From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753532AbcEWMFi (ORCPT ); Mon, 23 May 2016 08:05:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59450 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752296AbcEWMFh (ORCPT ); Mon, 23 May 2016 08:05:37 -0400 Date: Mon, 23 May 2016 07:05:34 -0500 From: Josh Poimboeuf To: Brian Gerst Cc: Andy Lutomirski , Thomas Gleixner , Denys Vlasenko , Ingo Molnar , "linux-kernel@vger.kernel.org" , Borislav Petkov , "H. Peter Anvin" , X86 ML Subject: Re: [PATCH 3/4] x86: Rewrite switch_to() code Message-ID: <20160523120534.lzphuumzemvrmuwg@treble> References: <1463846691-18498-1-git-send-email-brgerst@gmail.com> <1463846691-18498-4-git-send-email-brgerst@gmail.com> <20160523023414.laljasegs2cex5be@treble> <20160523114021.hog2qzmm2nhwbxa4@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.0.1 (2016-04-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Mon, 23 May 2016 12:05:36 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 23, 2016 at 07:49:37AM -0400, Brian Gerst wrote: > On Mon, May 23, 2016 at 7:40 AM, Josh Poimboeuf wrote: > > On Sun, May 22, 2016 at 09:47:22PM -0700, Andy Lutomirski wrote: > >> On Sun, May 22, 2016 at 7:34 PM, Josh Poimboeuf wrote: > >> > On Sun, May 22, 2016 at 10:59:38AM -0700, Andy Lutomirski wrote: > >> >> cc: Josh Poimboeuf: do you care about the exact stack layout of the > >> >> bottom of the stack of an inactive task? > >> > > >> > So there's one minor issue with this patch, relating to unwinding the > >> > stack of a newly forked task. For detecting reliable stacks, the > >> > unwinder needs to unwind all the way to the syscall pt_regs to make sure > >> > the stack is sane. But for newly forked tasks, that won't be possible > >> > here because the unwinding will stop at the fork_frame instead. > >> > > >> > So from an unwinder standpoint it might be nice for copy_thread_tls() to > >> > place a frame pointer on the stack next to the ret_from_fork return > >> > address, so that it would resemble an actual stack frame. The frame > >> > pointer could probably just be hard-coded to zero. And then the first > >> > bp in fork_frame would need to be a pointer to it instead of zero. That > >> > would make it nicely resemble the stack of any other task. > >> > > >> > Alternatively I could teach the unwinder that if the unwinding starts at > >> > the fork_frame offset from the end of the stack page, and the saved rbp > >> > is zero, it can assume that it's a newly forked task. But that seems a > >> > little more brittle to me, as it requires the unwinder to understand > >> > more of the internal workings of the fork code. > >> > > >> > But overall I think this patch is a really nice cleanup, and other than > >> > the above minor issue it should be fine with my reliable unwinder, since > >> > rbp is still at the top of the stack. > >> > >> Is this a regression or is there some reason that it works right > >> without the patch? > > > > Without the patch, it uses TIF_FORK to determine the stack is empty. > > Where is this code? I don't see it in the mainline kernel. Yeah, it hasn't been merged. Here's the last version: https://lkml.kernel.org/r/4d34d452bf8f85c7d6d5f93db1d3eeb4cba335c7.1461875890.git.jpoimboe@redhat.com -- Josh