From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755811AbcDDPq1 (ORCPT ); Mon, 4 Apr 2016 11:46:27 -0400 Received: from mail.kernel.org ([198.145.29.136]:58376 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752590AbcDDPq0 (ORCPT ); Mon, 4 Apr 2016 11:46:26 -0400 From: Andy Lutomirski To: X86 ML Cc: Paolo Bonzini , Peter Zijlstra , KVM list , Arjan van de Ven , xen-devel , linux-kernel@vger.kernel.org, Linus Torvalds , Andrew Morton , Borislav Petkov , Andy Lutomirski Subject: [PATCH] x86/extable: Add a comment about early exception handlers Date: Mon, 4 Apr 2016 08:46:22 -0700 Message-Id: <5f1dcd6919f4a5923959a8065cb2c04d9dac1412.1459784772.git.luto@kernel.org> X-Mailer: git-send-email 2.5.5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Borislav asked for a comment explaining why all exception handlers are allowed early. Signed-off-by: Andy Lutomirski --- arch/x86/mm/extable.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c index 98b5f45d9d79..36fe03bc81ee 100644 --- a/arch/x86/mm/extable.c +++ b/arch/x86/mm/extable.c @@ -132,6 +132,20 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr) if (regs->cs != __KERNEL_CS) goto fail; + /* + * The full exception fixup machinery is available as soon as + * the early IDT is loaded. This means that it is the + * responsibility of extable users to either function correctly + * when handlers are invoked early or to simply avoid causing + * exceptions before they're ready to handle them. + * + * This is better than filtering which handlers can be used, + * because refusing to call a handler here is guaranteed to + * result in a hard-to-debug panic. + * + * Keep in mind that not all vectors actually get here. Early + * fage faults, for example, are special. + */ if (fixup_exception(regs, trapnr)) return; -- 2.5.5