linux-kernel - Re: [PATCH v34 11/24] x86/sgx: Add SGX enclave driver

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Date:   Tue, 7 Jul 2020 05:39:04 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Seth Moore <sethmo@...gle.com>,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com,
        josh@...htriplett.org, kai.huang@...el.com, kai.svahn@...el.com,
        kmoy@...gle.com, ludloff@...gle.com, luto@...nel.org,
        nhorman@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com
Subject: Re: [PATCH v34 11/24] x86/sgx: Add SGX enclave driver

On Mon, Jul 06, 2020 at 09:29:04PM -0700, Sean Christopherson wrote:
> > > > +	idx_start = PFN_DOWN(start);
> > > > +	idx_end = PFN_DOWN(end - 1);
> > > > +
> > > > +	for (idx = idx_start; idx <= idx_end; ++idx) {
> > > > +		mutex_lock(&encl->lock);
> > > > +		page = radix_tree_lookup(&encl->page_tree, idx);
> > > > +		mutex_unlock(&encl->lock);
> > > > +
> > > > +		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > > > +			return -EACCES;
> > > 
> > > You should really use an iterator here instead of repeated lookups.
> > > xas_for_each() will probably be what you want.
> > 
> > Thank you for your remarks. I'll look into using xarray for this.
> 
> Question for Matthew:
> 
> To enforce the "page must be populated" rule, is there a clean way to retrieve
> the index of the current entry?  Our entries/pages don't have information
> about their index.  Or should we just count the number of entries and check
> 'em at the end? E.g.
> 
>         xas_for_each(...) {
>                 if (~page->vm_max_prot_bits & vm_prot_bits)
>                         return -EACCES;
>                 nr_entries++;
>         }
> 
>         if (nr_entries != (end_index - start_index))
>                 return -EACCES;

Probably best just to steal the implementation from here:

pgoff_t page_cache_next_miss(struct address_space *mapping,
                             pgoff_t index, unsigned long max_scan)
{
        XA_STATE(xas, &mapping->i_pages, index);

        while (max_scan--) {
                void *entry = xas_next(&xas);
                if (!entry || xa_is_value(entry))
                        break;
                if (xas.xa_index == 0)
                        break;
        }

        return xas.xa_index;
}

although I think you have a simpler task.

	XA_STATE(xas, ..., start_index);

	for (;;) {
		struct page *page = xas_next(&xas);

		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
			return -EACCES;
	}

	return 0;

should do the trick, I think.