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Re: [PATCH] PR ld/19636: pie changes program behavior and generate unnecessary dynamic symbols
- From: Alan Modra <amodra at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Binutils <binutils at sourceware dot org>
- Date: Thu, 18 Feb 2016 17:01:24 +1030
- Subject: Re: [PATCH] PR ld/19636: pie changes program behavior and generate unnecessary dynamic symbols
- Authentication-results: sourceware.org; auth=none
- References: <20160215150032 dot GA26273 at gmail dot com> <20160217052322 dot GA14941 at gmail dot com> <CAMe9rOoOKPsrxf51pzBzouREZYP0TUyFNq2XTZGY61p3kzrEgA at mail dot gmail dot com> <20160218004245 dot GG31757 at bubble dot grove dot modra dot org> <CAMe9rOqhYY6R7gUgaw7L5ab-Rs=3VDCVdBEsGcGCWde4aA6a8Q at mail dot gmail dot com>
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 <firstname.lastname@example.org> 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 <email@example.com> 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.
Australia Development Lab, IBM