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

[Dave Hansen]

On 9/9/20 3:08 PM, Yu, Yu-cheng wrote:
> After looking at this more, I found the changes are more similar to
> mprotect() than madvise().  We are going to change an anonymous mapping
> to a read-only mapping, and add the VM_SHSTK flag to it.  Would an
> x86-specific mprotect(PROT_SHSTK) make more sense?
> 
> One alternative would be requiring a read-only mapping for
> madvise(MADV_SHSTK).  But that is inconvenient for the application.

Why?  It's just:

	mmap()/malloc();
	mprotect(PROT_READ);
	madvise(MADV_SHSTK);

vs.

	mmap()/malloc();
	mprotect(PROT_SHSTK);

I'm not sure a single syscall counts as inconvenient.

I don't quite think we should use a PROT_ bit for this.  It seems like
the kind of thing that could be fragile and break existing expectations.
 I don't care _that_ strongly though.