[PATCH] gold: --export-dynamic-symbol: don't imply -u

[Fangrui Song]

On Sat, Jun 13, 2020 at 6:08 PM Fāng-ruì Sòng via Binutils
<binutils@sourceware.org> wrote:
>
> On Sat, Jun 13, 2020 at 5:23 PM Cary Coutant <ccoutant@gmail.com> wrote:
> >
> > > to match GNU ld.
> > >
> > > gold/
> > >
> > >         * archive.cc (Library_base::should_include_member): Don't handle
> > >         --export-dynamic-symbol.
> > >         * symtab.cc (Symbol_table::do_add_undefined_symbols_from_command_line):
> > >         Likewise.
> >
> > You say we're doing this to match Gnu ld, but Gnu ld didn't implement
> > this option until several years after it was added to Gold. Shouldn't
> > the burden of compatibility be in the other direction? I'm not
> > comfortable changing the behavior of --export-dynamic-symbol like
> > this, and my memory isn't good enough to remember what the rationale
> > was for making it work like -u -- whether it was just "it seems like
> > it makes sense" or "there's a specific need for it to work that way".
> >
> > -cary
>
> I did try to be compatible with Gold for my initial patch but I
> quickly realized not implying -u makes more sense.
> https://sourceware.org/pipermail/binutils/2020-May/110953.html
>
> This option does not appear to have open source uses (according to
> Debian Code Search) since GNU ld did not implement it until a few days
> ago.
> The updated semantics did cause some trouble to me - I had to fix a
> few --export-dynamic-symbol= uses for my company. Everything is good
> now.
>
> My conjecture about the design of implied -u: we do ugly things like
> --export-dynamic-symbol=_Unwind_Resume (and a few other symbols), so
> that the executable exposes unwind symbols
> which can prevent glibc's ugly dlopen("libgcc.so.1") . If
> --export-dynamic-symbol= implies -u, things are easier:
>
> we can write: -Wl,--export-dynamic-symbol=_Unwind_Backtrace
> -Wl,--export-dynamic-symbol=_Unwind_Resume
> instead of: -Wl,--export-dynamic-symbol=_Unwind_Backtrace
> -Wl,--export-dynamic-symbol=_Unwind_Resume -Wl,-u,_Unwind_Backtrace
> -Wl,-u,_Unwind_Resume
>
> -----
>
> There are still a few differences between gold and GNU ld:
>
> * --export-dynamic-symbol=non_exist adds an undefined symbol in .symtab
> * --export-dynamic-symbol does not take a glob in gold
> * --export-dynamic-symbol does not make symbol preemptable in -shared
> link if -Bsymbolic or -Bsymbolic-functions or --dynamic-list is
> specified.
>
> ---
>
> A short summary of --export-dynamic-symbol's intended behavior:
>
> -no-pie or -pie: matched non-local defined symbols will be added to
> the dynamic symbol table.
> -shared: matched non-local STV_DEFAULT symbols will not be bound to
> definitions within the shared object even if they would otherwise be
> due to -Bsymbolic, -Bsymbolic-functions, or --dynamic-list.

Ping :)