From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs Date: Tue, 3 Jul 2018 13:26:39 -0400 (EDT) Message-ID: <1047204530.11737.1530638799580.JavaMail.zimbra@efficios.com> References: <8B2E4CEB-3080-4602-8B62-774E400892EB@amacapital.net> <20180703081449.GT2494@hirez.programming.kicks-ass.net> <20180703082955.GH3704@osiris> <20180703084312.GU2494@hirez.programming.kicks-ass.net> <20180703085546.GJ3704@osiris> <20180703092113.GV2494@hirez.programming.kicks-ass.net> <20180703164048.i2te5gjemcafqzwf@two.firstfloor.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Linus Torvalds Cc: Andi Kleen , Peter Zijlstra , heiko carstens , Andy Lutomirski , Thomas Gleixner , linux-kernel , linux-api , "Paul E. McKenney" , Boqun Feng , Dave Watson , Paul Turner , Andrew Morton , Russell King , Ingo Molnar , "H. Peter Anvin" , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Catalin Marinas Will Deacon List-Id: linux-api@vger.kernel.org ----- On Jul 3, 2018, at 1:10 PM, Linus Torvalds torvalds@linux-foundation.org wrote: > On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen wrote: >> >> So it sounds like architectures that don't have an instruction atomic u64 >> *_user need to disable interrupts during the access, and somehow handle that >> case when a page fault happens? > > No. It's actually the store by *user* space that is the critical one. > Not the whole 64-bit value, just the low pointer part. > > The kernel could do it as a byte-by-byte load, really. It's > per-thread, and once the kernel is running, it's not going to change. > The kernel never changes the value, it just loads it from user space. > > So all the atomicity worries for the kernel are a red herring. They'd > arguably be nice to have - but only for an insane case that makes > absolutely no sense (a different thread trying to change the value). > > Can we please stop the idiocy already? The kernel could read the rseq > pointer one bit at a time, and do a little dance with "yield()" in > between, and take interrupts and page faults, and it wouldn't matter > AT ALL. > > It's not even that we read the value from an interrupt context, it's > that as we return to user space (which can be the result of an > interrupt) we can read the value. > > This whole thread has been filled with crazy "what if" things that don't matter. Sorry to come back in the thread late, looks like I've missed all the fun. I agree with Linus: we can simply document that updates to rseq->rseq_cs should be thread-local in the rseq uapi and be done with it. This would allow using get_user(u64) even on 32-bit architectures, because we cannot care less if an architecture chooses to read the u64 byte-wise while standing on its feet. With this added requirement, Andy's idea of using a union between __u64 and upper/lower __u32 would fit very nicely. If everyone is OK with that approach, I can prepare an updated patch. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com