Re: [PATCH 21/25] mm: implement new mprotect_key() system call

On Mon, 2015-09-28 at 12:18 -0700, Dave Hansen wrote:

From: Dave Hansen <dave.hansen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>

mprotect_key() is just like mprotect, except it also takes a
protection key as an argument.  On systems that do not support
protection keys, it still works, but requires that key=0.

I'm not sure how userspace is going to use the key=0 feature? ie. userspace
will still have to detect that keys are not supported and use key 0 everywhere.
At that point it could just as well skip the mprotect_key() syscalls entirely
couldn't it?

I expect it to get used like this, if you want to guarantee that
any mapping you create can *never* be accessed without the right
protection keys set up.

	pkey_deny_access(11); // random pkey
	int real_prot = PROT_READ|PROT_WRITE;
	ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
	ret = mprotect_key(ptr, PAGE_SIZE, real_prot, 11);

This way, there is *no* window where the mapping is accessible
since it was always either PROT_NONE or had a protection key set.

We settled on 'unsigned long' for the type of the key here.  We
only need 4 bits on x86 today, but I figured that other
architectures might need some more space.

If the existing mprotect() syscall had a flags argument you could have just
used that. So is it worth just adding mprotect2() now and using it for this? ie:

int mprotect2(unsigned long start, size_t len, unsigned long prot, unsigned long flags) ..

And then you define bit zero of flags to say you're passing a pkey, and it's in
bits 1-63?

That way if other arches need to do something different you at least have the
flags available?

cheers