[1/9][RFC][DWARF] Reserve three DW_OP numbers in vendor extension space

Richard Earnshaw (lists) Richard.Earnshaw@arm.com
Thu Dec 1 10:42:00 GMT 2016


On 30/11/16 21:43, Cary Coutant wrote:
> How about if instead of special DW_OP codes, you instead define a new
> virtual register that contains the mangled return address? If the rule
> for that virtual register is anything other than DW_CFA_undefined,
> you'd expect to find the mangled return address using that rule;
> otherwise, you would use the rule for LR instead and expect an
> unmangled return address. The earlier example would become (picking an
> arbitrary value of 120 for the new virtual register number):
> 
>         .cfi_startproc
>    0x0  paciasp (this instruction sign return address register LR/X30)
>         .cfi_val 120, DW_OP_reg30
>    0x4  stp     x29, x30, [sp, -32]!
>         .cfi_offset 120, -16
>         .cfi_offset 29, -32
>         .cfi_def_cfa_offset 32
>    0x8  add     x29, sp, 0
> 
> Just a suggestion...

What about signing other registers?  And what if the value is then
copied to another register?  Don't you end up with every possible
register (including the FP/SIMD registers) needing a shadow copy?

R.

> 
> -cary
> 
> 
> On Wed, Nov 16, 2016 at 6:02 AM, Jakub Jelinek <jakub@redhat.com> 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



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