Re: [PATCH v34 10/24] mm: Add vm_ops->mprotect()

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Wed, Jul 08, 2020 at 03:37:08PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 08, 2020 at 05:33:20PM +0300, Jarkko Sakkinen wrote:
> > I get the point but I don't think that your proposal could work given
> > that mprotect-callback takes neither 'prev' nor 'newflags' as its
> > parameters. The current callback has no means to call mprotect_fixup()
> > properly.
> > 
> > It would have to be extended
> > 
> > 	int (*mprotect)(struct vm_area_struct *vma,
> > 			struct vm_area_struct **pprev, unsigned long start,
> > 			unsigned long end, unsigned long prot,
> > 			unsigned long newflags);
> > 
> > Is this what you want?
> 
> https://lore.kernel.org/linux-mm/20200625173050.GF7703@casper.infradead.org/

Ugh, it's there as it should be. I'm sorry - I just misread the code.

I think that should work, and we do not have to do extra conversion
inside the callback.

There is still one thing that I'm wondering. 'page->vm_max_prot_bits'
contains some an union of subset of {VM_READ, VM_WRITE, VM_EXEC}, and
'newflags' can contain other bits set too.

The old implementation of sgx_vma_mprotect() is like this:

static int sgx_vma_mprotect(struct vm_area_struct *vma, unsigned long start,
			    unsigned long end, unsigned long prot)
{
	return sgx_encl_may_map(vma->vm_private_data, start, end,
				calc_vm_prot_bits(prot, 0));
}

The new one should be probably the implemented along the lines of

static int sgx_vma_mprotect(struct vm_area_struct *vma,
			    struct vm_area_struct **pprev, unsigned long start,
			    unsigned long end, unsigned long newflags)
{
	unsigned long masked_newflags = newflags &
					(VM_READ | VM_WRITE | VM_EXEC);
	int ret;

	ret = sgx_encl_may_map(vma->vm_private_data, start, end,
				   masked_newflags);
	if (ret)
		return ret;

	return mprotect_fixup(vma, pprev, start, end, newflags);
}

Alternatively the filtering can be inside sgx_encl_may_map(). Perhaps
that is a better place for it. This was just easier function to
represent the idea.

/Jarkko