From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73273C54EAA for ; Mon, 30 Jan 2023 10:54:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236173AbjA3Kyt (ORCPT ); Mon, 30 Jan 2023 05:54:49 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43920 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230416AbjA3Kyq (ORCPT ); Mon, 30 Jan 2023 05:54:46 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id B48E617CC0 for ; Mon, 30 Jan 2023 02:54:45 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 70FD316A3; Mon, 30 Jan 2023 02:55:27 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.13.128]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E89D93F71E; Mon, 30 Jan 2023 02:54:42 -0800 (PST) Date: Mon, 30 Jan 2023 10:54:40 +0000 From: Mark Rutland To: Guo Ren Cc: anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, conor.dooley@microchip.com, heiko@sntech.de, rostedt@goodmis.org, mhiramat@kernel.org, jolsa@redhat.com, bp@suse.de, jpoimboe@kernel.org, suagrfillet@gmail.com, andy.chiu@sifive.com, e.shatokhin@yadro.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next V7 1/7] riscv: ftrace: Fixup panic by disabling preemption Message-ID: References: <20230112090603.1295340-1-guoren@kernel.org> <20230112090603.1295340-2-guoren@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 28, 2023 at 05:37:46PM +0800, Guo Ren wrote: > On Thu, Jan 12, 2023 at 8:16 PM Mark Rutland wrote: > > > > Hi Guo, > > > > On Thu, Jan 12, 2023 at 04:05:57AM -0500, guoren@kernel.org wrote: > > > From: Andy Chiu > > > > > > In RISCV, we must use an AUIPC + JALR pair to encode an immediate, > > > forming a jump that jumps to an address over 4K. This may cause errors > > > if we want to enable kernel preemption and remove dependency from > > > patching code with stop_machine(). For example, if a task was switched > > > out on auipc. And, if we changed the ftrace function before it was > > > switched back, then it would jump to an address that has updated 11:0 > > > bits mixing with previous XLEN:12 part. > > > > > > p: patched area performed by dynamic ftrace > > > ftrace_prologue: > > > p| REG_S ra, -SZREG(sp) > > > p| auipc ra, 0x? ------------> preempted > > > ... > > > change ftrace function > > > ... > > > p| jalr -?(ra) <------------- switched back > > > p| REG_L ra, -SZREG(sp) > > > func: > > > xxx > > > ret > > > > As mentioned on the last posting, I don't think this is sufficient to fix the > > issue. I've replied with more detail there: > > > > https://lore.kernel.org/lkml/Y7%2F3hoFjS49yy52W@FVFF77S0Q05N/ > > > > Even in a non-preemptible SMP kernel, if one CPU can be in the middle of > > executing the ftrace_prologue while another CPU is patching the > > ftrace_prologue, you have the exact same issue. > > > > For example, if CPU X is in the prologue fetches the old AUIPC and the new > > JALR (because it races with CPU Y modifying those), CPU X will branch to the > > wrong address. The race window is much smaller in the absence of preemption, > > but it's still there (and will be exacerbated in virtual machines since the > > hypervisor can preempt a vCPU at any time). > > > > Note that the above is even assuming that instruction fetches are atomic, which > > I'm not sure is the case; for example arm64 has special CMODX / "Concurrent > > MODification and eXecutuion of instructions" rules which mean only certain > > instructions can be patched atomically. > > > > Either I'm missing something that provides mutual exclusion between the > > patching and execution of the ftrace_prologue, or this patch is not sufficient. > This patch is sufficient because riscv isn't the same as arm64. It > uses default arch_ftrace_update_code, which uses stop_machine. > See kernel/trace/ftrace.c: > void __weak arch_ftrace_update_code(int command) > { > ftrace_run_stop_machine(command); > } Ah; sorry, I had misunderstood here, since the commit message spoke in terms of removing that. As long as stop_machine() is used I agree this is safe; sorry for the noise. > ps: > Yes, it's not good, and it's expensive. We can't have everything! :) Thanks, Mark.