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Re: RFC: Update x86 psABI to support shadow stac

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

>> It is hard
>> to tell just by looking at assembly instructions.  If shadow stack is enabled,
>> compiler should turn on the SHSTK bit in output:
>>
>> [hjl@gnu-tools-1 32]$ readelf -n crtprec32.o
>>
>> Displaying notes found in: .note.gnu.property
>>   Owner                 Data size Description
>>   GNU                  0x0000000c NT_GNU_PROPERTY_TYPE_0
>>       Properties: x86 feature: IBT
>>   GNU                  0x0000000c NT_GNU_PROPERTY_TYPE_0
>>       Properties: x86 feature: SHSTK
>> [hjl@gnu-tools-1 32]$
>>
>> I don't know if it is sufficient for verification.
>
> The ABI document needs to specify what the flag means.  I don't think
> it's sufficient to essentially say, “the toolchain did or did not do
> some unspecified stuff and we believe the binary is now compatible with
> the shadow stack feature”.
>

Please see CET x86-64 psABI:

https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI

and let me know if they are sufficient.

Thanks.

-- 
H.J.