Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752153AbaAPJT2 (ORCPT ); Thu, 16 Jan 2014 04:19:28 -0500 Received: from smarthost01c.mail.zen.net.uk ([212.23.1.5]:47351 "EHLO smarthost01c.mail.zen.net.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751957AbaAPJTY (ORCPT ); Thu, 16 Jan 2014 04:19:24 -0500 Message-ID: <1389863932.3530.12.camel@linaro1.home> Subject: Re: [PATCH v4 13/16] ARM: Add an emulate flag to the kprobes/uprobes instruction decode functions From: "Jon Medhurst (Tixy)" To: David Long Cc: linux-arm-kernel@lists.infradead.org, Russell King , Rabin Vincent , Oleg Nesterov , Srikar Dronamraju , Ingo Molnar , Masami Hiramatsu , Ananth N Mavinakayanahalli , Anil S Keshavamurthy , davem@davemloft.net, Peter Zijlstra , Paul Mackerras , Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org Date: Thu, 16 Jan 2014 09:18:52 +0000 In-Reply-To: <52D6E1FE.8000303@linaro.org> References: <1387166930-13182-1-git-send-email-dave.long@linaro.org> <1387166930-13182-14-git-send-email-dave.long@linaro.org> <1387551482.3404.63.camel@linaro1.home> <52D6E1FE.8000303@linaro.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4-3 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Originating-smarthost01c-IP: [82.69.122.217] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2014-01-15 at 14:31 -0500, David Long wrote: > On 12/20/13 09:58, Jon Medhurst (Tixy) wrote: > > On Sun, 2013-12-15 at 23:08 -0500, David Long wrote: [...] > >> { > >> #ifdef CONFIG_THUMB2_KERNEL > >> if (thumb) { > >> @@ -253,7 +253,7 @@ set_emulated_insn(probes_opcode_t insn, struct arch_specific_insn *asi, > >> * non-zero value, the corresponding nibble in pinsn is validated and modified > >> * according to the type. > >> */ > >> -static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs) > >> +static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs, bool modify) > >> { > >> probes_opcode_t insn = *pinsn; > >> probes_opcode_t mask = 0xf; /* Start at least significant nibble */ > >> @@ -317,9 +317,16 @@ static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs) > >> /* Replace value of nibble with new register number... */ > >> insn &= ~mask; > >> insn |= new_bits & mask; > >> + if (modify) { > >> + /* Replace value of nibble with new register number */ > >> + insn &= ~mask; > >> + insn |= new_bits & mask; > >> + } > > > > Huh? As is, the above addition doesn't do anything because insn has > > already been modified. I guess you played with the idea that you needed > > to avoid changing insn (you don't) and then didn't undo the experiment > > quite right. :-) > > > > The conditional modification of the instruction was part of Rabin's > original work for uprobes, but I messed up the merge from an earlier > working version of my patches. My intention was/is to delete the old > unconditional code. Sounds like maybe you disagree though. The intent > is to only modify the instruction in the kprobes case. 'insn' is the local variable containing the instruction value we're processing. It doesn't matter if we change that, we just need to avoid updating the instruction in memory, which the code in the next chunk already correctly checks for... > >> } > >> > >> - *pinsn = insn; > >> + if (modify) > >> + *pinsn = insn; > >> + > >> return true; > >> So only one of these 'if (modify)' checks is required for code correctness, and I suggest keeping the second one as it's more explicit and defensive. -- Tixy -- 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/