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

[Dave Hansen <dave.hansen@intel.com>]

On 8/26/20 11:49 AM, Yu, Yu-cheng wrote:
>> I would expect things like Go and various JITs to call it directly.
>>
>> If we wanted to be fancy and add a potentially more widely useful
>> syscall, how about:
>>
>> mmap_special(void *addr, size_t length, int prot, int flags, int type);
>>
>> Where type is something like MMAP_SPECIAL_X86_SHSTK.  Fundamentally,
>> this is really just mmap() except that we want to map something a bit
>> magical, and we don't want to require opening a device node to do it.
> 
> One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*.
> Does ARM have similar needs for memory mapping, Dave?

No idea.

But, mmap_special() is *basically* mmap2() with extra-big flags space.
I suspect it will grow some more uses on top of shadow stacks.  It could
have, for instance, been used to allocate MPX bounds tables.