Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753965AbbFAV6S (ORCPT ); Mon, 1 Jun 2015 17:58:18 -0400 Received: from mail-lb0-f181.google.com ([209.85.217.181]:36157 "EHLO mail-lb0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752245AbbFAV6S (ORCPT ); Mon, 1 Jun 2015 17:58:18 -0400 MIME-Version: 1.0 In-Reply-To: <556CD37E.9000501@hitachi.com> References: <1433176331-479-1-git-send-email-eugene.shatokhin@rosalab.ru> <1433176331-479-3-git-send-email-eugene.shatokhin@rosalab.ru> <556CD37E.9000501@hitachi.com> From: Andy Lutomirski Date: Mon, 1 Jun 2015 14:57:55 -0700 Message-ID: Subject: Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again To: Masami Hiramatsu Cc: Eugene Shatokhin , Ingo Molnar , Ingo Molnar , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2380 Lines: 55 On Mon, Jun 1, 2015 at 2:49 PM, Masami Hiramatsu wrote: > On 2015/06/02 2:04, Andy Lutomirski wrote: >> On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin >> wrote: >>> Commit 91e5ed49fca0 ("x86/asm/decoder: Fix and enforce max instruction >>> size in the insn decoder") has changed MAX_INSN_SIZE from 16 to 15 bytes >>> on x86. >>> >>> As a side effect, the slots Kprobes use to store the instructions became >>> 1 byte shorter. This is unfortunate because, for example, the Kprobes' >>> "boost" feature can not be used now for the instructions of length 11, >>> like a quite common kind of MOV: >>> * movq $0xffffffffffffffff,-0x3fe8(%rax) (48 c7 80 18 c0 ff ff ff ff ff ff) >>> * movq $0x0,0x88(%rdi) (48 c7 87 88 00 00 00 00 00 00 00) >>> and so on. >>> >>> This patch makes the insn slots 16 bytes long, like they were before while >>> keeping MAX_INSN_SIZE intact. >>> >>> Other tools may benefit from this change as well. >> >> What is a "slot" and why does this patch make sense? Naively, I'd >> expect that the check you're patching is entirely unnecessary -- I >> don't see what the size of the instruction being probed has to do with >> the safety of executing it out of line and then jumping back. >> >> Is there another magic 16 somewhere that this is enforcing that we >> don't overrun? > > The kprobe-"booster" adds a jump back code (jmp ) > right after the instruction in the out-of-code buffer(slot). So we need at least > the insn-length + 5 bytes for the slot, it's the trick of the magic :) This still doesn't explain what a "slot" is. I broke (?) something because I didn't see anything that looked relevant that I was changing. But now I see it: - .insn_size = MAX_INSN_SIZE, + .insn_size = KPROBE_INSN_SLOT_SIZE, Would it make sense to clean this up? insn_size isn't the size of an instruction at all -- it's the size of a kprobe jump target in units of sizeof(kprobe_opcode_t). How about renaming insn_size to something sensible (and maybe specifying the size in *bytes*)? --Andy -- 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/