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Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kfn0W-0000Oi-71; Thu, 19 Nov 2020 16:42:04 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2F88D3019CE; Thu, 19 Nov 2020 17:42:03 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 193C9200E0A43; Thu, 19 Nov 2020 17:42:03 +0100 (CET) Date: Thu, 19 Nov 2020 17:42:03 +0100 From: Peter Zijlstra To: Will Deacon Subject: Re: [PATCH v3 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Message-ID: <20201119164203.GU3121392@hirez.programming.kicks-ass.net> References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-9-will@kernel.org> <20201119161448.GR3121392@hirez.programming.kicks-ass.net> <20201119162822.GA4582@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201119162822.GA4582@willie-the-truck> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Marc Zyngier , kernel-team@android.com, Vincent Guittot , Juri Lelli , Quentin Perret , Catalin Marinas , Johannes Weiner , linux-kernel@vger.kernel.org, Qais Yousef , Ingo Molnar , Li Zefan , Greg Kroah-Hartman , Tejun Heo , Suren Baghdasaryan , Morten Rasmussen , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Nov 19, 2020 at 04:28:23PM +0000, Will Deacon wrote: > On Thu, Nov 19, 2020 at 05:14:48PM +0100, Peter Zijlstra wrote: > > On Fri, Nov 13, 2020 at 09:37:13AM +0000, Will Deacon wrote: > > > When exec'ing a 32-bit task on a system with mismatched support for > > > 32-bit EL0, try to ensure that it starts life on a CPU that can actually > > > run it. > > > > > > Signed-off-by: Will Deacon > > > --- > > > arch/arm64/kernel/process.c | 12 +++++++++++- > > > 1 file changed, 11 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > > > index 1540ab0fbf23..17b94007fed4 100644 > > > --- a/arch/arm64/kernel/process.c > > > +++ b/arch/arm64/kernel/process.c > > > @@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp) > > > return sp & ~0xf; > > > } > > > > > > +static void adjust_compat_task_affinity(struct task_struct *p) > > > +{ > > > + const struct cpumask *mask = system_32bit_el0_cpumask(); > > > + > > > + if (restrict_cpus_allowed_ptr(p, mask)) > > > + set_cpus_allowed_ptr(p, mask); > > > > This silently destroys user state, at the very least that ought to go > > with a WARN or something. Ideally SIGKILL though. What's to stop someone > > from doing a sched_setaffinity() right after the execve, same problem. > > So why bother.. > > It's no different to CPU hot-unplug though, is it? From the perspective of > the 32-bit task, the 64-bit-only cores were hot-unplugged at the point of > execve(). Calls to sched_setaffinity() for 32-bit tasks will reject attempts > to include 64-bit-only cores. select_fallback_rq() has a printk() in to at least notify things went bad. But I don't particularly like the current hotplug semantics; I've wanted to disallow the hotplug when it would result in this case, but computing that is tricky. It's one of those things that's forever on the todo list ... :/ > I initially wanted to punt this all to userspace, but one of the big > problems with that is when a 64-bit task is running on a CPU only capable > of running 64-bit tasks and it execve()s a 32-bit task. At the point, we > have to do something because we can't even run the new task for it to do > a sched_affinity() call (and we also can't deliver SIGILL). Userspace can see that one coming though... I suppose you can simply make the execve fail before the point of no return. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel