On Wed, Apr 8, 2020 at 5:53 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Apr 7, 2020 at 4:58 PM Fangrui Song <i@maskray.me> wrote:
> >
> > On 2020-04-07, H.J. Lu wrote:
> > >On Tue, Apr 7, 2020 at 2:23 PM Fangrui Song <i@maskray.me> wrote:
> > >>
> > >> On 2020-04-07, H.J. Lu via Binutils wrote:
> > >> >On Tue, Apr 7, 2020 at 5:55 AM Andreas Schwab <schwab@linux-m68k.org> wrote:
> > >> >>
> > >> >> On Apr 07 2020, H.J. Lu wrote:
> > >> >>
> > >> >> > The optional argument VISIBILITY updates the visibility of
> > >> >> > the original symbol.  The valid visibilities are 'local', 'hidden', and
> > >> >> > 'remove'.  The 'local' visibility makes the original symbol a local
> > >> >> > symbol (*note Local::).  The 'hidden' visibility sets the visibility of
> > >> >> > the original symbol to 'hidden' (*note Hidden::).  The 'remove'
> > >> >> > visibility removes the original symbol from the symbol table.  If
> > >> >> > visibility isn't specified, the original symbol is unchanged.
> > >> >>
> > >> >> Looks good.
> > >> >
> > >> >Here is the updated patch.
> > >> >
> > >> >--
> > >> >H.J.
> > >>
> > >> Let me summarize the current status:
> > >>
> > >> @@@ has the renaming semantics. (invented in 2000)
> > >> @ and @@ has the copying semantics. The original symbol remains which is usually cumbersome.
> > >>
> > >> We have received some requests:
> > >>
> > >> * Have a way to not retain the original symbol
> > >> * Have a way to define multiple versions `.symver something,foo@v1; .symver something,foo@v2. symver something,foo@@v3`
> > >>
> > >>
> > >> We have many choices.
> > >>
> > >> A. make @ @@ similar to @@@
> > >>    For @@, because of the linking rule (a @@ definition can satisfy a
> > >>    referenced without a version), there should be no difference.
> > >>
> > >>    For @, this will be a semantic break. Personally I don't think this
> > >>    matters. I believe 99% projects don't need the original symbol and
> > >>    will not notice anything. I also checked with FreeBSD developers.
> > >
> > >The original symbol name is used in glibc to bypass PLT within
> > >libc.so.
> >
> > This does not seem correct. By convention, the hidden aliases are those prefixed with __
> > They are called to bypass PLT (STV_HIDDEN implies the non-preemptible property).
> > The original symbol does not have the prefix.
> >
> > When a linker sees memcpy@@GLIBC_2.14 , basically it inserts both "memcpy" and
> > "memcpy@GLIBC_2.14" into the symbol table.  Both a reference without a version
> > "memcpy" and a reference with a version "memcpy@GLIBC_2.14" can be satisfied.
>
> As I said before, the original purpose of .symver is for glibc to
> implement symbol
> versioning.  Given:
>
> ---
> const int _sys_nerr_internal = 200;
> __asm__ (".symver _sys_nerr_internal, sys_nerr@@GLIBC_2.12");
> ---
>
> _sys_nerr_internal is used internally in glibc:
>
> File: libc_pic.a(_strerror.os)
>
> Relocation section '.rela.text' at offset 0x14e0 contains 17 entries:
>     Offset             Info             Type               Symbol's
> Value  Symbol's Name + Addend
> 000000000000001c  0000001500000002 R_X86_64_PC32
> 0000000000000000 _sys_nerr_internal - 4
> 000000000000002c  0000001600000002 R_X86_64_PC32
> 0000000000000000 _sys_errlist_internal - 4
>
> ---
> char *
> __strerror_r (int errnum, char *buf, size_t buflen)
> {
>   ...
>   return (char *) _(_sys_errlist_internal[errnum]);
> }
> ---
>
> Also with -g, _sys_nerr_internal is also referenced in debug info.  We
> just can't change .symver to rename.
>
> > If the definition of "memcpy" also exists, I think it is somewhat special cased
> > in GNU ld and/or gold. In GNU ld, the actual implementation may be more complex
> > with indirect symbol involved. I believe the whole thing can be simplified a
> > lot by using renaming semantic. I debugged this area last year and may
> > misremember something.
>
> It is OK to extend .symver directive.  Change it to rename semantic isn't an
> option.
>

Updated patch to call symbol_remove to remove the symbol
if it isn't used in relocation.

-- 
H.J.