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 C8811C6FA8E for ; Mon, 26 Sep 2022 18:03:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229522AbiIZSDy (ORCPT ); Mon, 26 Sep 2022 14:03:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35214 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229868AbiIZSDI (ORCPT ); Mon, 26 Sep 2022 14:03:08 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 3133CDF7A; Mon, 26 Sep 2022 10:44:02 -0700 (PDT) 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 4F7921CE2; Mon, 26 Sep 2022 10:44:08 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.81.104]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9DA163F73B; Mon, 26 Sep 2022 10:43:57 -0700 (PDT) Date: Mon, 26 Sep 2022 18:43:51 +0100 From: Mark Rutland To: Catalin Marinas Cc: Daniel Borkmann , Xu Kuohai , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Florent Revest , Will Deacon , Jean-Philippe Brucker , Steven Rostedt , Ingo Molnar , Oleg Nesterov , Alexei Starovoitov , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Zi Shen Lim , Pasha Tatashin , Ard Biesheuvel , Marc Zyngier , Guo Ren , Masami Hiramatsu Subject: Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64 Message-ID: References: <20220913162732.163631-1-xukuohai@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Mon, Sep 26, 2022 at 03:40:20PM +0100, Catalin Marinas wrote: > On Thu, Sep 22, 2022 at 08:01:16PM +0200, Daniel Borkmann wrote: > > On 9/13/22 6:27 PM, Xu Kuohai wrote: > > > This series adds ftrace direct call for arm64, which is required to attach > > > bpf trampoline to fentry. > > > > > > Although there is no agreement on how to support ftrace direct call on arm64, > > > no patch has been posted except the one I posted in [1], so this series > > > continues the work of [1] with the addition of long jump support. Now ftrace > > > direct call works regardless of the distance between the callsite and custom > > > trampoline. > > > > > > [1] https://lore.kernel.org/bpf/20220518131638.3401509-2-xukuohai@huawei.com/ > > > > > > v2: > > > - Fix compile and runtime errors caused by ftrace_rec_arch_init > > > > > > v1: https://lore.kernel.org/bpf/20220913063146.74750-1-xukuohai@huaweicloud.com/ > > > > > > Xu Kuohai (4): > > > ftrace: Allow users to disable ftrace direct call > > > arm64: ftrace: Support long jump for ftrace direct call > > > arm64: ftrace: Add ftrace direct call support > > > ftrace: Fix dead loop caused by direct call in ftrace selftest > > > > Given there's just a tiny fraction touching BPF JIT and most are around core arm64, > > it probably makes sense that this series goes via Catalin/Will through arm64 tree > > instead of bpf-next if it looks good to them. Catalin/Will, thoughts (Ack + bpf-next > > could work too, but I'd presume this just results in merge conflicts)? > > I think it makes sense for the series to go via the arm64 tree but I'd > like Mark to have a look at the ftrace changes first. >From a quick scan, I still don't think this is quite right, and as it stands I believe this will break backtracing (as the instructions before the function entry point will not be symbolized correctly, getting in the way of RELIABLE_STACKTRACE). I think I was insufficiently clear with my earlier feedback there, as I have a mechanism in mind that wa a little simpler. I'll try to reply with some more detail tomorrow, but I don't think this is the right approach, and as mentioned previously (and e.g. at LPC) I'd strongly prefer to *not* implement direct calls, so that we can have more consistent entry/exit handling. Thanks, Mark.