[PATCH 14/17] prmem: llist, hlist, both plain and rcu

[Tycho Andersen]

On Wed, Oct 24, 2018 at 05:03:01PM +0300, Igor Stoppa wrote:
> On 24/10/18 14:37, Mathieu Desnoyers wrote:
> > Also, is it the right approach to duplicate existing APIs, or should we
> > rather hook into page fault handlers and let the kernel do those "shadow"
> > mappings under the hood ?
> 
> This question is probably a good candidate for the small Q&A section I have
> in the 00/17.
> 
> 
> > Adding a new GFP flags for dynamic allocation, and a macro mapping to
> > a section attribute might suffice for allocation or definition of such
> > mostly-read-only/seldom-updated data.
> 
> I think what you are proposing makes sense from a pure hardening standpoint.
> From a more defensive one, I'd rather minimise the chances of giving a free
> pass to an attacker.
> 
> Maybe there is a better implementation of this, than what I have in mind.
> But, based on my current understanding of what you are describing, there
> would be few issues:
> 
> 1) where would the pool go? The pool is a way to manage multiple vmas and
> express common property they share. Even before a vma is associated to the
> pool.
> 
> 2) there would be more code that can seamlessly deal with both protected and
> regular data. Based on what? Some parameter, I suppose.
> That parameter would be the new target.
> If the code is "duplicated", as you say, the actual differences are baked in
> at compile time. The "duplication" would also allow to have always inlined
> functions for write-rare and leave more freedom to the compiler for their
> non-protected version.
> 
> Besides, I think the separate wr version also makes it very clear, to the
> user of the API, that there will be a price to pay, in terms of performance.
> The more seamlessly alternative might make this price less obvious.

What about something in the middle, where we move list to list_impl.h,
and add a few macros where you have list_set_prev() in prlist now, so
we could do,

// prlist.h

#define list_set_next(head, next) wr_ptr(&head->next, next)
#define list_set_prev(head, prev) wr_ptr(&head->prev, prev)

#include <linux/list_impl.h>

// list.h

#define list_set_next(next) (head->next = next)
#define list_set_next(prev) (head->prev = prev)

#include <linux/list_impl.h>

I wonder then if you can get rid of some of the type punning too? It's
not clear exactly why that's necessary from the series, but perhaps
I'm missing something obvious :)

I also wonder how much the actual differences being baked in at
compile time makes. Most (all?) of this code is inlined.

Cheers,

Tycho