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Re: [PATCH] PR ld/19636: pie changes program behavior and generate unnecessary dynamic symbols

On Wed, Feb 17, 2016 at 08:24:06PM -0800, H.J. Lu wrote:
> On Wed, Feb 17, 2016 at 4:42 PM, Alan Modra <> wrote:
> > On Wed, Feb 17, 2016 at 03:33:51PM -0800, H.J. Lu wrote:
> >> On Tue, Feb 16, 2016 at 9:23 PM, H.J. Lu <> wrote:
> >> > On Mon, Feb 15, 2016 at 07:00:32AM -0800, H.J. Lu wrote:
> >> >> I fixed x86.  Other backends need similar fix.  Any comments?
> >
> > If you aren't going to fix other backends too, have you at least run
> > the testsuite for all the other ELF targets that use SYMBOLIC_BIND?
> Should I enable them for all ELF target with PIE/PIC support?
> All of them fail since none of them handle PIE correctly.

No, I meant the existing testsuite, not your new tests.  These changes
are quite likely to break existing tests.

> >> -/* Will a symbol be bound to the definition within the shared
> >> -   library, if any.  A unique symbol can never be bound locally.  */
> >> -#define SYMBOLIC_BIND(INFO, H) \
> >> -    (!(H)->unique_global \
> >> -     && ((INFO)->symbolic || ((INFO)->dynamic && !(H)->dynamic)))
> >> +/* Will a symbol be bound to the definition within the PIC object, if
> >> +   any.  A unique symbol can never be bound locally.  Symbols are always
> >> +   bound locally in PIE, similar to -shared -Bsymbolic.  */
> >> +#define SYMBOLIC_BIND(INFO, H)                                               \
> >> +    (!(H)->unique_global                                             \
> >> +     && ((INFO)->symbolic                                            \
> >> +      || ((INFO)->dynamic && !(H)->dynamic)                          \
> >> +      || bfd_link_pie (INFO)))
> >
> > This probably should be bfd_link_executable rather than bfd_link_pie.
> SYMBOLIC_BIND was intended for shared library and is used only when
> PIC is true.  It is never applied to bfd_link_hash_undefweak.  That is why
> I added bfd_link_pie.  Change it to bfd_link_executable is a good start if
> we want to extend it to executable where bfd_link_hash_undefweak is a
> special case.  bfd_link_hash_undefweak always binds locally in executable
> and never binds locally in shared library.   Should we extend SYMBOLIC_BIND
> to cover all cases so that we don't have check PIC nor undefweak before
> using it?

The reason I suggested bfd_link_executable was simply that it is a
property of ELF executables in general that symbols bind locally, not
just a property of PIEs.  I wouldn't try to extend SYMBOLIC_BIND to
other cases just yet, and your claim about bfd_link_hash_undefweak is
not true.  That was the whole point of my comment below.

> > This change is incorrect.  Some targets (even x86_64 with -fPIC)
> > support
> >   if (fun)
> >     fun ();
> > for an undefined weak fun at link time, making fun dynamic and
> > emitting dynamic relocs against fun.  So when the executable is linked
> > against a new shared library that provides fun, fun will be called.

> Yes, it should be moved before

No, that change should not be added at all!

> It exposed more issues in x86 backends :-).

I'm not surprised.  :)  I tried something similar many years ago, but
ran into "issues" in the powerpc backends, and decided I didn't care
enough about a few extra symbols.

Alan Modra
Australia Development Lab, IBM

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