From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752792AbcEWCeS (ORCPT ); Sun, 22 May 2016 22:34:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52961 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752672AbcEWCeR (ORCPT ); Sun, 22 May 2016 22:34:17 -0400 Date: Sun, 22 May 2016 21:34:14 -0500 From: Josh Poimboeuf To: Andy Lutomirski Cc: Brian Gerst , 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: <20160523023414.laljasegs2cex5be@treble> References: <1463846691-18498-1-git-send-email-brgerst@gmail.com> <1463846691-18498-4-git-send-email-brgerst@gmail.com> 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 [10.5.110.26]); Mon, 23 May 2016 02:34:16 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Josh