Date: Thu, 25 Oct 2018 01:52:11 +0300 From: Igor Stoppa <igor.stoppa@...il.com> To: Tycho Andersen <tycho@...ho.ws> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Kees Cook <keescook@...omium.org>, Matthew Wilcox <willy@...radead.org>, Dave Chinner <david@...morbit.com>, James Morris <jmorris@...ei.org>, Michal Hocko <mhocko@...nel.org>, kernel-hardening@...ts.openwall.com, linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org, igor stoppa <igor.stoppa@...wei.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Jonathan Corbet <corbet@....net>, Laura Abbott <labbott@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Kate Stewart <kstewart@...uxfoundation.org>, "David S. Miller" <davem@...emloft.net>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Philippe Ombredanne <pombredanne@...b.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Josh Triplett <josh@...htriplett.org>, rostedt <rostedt@...dmis.org>, Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 14/17] prmem: llist, hlist, both plain and rcu 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 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? Or maybe I miss your point? -- igor
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