Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751761AbaJTQtw (ORCPT ); Mon, 20 Oct 2014 12:49:52 -0400 Received: from mail-lb0-f177.google.com ([209.85.217.177]:40475 "EHLO mail-lb0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750798AbaJTQtu (ORCPT ); Mon, 20 Oct 2014 12:49:50 -0400 MIME-Version: 1.0 In-Reply-To: <20141020083326.GA3219@twins.programming.kicks-ass.net> References: <20141019213341.GF23531@worktop.programming.kicks-ass.net> <20141019222004.GI23531@worktop.programming.kicks-ass.net> <20141020083326.GA3219@twins.programming.kicks-ass.net> From: Andy Lutomirski Date: Mon, 20 Oct 2014 09:49:28 -0700 Message-ID: Subject: Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped To: Peter Zijlstra Cc: Erik Bosman , Ingo Molnar , "linux-kernel@vger.kernel.org" , Paul Mackerras , Kees Cook , Arnaldo Carvalho de Melo , Andrea Arcangeli , Valdis Kletnieks Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 20, 2014 at 1:33 AM, Peter Zijlstra wrote: > On Sun, Oct 19, 2014 at 03:57:54PM -0700, Andy Lutomirski wrote: >> > Maybe, but at that point we commit to yet another ABI... I'd rather just >> > put a 'sane' implementation in a library or so. >> >> This cuts both ways, though. For vdso timekeeping, the underlying >> data structure has changed repeatedly, sometimes to add features, and >> sometimes for performance, and the vdso has done a good job insulating >> userspace from it. (In fact, until 3.16, even the same exact kernel >> version couldn't be relied on to have the same data structure with >> different configs, and even now, no one really wants to teach user >> libraries how to parse the pvclock data structures.) > > Fair enough, but as it stands we've already committed to the data > structure exposed to userspace. True. OTOH, if a vdso function gets added, a few releases go by, and all the userspace tools get updated, then the old data structure could be dropped if needed by clearing cap_user_rdpmc. Anyway, this is so far out of scope for the current project that I'm going to ignore it. >> FWIW, something should probably specify exactly when it's safe to try >> a userspace rdpmc. I think that the answer is that, for a perf event >> watching a pid, only that pid can do it (in particular, other threads >> must not try). For a perf event monitoring a whole cpu, the answer is >> less clear to me. > > This all was really only meant to be used for self-monitoring, so where > an event is attached to the very same task, anything else and I'm find > disabling it. Actually implementing this might be a touch awkward. I can check whether an event has a task context that matches the creating task, but that's not necessarily the same thing as the task that mmaps it. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/