On Thu, Oct 25, 2018 at 01:52:11AM +0300, Igor Stoppa wrote: > On 24/10/2018 17:56, Tycho Andersen wrote: > > 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 :) > > nothing obvious, probably there is only half a reference in the slides I > linked-to in the cover letter :-) > > So far I have minimized the number of "intrinsic" write rare functions, > mostly because I would want first to reach an agreement on the > implementation of the core write-rare. > > However, once that is done, it might be good to convert also the prlists to > be "intrinsics". A list node is 2 pointers. > If that was the alignment, i.e. __align(sizeof(list_head)), it might be > possible to speed up a lot the list handling even as write rare. > > Taking as example the insertion operation, it would be probably sufficient, > in most cases, to have only two remappings: > - one covering the page with the latest two nodes > - one covering the page with the list head > > That is 2 vs 8 remappings, and a good deal of memory barriers less. > > This would be incompatible with what you are proposing, yet it would be > justifiable, I think, because it would provide better performance to prlist, > potentially widening its adoption, where performance is a concern. I guess the writes to these are rare, right? So perhaps it's not such a big deal :) > > I also wonder how much the actual differences being baked in at > > compile time makes. Most (all?) of this code is inlined. > > If the inlined function expects to receive a prlist_head *, instead of a > list_head *, doesn't it help turning runtime bugs into buildtime bugs? In principle it's not a bug to use the prmem helpers where the regular ones would do, it's just slower (assuming the types are the same). But mostly, it's a way to avoid actually copying and pasting most of the implementations of most of the data structures. I see some other replies in the thread already, but this seems not so good to me. Tycho
