Re: [Ping~][1/9][RFC][DWARF] Reserve three DW_OP numbers in vendor extension space

[Jiong Wang <jiong dot wang at foss dot arm dot com>]

Jiong Wang writes:

> On 16/11/16 14:02, Jakub Jelinek wrote:
>> On Wed, Nov 16, 2016 at 02:54:56PM +0100, Mark Wielaard wrote:
>>> On Wed, 2016-11-16 at 10:00 +0000, Jiong Wang wrote:
>>>>   The two operations DW_OP_AARCH64_paciasp and DW_OP_AARCH64_paciasp_deref were
>>>> designed as shortcut operations when LR is signed with A key and using
>>>> function's CFA as salt.  This is the default behaviour of return address
>>>> signing so is expected to be used for most of the time.  DW_OP_AARCH64_pauth
>>>> is designed as a generic operation that allow describing pointer signing on
>>>> any value using any salt and key in case we can't use the shortcut operations
>>>> we can use this.
>>>
>>> I admit to not fully understand the salting/keying involved. But given
>>> that the DW_OP space is really tiny, so we would like to not eat up too
>>> many of them for new opcodes. And given that introducing any new DW_OPs
>>> using for CFI unwinding will break any unwinder anyway causing us to
>>> update them all for this new feature. Have you thought about using a new
>>> CIE augmentation string character for describing that the return
>>> address/link register used by a function/frame is salted/keyed?
>>>
>>> This seems a good description of CIE records and augmentation
>>> characters:http://www.airs.com/blog/archives/460
>>>
>>> It obviously also involves updating all unwinders to understand the new
>>> augmentation character (and possible arguments). But it might be more
>>> generic and saves us from using up too many DW_OPs.
>>
>> From what I understood, the return address is not always scrambled, so
>> it doesn't apply to the whole function, just to most of it (except for
>> an insn in the prologue and some in the epilogue).  So I think one op is
>> needed.  But can't it be just a toggable flag whether the return address
>> is scrambled + some arguments to it?
>> Thus DW_OP_AARCH64_scramble .uleb128 0 would mean that the default
>> way of scrambling starts here (if not already active) or any kind of
>> scrambling ends here (if already active), and
>> DW_OP_AARCH64_scramble .uleb128 non-zero would be whatever encoding you need
>> to represent details of the less common variants with details what to do.
>> Then you'd just hook through some MD_* macro in the unwinder the
>> descrambling operation if the scrambling is active at the insns you unwind
>> on.
>>
>>       Jakub
>
> Hi Mark, Jakub:
>
>    Thanks very much for the suggestions.
>
>    I have done some experiments on your ideas and am thinking it's good to
>    combine them together.  The use of DW_CFA instead of DW_OP can avoid building
>    all information from scratch at each unwind location, while we can indicate
>    the signing key index through new AArch64 CIE augmentation 'B'. This new
>    approach reduce the unwind table size overhead from ~25% to ~5% when return
>    address signing enabled, it also largely simplified dwarf generation code for
>    return address signing.
>
>    As one new DWARF call frame instruction is needed for AArch64, I want to reuse
>    DW_CFA_GNU_window_save to save the space.  It is in vendor extension space and
>    used for Sparc only, I think it make sense to reuse it for AArch64. On
>    AArch64, DW_CFA_GNU_window_save toggle return address sign status which kept
>    in a new boolean type column in DWARF table,  so DW_CFA_GNU_window_save takes
>    no argument on AArch64, the same as on Sparc, this makes no difference to those
>    existed encoding, length calculation code.
>
>    Meanwhile one new DWARF expression operation number is still needed for
>    AArch64, it's useful for describing those complex pointer signing scenarios
>    and it will be used to multiplex some further extensions on AArch64.
>
>    OK on this proposal and to install this patch to gcc trunk?
>
> Hi GDB, Binutils maintainer:
>
>    OK on this proposal and install this patch to binutils-gdb master?
>
> include/
> 2016-11-29   Richard Earnshaw  <rearnsha@arm.com>
>               Jiong Wang  <jiong.wang@arm.com>
>
>          * dwarf2.def (DW_OP_AARCH64_operation): Reserve the number 0xea.

Ping~

Thanks.

-- 
Regards,
Jiong