Re: RFC: Update x86 psABI to support shadow stac

[Florian Weimer <fweimer@redhat.com>]

On 07/27/2017 06:21 PM, H.J. Lu wrote:
> On Thu, Jul 27, 2017 at 8:44 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 06/28/2017 01:21 PM, H.J. Lu wrote:
>>> On Wed, Jun 28, 2017 at 2:58 AM, Florian Weimer <fweimer@redhat.com> wrote:
>>>> On 06/22/2017 08:44 PM, H.J. Lu wrote:
>>>>>> The responsibilities for compliance are split between caller and callee,
>>>>>> which can live in different shared objects.  I think it would be prudent
>>>>>> to formulate the requirement in such a way that compliance can be
>>>>>> checked by looking at one DSO in isolation.
>>>>
>>>>> What do you mean by it?
>>>>
>>>> I suggest to word the ABI requirement in such a way that it is possible
>>>> to verify if a shared object complies with it isolation, independent of
>>>> how its functions are called.
>>>>
>>>
>>> 99% of existing binaries are compatible with shadow stack.
>>
>> I find that surprising, or does this number to refer to x86-64 binaries
>> only?
> 
> CET is x86 specific.  You can take a look at the current CET changes for
> GCC at
> 
> https://github.com/hjl-tools/gcc/tree/hjl/cet/reorg16

So i386 is supported?  Then I find your claim about 99% compatibility
surprising because LLVM uses this instruction sequence

	calll	.L0$pb
.L0$pb:
	popl	%ebx
.Ltmp0:
	addl	$_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %ebx

to set %ebx to the GOT pointer.

Older GCC did as well for some CPU tunings, until:

  https://gcc.gnu.org/ml/gcc-patches/2011-06/msg02295.html

Thanks,
Florian