V3 [PATCH] gas: Extend .symver directive

[Fangrui Song]

On 2020-04-08, H.J. Lu wrote:
>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.

OK.

>> > 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.

Is the second .symver required to be after the definition?

Case A

.symver foo,foo@@v2
.symver foo,foo@v1

.globl foo
foo:
   ret

% as-new a.s
a.s: Assembler messages:
a.s: Error: local label `"0" (instance number 0 of a dollar label)' is not defined

Case B

.globl foo
foo:
   ret

.symver foo,foo@@v2
.symver foo,foo@v1
.hidden foo

      5: 0000000000000000     0 NOTYPE  GLOBAL HIDDEN     1 foo
      6: 0000000000000000     0 NOTYPE  GLOBAL HIDDEN     1 foo@@v2
      7: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    1 foo@v1