[PATCH] "used" attribute saves decl from linker garbage collection

[H.J. Lu]

On Fri, Nov 6, 2020 at 4:01 PM Jeff Law <law@redhat.com> wrote:
>
>
> On 11/6/20 4:45 PM, H.J. Lu wrote:
> > On Fri, Nov 6, 2020 at 3:37 PM Jeff Law <law@redhat.com> wrote:
> >>
> >> On 11/6/20 4:29 PM, H.J. Lu wrote:
> >>> On Fri, Nov 6, 2020 at 3:22 PM Jeff Law <law@redhat.com> wrote:
> >>>> On 11/5/20 7:34 AM, H.J. Lu via Gcc-patches wrote:
> >>>>> On Thu, Nov 5, 2020 at 3:37 AM Jozef Lawrynowicz
> >>>>> <jozef.l@mittosystems.com> wrote:
> >>>>>> On Thu, Nov 05, 2020 at 06:21:21AM -0500, Hans-Peter Nilsson wrote:
> >>>>>>> On Wed, 4 Nov 2020, H.J. Lu wrote:
> >>>>>>>> .retain is ill-defined.   For example,
> >>>>>>>>
> >>>>>>>> [hjl@gnu-cfl-2 gcc]$ cat /tmp/x.c
> >>>>>>>> static int xyzzy __attribute__((__used__));
> >>>>>>>> [hjl@gnu-cfl-2 gcc]$ ./xgcc -B./ -S /tmp/x.c -fcommon
> >>>>>>>> [hjl@gnu-cfl-2 gcc]$ cat x.s
> >>>>>>>> .file "x.c"
> >>>>>>>> .text
> >>>>>>>> .retain xyzzy  <<<<<<<<< What does it do?
> >>>>>>>> .local xyzzy
> >>>>>>>> .comm xyzzy,4,4
> >>>>>>>> .ident "GCC: (GNU) 11.0.0 20201103 (experimental)"
> >>>>>>>> .section .note.GNU-stack,"",@progbits
> >>>>>>>> [hjl@gnu-cfl-2 gcc]$
> >>>>>>> To answer that question: it's up to the assembler, but for ELF
> >>>>>>> and SHF_GNU_RETAIN, it seems obvious it'd tell the assembler to
> >>>>>>> set SHF_GNU_RETAIN for the section where the symbol ends up.
> >>>>>>> We both know this isn't rocket science with binutils.
> >>>>>> Indeed, and my patch handles it trivially:
> >>>>>> https://sourceware.org/pipermail/binutils/2020-November/113993.html
> >>>>>>
> >>>>>>   +void
> >>>>>>   +obj_elf_retain (int arg ATTRIBUTE_UNUSED)
> >>>>>>   .... snip ....
> >>>>>>   +  sym = get_sym_from_input_line_and_check ();
> >>>>>>   +  symbol_get_obj (sym)->retain = 1;
> >>>>>>
> >>>>>>   @@ -2624,6 +2704,9 @@ elf_frob_symbol (symbolS *symp, int *puntp)
> >>>>>>     }
> >>>>>>        }
> >>>>>>
> >>>>>>   +  if (symbol_get_obj (symp)->retain)
> >>>>>>   +    elf_section_flags (S_GET_SEGMENT (symp)) |= SHF_GNU_RETAIN;
> >>>>>>   +
> >>>>>>      /* Double check weak symbols.  */
> >>>>>>      if (S_IS_WEAK (symp))
> >>>>>>        {
> >>>>>>
> >>>>>> We could check that the symbol named in the .retain directive has
> >>>>>> already been defined, however this isn't compatible with GCC
> >>>>>> mark_decl_preserved handling, since mark_decl_preserved is called
> >>>>>> emitted before the local symbols are defined in the assembly output
> >>>>>> file.
> >>>>>>
> >>>>>> GAS should at least validate that the symbol named in the .retain
> >>>>>> directive does end up as a symbol though.
> >>>>>>
> >>>>> Don't add .retain.
> >>>> Why?  I don't see why you find it so objectionable.
> >>>>
> >>> An ELF symbol directive should operate on symbol table:
> >>>
> >>> http://www.sco.com/developers/gabi/latest/ch4.symtab.html
> >>>
> >>> not the section flags where the symbol is defined.
> >> I agree in general, but I think this is one of those cases where it's
> >> not so clear.  And what you're talking about is an implementation detail.
> > There is no need for such a hack.  The proper thing to do in ELF is
> > to place such a symbol in a section with SHF_GNU_RETAIN flag.   This
> > also avoids the question what to do with SHN_COMMON.
>
> I'm not sure that's a good idea either.  Moving symbols into a section
> other than they'd normally live doesn't seem all that wise.

In ELF, a symbol must be defined in a section.  If we want to keep a symbol,
we should place it in an SHF_GNU_RETAIN section.

>
> Let's face it, there's not a great solution here.  If we mark its
> existing section, then everything in that section gets kept.  If we put

FWIW, this is what .retain direct does and is one reason why I object
it.

> the object into a different section than it would normally live, then
> that opens a whole new can of worms.

We should place it in a section which it normally lives in and mark the
section with SHF_GNU_RETAIN.

-- 
H.J.