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

Alan Modra amodra@gmail.com
Mon Feb 22 02:53:00 GMT 2016 
On Sat, Feb 20, 2016 at 09:59:24PM -0800, H.J. Lu wrote:
> On Wed, Feb 17, 2016 at 10:31 PM, Alan Modra <amodra@gmail.com> wrote:
> > 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 <amodra@gmail.com> wrote:
> >> > 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!
> 
> I moved it to x86 backend via elf_backend_fixup_symbol.

Where it is just as wrong as anywhere else.  You are changing the
behaviour of the following program.

cat > undefweak.c <<EOF
extern void __attribute__ ((weak)) fun (void);
int main (void)
{
  if (&fun != 0)
    fun ();
  return 0;
}
EOF
gcc -c -fPIC -O2 undefweak.c
gcc -o undefweak undefweak.o

If some later version of libc.so defines "fun", then with your change,
"fun" won't be called.  I can't cite real-world examples of this
behaviour off the top of my head, where "fun" is likely to be some
function multiple levels deep in a tree of user shared libraries, but
I'm almost certain you will break some people's expectations.

-- 
Alan Modra
Australia Development Lab, IBM
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