Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754175Ab0H3AZS (ORCPT ); Sun, 29 Aug 2010 20:25:18 -0400 Received: from mail-vw0-f46.google.com ([209.85.212.46]:45576 "EHLO mail-vw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753357Ab0H3AZR convert rfc822-to-8bit (ORCPT ); Sun, 29 Aug 2010 20:25:17 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=wapMNPrjXIBbSMNnAyz0s8TsUR98Qs4EYHgXGChi4WdEp3DV/1AyZR8Kls8VqWK3z4 ND17LcYFE3l3RWsyPO+hgQVYf9JS90dHlQHc8PgY3lylMqWf9XdmjXx4qhERXLwkBg+x NvyPDt7Gty7avzYOZp+q/WjT31tbOGjk3IHB0= MIME-Version: 1.0 In-Reply-To: References: <1283011478-27237-1-git-send-email-brgerst@gmail.com> <1283011478-27237-7-git-send-email-brgerst@gmail.com> Date: Sun, 29 Aug 2010 20:25:16 -0400 Message-ID: Subject: Re: [PATCH 06/11] x86-64: Fix %cs value in convert_from_fxsr() From: Brian Gerst To: Pekka Enberg Cc: hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1494 Lines: 31 On Sun, Aug 29, 2010 at 2:41 PM, Pekka Enberg wrote: > On Sat, Aug 28, 2010 at 7:04 PM, Brian Gerst wrote: >> While %ds still contains the userspace selector, %cs is KERNEL_CS >> at this point.  Always get %cs from pt_regs. >> >> It actually is possible to get the correct segments for compat tasks, >> but that involves using the [f]xsave instruction without a REX.W prefix. >> >> Signed-off-by: Brian Gerst > > It might be just me but the above description doesn't explain > anything. What's the problem here? What is this fixing? The %cs segment being reported to a compat task is flat out wrong. It is getting KERNEL_CS when it should be some userspace segment. The code segment may still be wrong, because the %cs in pt_regs may not have been the segment where the instruction that flagged the exception executed from. That could be fixed by using fxsave without a REX.W prefix when saving the state of compat tasks, which would save the segment and 32-bit offset instead of the 64-bit offset for the code and data pointers. This is such a corner case that it probably isn't worth putting much effort into fixing unless someone demonstrates a real need for it. -- Brian Gerst -- 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/