From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751723AbdJYLZJ (ORCPT ); Wed, 25 Oct 2017 07:25:09 -0400 Received: from mx2.suse.de ([195.135.220.15]:58379 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751589AbdJYLZF (ORCPT ); Wed, 25 Oct 2017 07:25:05 -0400 Subject: Re: [PATCH 10/13] x86/alternative: Support indirect call replacement To: Josh Poimboeuf , x86@kernel.org Cc: linux-kernel@vger.kernel.org, Andy Lutomirski , Linus Torvalds , Sasha Levin , live-patching@vger.kernel.org, Jiri Slaby , Ingo Molnar , "H. Peter Anvin" , Peter Zijlstra , Mike Galbraith , Chris Wright , Alok Kataria , Rusty Russell , virtualization@lists.linux-foundation.org, Boris Ostrovsky , xen-devel@lists.xenproject.org, Thomas Gleixner , Borislav Petkov References: From: Juergen Gross Message-ID: Date: Wed, 25 Oct 2017 13:25:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: de-DE Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/10/17 17:58, Josh Poimboeuf wrote: > Add alternative patching support for replacing an instruction with an > indirect call. This will be needed for the paravirt alternatives. > > Signed-off-by: Josh Poimboeuf > --- > arch/x86/kernel/alternative.c | 22 +++++++++++++++------- > 1 file changed, 15 insertions(+), 7 deletions(-) > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 3344d3382e91..81c577c7deba 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -410,20 +410,28 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, > insnbuf_sz = a->replacementlen; > > /* > - * 0xe8 is a relative jump; fix the offset. > - * > - * Instruction length is checked before the opcode to avoid > - * accessing uninitialized bytes for zero-length replacements. > + * Fix the address offsets for call and jump instructions which > + * use PC-relative addressing. > */ > if (a->replacementlen == 5 && *insnbuf == 0xe8) { > + /* direct call */ > *(s32 *)(insnbuf + 1) += replacement - instr; > - DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx", > + DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx", > *(s32 *)(insnbuf + 1), > (unsigned long)instr + *(s32 *)(insnbuf + 1) + 5); > - } > > - if (a->replacementlen && is_jmp(replacement[0])) > + } else if (a->replacementlen == 6 && *insnbuf == 0xff && > + *(insnbuf+1) == 0x15) { > + /* indirect call */ > + *(s32 *)(insnbuf + 2) += replacement - instr; > + DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx", > + *(s32 *)(insnbuf + 2), > + (unsigned long)instr + *(s32 *)(insnbuf + 2) + 6); > + > + } else if (a->replacementlen && is_jmp(replacement[0])) { Is this correct? Without your patch this was: if (*insnbuf == 0xe8) ... if (is_jmp(replacement[0])) ... Now you have: if (*insnbuf == 0xe8) ... else if (*insnbuf == 0xff15) ... else if (is_jmp(replacement[0])) ... So only one or none of the three variants will be executed. In the past it could be none, one or both. Juergen