Re: [RFC PATCH 00/17] objtool: add base support for arm64

[Madhavan T. Venkataraman]

On 1/22/21 3:43 PM, Ard Biesheuvel wrote:
> On Fri, 22 Jan 2021 at 22:15, Madhavan T. Venkataraman
> <madvenka@xxxxxxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 1/22/21 11:43 AM, Mark Brown wrote:
>>> On Thu, Jan 21, 2021 at 12:54:52PM -0600, Josh Poimboeuf wrote:
>>>
>>>> 2) The shadow stack idea sounds promising -- how hard would it be to
>>>>    make a prototype reliable unwinder?
>>>
>>> In theory it doesn't look too hard and I can't see a particular reason
>>> not to try doing this - there's going to be edge cases but hopefully for
>>> reliable stack trace they're all in areas where we would be happy to
>>> just decide the stack isn't reliable anyway, things like nesting which
>>> allocates separate shadow stacks for each nested level for example.
>>> I'll take a look.
>>>
>>
>> I am a new comer to this discussion and I am learning. Just have some
>> questions. Pardon me if they are obvious or if they have already been
>> asked and answered.
>>
>> Doesn't Clang already have support for a shadow stack implementation for ARM64?
>> We could take a look at how Clang does it.
>>
>> Will there not be a significant performance hit? May be, some of it can be
>> mitigated by using a parallel shadow stack rather than a compact one.
>>
>> Are there any longjmp style situations in the kernel where the stack is
>> unwound by several frames? In these cases, the shadow stack must be unwound
>> accordingly.
>>
> 
> Hello Madhavan,
> 
> Let's discuss the details of shadow call stacks on a separate thread,
> instead of further hijacking Julien's series.
> 

OK. Sounds good.