 2022 -0300
>> >
>> >     libio: Remove the usage of __libc_IO_vtables
>> >
>> > added
>> >
>> >  #define libio_static_fn_required(name) __asm (".globl " #name);
>> >
>> > to link in libio functions in static binaries.  But there is no relocation
>> > in
>> >       .globl  _IO_file_open
>> >
>> > and "strip --strip-unneeded" will remove such unreferenced symbols which
>> > breaks static binaries.  Redefine libio_static_fn_required to create a
>> > reference to the required function with
>> >
>> >       .pushsection ".data.rel.ro.local","aw"
>> > .L_IO_file_open:
>> >       .dc.a   _IO_file_open
>> >       .popsection
>> >
>> > This fixes BZ #33300.
>>
>> Is it possible to use SHF_RETAIN instead?
>
> SHF_GNU_RETAIN is used to keep an unused section for --gc-sections while
> this one is to include an object from libc.a.  Even if the section with
> _IO_file_doallocate is marked with SHF_GNU_RETAIN, it won't be linked in
> without a reference to _IO_file_doallocate.

Hmm.  The --strip-unneeded option says this:

       --strip-unneeded
           Remove all symbols that are not needed for relocation processing in
           addition to debugging symbols and sections stripped by
           --strip-debug.

I think it's a user error to use this on libc.a.  What's the benefit of
using this option?

Thanks,
Florian