stack arenas using alloca

[Florian Weimer]

* Michael Clark via Gcc:

> On 8/23/24 15:57, Michael Clark wrote:
>> On 8/23/24 15:46, Michael Clark wrote:
>>> one more thing. it doesn't require PT_GNU_STACK or writable stacks
>>> like GCC nested functions. 🙂 so I think it is safer but it does
>>> have safety issues, mostly related to stack overflows but its going
>>> to need some careful analysis with respect to ROP.
>> brain isn't working. of course it needs writable stacks. it doesn't
>> need executable stacks. starting to think about [[allocareturn]]
>> ROP...
>
> if you loaded the return address from the fixed frame in the epilogue
> into a temporary during restore of callee save registers which usually
> unwinds the dynamic offset, you could translate the return instruction
> into an indirect branch.

It poisons the return branch predictor, so it's really bad for
performance and not just incompatible with shadow stacks.

For architectures without a link register, it's probably better to copy
the return address to an elevated position on the stack and use a
regular return instruction.

Thanks,
Florian