Re: [PATCH] i386: Remove PLT0 and use non-lazy PLT if PLT0 is unused

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, May 6, 2017 at 11:55 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 05/06/2017 05:11 PM, H.J. Lu wrote:
>>
>> 1. Call external function.
>> 2. Call internal IFUNC function.  The best implementation is selected
>> for the target processor at run-time.
>> 3. Act as the canonical function address.
>
>
> Audit support is missing from this list.  Audit support needs a PLT stub
> which calls the audit trampoline with some sort of function index or
> address.

This behavior disables LD_AUDIT support with -z now, in addition to

export LD_BIND_NOW=1

or compiling with -fno-plt.  Also should LD_AUDIT work on external
calls inside shared objects linked with -z now?

> I'm concerned that -z now in recent binutils (even without this patch)
> removes audit support, when before, audit support was not impacted by -z now
> (I think, I'm not sure on this point).
>
> We really must avoid hardening flags which have profound semantic
> implications on generated binaries because it discourages people from
> enabling them.

The impacts for removing PLT0 with -z now are

Cons:

1. LD_AUDIT no longer works.

Pros:

1. Make the run-time behavior the same with "export LD_BIND_NOW=1".
2. Disable LD_AUDIT to further improve security.

I can add a new "-z audit" option to enable LD_AUDIT as much as
possible, including

--
extern void foo (void);

void *
foo_p ()
{
  foo ();
  return foo;
}
--

-- 
H.J.