Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Dave Hansen
2020-08-25T19:19:45Z

On 8/25/20 11:43 AM, Yu, Yu-cheng wrote:
>>> arch_prctl(ARCH_X86_CET_MMAP_SHSTK, u64 *args)
>>>      Allocate a new shadow stack.
>>>
>>>      The parameter 'args' is a pointer to a user buffer.
>>>
>>>      *args = desired size
>>>      *(args + 1) = MAP_32BIT or MAP_POPULATE
>>>
>>>      On returning, *args is the allocated shadow stack address.
>>
>> This is hideous.  Would this be better as a new syscall?
>
> Could you point out why this is hideous, so that I can modify the
> arch_prctl?

Passing values in memory is hideous when we don't have to. A syscall
would let you have separate arguments for size and flags and would also
let you have a nice return value instead of needing to do that in memory
too.

> I think this is more arch-specific.  Even if it becomes a new syscall,
> we still need to pass the same parameters.

Right, but without the copying in and out of memory.