From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754545AbcEBQhS (ORCPT ); Mon, 2 May 2016 12:37:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42939 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754108AbcEBQhF (ORCPT ); Mon, 2 May 2016 12:37:05 -0400 Date: Mon, 2 May 2016 17:35:08 +0200 From: Oleg Nesterov To: Andy Lutomirski Cc: X86 ML , "linux-kernel@vger.kernel.org" , Roland McGrath , Andi Kleen , Borislav Petkov , Brian Gerst Subject: Re: ptrace vs FSGSBASE Message-ID: <20160502153508.GA7874@redhat.com> References: <20160502142746.GA7142@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/02, Andy Lutomirski wrote: > > On Mon, May 2, 2016 at 7:27 AM, Oleg Nesterov wrote: > >> > >> 1. I read fs_base using ptrace. I think I should get the actual > >> fs_base without any nonsense. > > > > Which fs_base? The member of user_regs_struct? But this structure/layout > > is just the ABI, so to me it seems correct that getreg() tries to look > > at ->fs and/or ->fsindex. > > Yeah, the member of user_regs_struct. Still can't understand this... user_regs_struct is just the set of offsets we use to "name" the registers for getreg/putreg. We simply do not have "the actual fs_base" we could use in getreg(), we need to calculate it. > > I can't understand what does "atomically" mean in this context. > > I mean "change fs and fs_base to these two values in a single syscall > so that the kernel can do something intelligent." > > Let me give some background: > [... snip ...] Thanks Andy. I need to re-read your explanation, but it seems I am starting to understand. And yes, I didn't bother to look at putreg() when I wrote my reply. > If you write, say, 0x2b to > fs and 12345 to fs_base using the ptrace API, you'd end up with FS == > 0x2b and FSBASE == 0, Hmm. I can be easily wrong again but afaics in this case do_arch_prctl() will change fs/fs_base first and set fsindex = FS_TLS_SEL fs = 0 and then... and then I simply can't understand what set_segment_reg(fs) will/should do in this case. Nor I can understand the "thread.fs != value" check before do_arch_prctl(ARCH_SET_FS). Confused. Oleg.