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 X-Spam-Level: X-Spam-Status: No, score=-8.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_NEOMUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 36F22C31E45 for ; Fri, 14 Jun 2019 01:53:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1004F20B7C for ; Fri, 14 Jun 2019 01:53:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726187AbfFNBwy (ORCPT ); Thu, 13 Jun 2019 21:52:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51620 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725813AbfFNBwy (ORCPT ); Thu, 13 Jun 2019 21:52:54 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1DED081F0C; Fri, 14 Jun 2019 01:52:54 +0000 (UTC) Received: from treble (ovpn-121-232.rdu2.redhat.com [10.10.121.232]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EEE8460A9A; Fri, 14 Jun 2019 01:52:52 +0000 (UTC) Date: Thu, 13 Jun 2019 20:52:51 -0500 From: Josh Poimboeuf To: Alexei Starovoitov Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , netdev@vger.kernel.org, bpf@vger.kernel.org, Peter Zijlstra , Song Liu , Kairui Song Subject: Re: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage Message-ID: <20190614015251.xyfzl5djr7zurtvj@treble> References: <03ddea21a533b7b0e471c1d73ebff19dacdcf7e3.1560431531.git.jpoimboe@redhat.com> <20190613215807.wjcop6eaadirz5xm@ast-mbp.dhcp.thefacebook.com> <20190614012248.ztruzocusb2vu7bl@treble> <20190614013904.v2tpiunrjukzlxsu@ast-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190614013904.v2tpiunrjukzlxsu@ast-mbp.dhcp.thefacebook.com> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Fri, 14 Jun 2019 01:52:54 +0000 (UTC) Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, Jun 13, 2019 at 06:39:05PM -0700, Alexei Starovoitov wrote: > On Thu, Jun 13, 2019 at 08:22:48PM -0500, Josh Poimboeuf wrote: > > On Thu, Jun 13, 2019 at 02:58:09PM -0700, Alexei Starovoitov wrote: > > > On Thu, Jun 13, 2019 at 08:21:03AM -0500, Josh Poimboeuf wrote: > > > > The BPF JIT code clobbers RBP. This breaks frame pointer convention and > > > > thus prevents the FP unwinder from unwinding through JIT generated code. > > > > > > > > RBP is currently used as the BPF stack frame pointer register. The > > > > actual register used is opaque to the user, as long as it's a > > > > callee-saved register. Change it to use R12 instead. > > > > > > > > Fixes: d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER") > > > > Reported-by: Song Liu > > > > Signed-off-by: Josh Poimboeuf > > > > --- > > > > arch/x86/net/bpf_jit_comp.c | 43 +++++++++++++++++++++---------------- > > > > 1 file changed, 25 insertions(+), 18 deletions(-) > > > > > > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c > > > > index e649f977f8e1..bb1968fea50a 100644 > > > > --- a/arch/x86/net/bpf_jit_comp.c > > > > +++ b/arch/x86/net/bpf_jit_comp.c > > > > @@ -100,9 +100,8 @@ static int bpf_size_to_x86_bytes(int bpf_size) > > > > /* > > > > * The following table maps BPF registers to x86-64 registers. > > > > * > > > > - * x86-64 register R12 is unused, since if used as base address > > > > - * register in load/store instructions, it always needs an > > > > - * extra byte of encoding and is callee saved. > > > > + * RBP isn't used; it needs to be preserved to allow the unwinder to move > > > > + * through generated code stacks. > > > > > > Extra register save/restore is kinda annoying just to fix ORC. > > > > It's not just for the ORC unwinder. It also fixes the frame pointer > > unwinder (see above commit msg). And it's standard frame pointer > > practice to not clobber RBP. > > not true. > generated JITed code has no issues with regular stack unwinder. > it breaks down under ORC only. > > > > Also every stack access from bpf prog will be encoded via r12 and consume > > > extra byte of encoding. I really don't like this approach. > > > > Do you have another callee-saved register you'd prefer to use as the > > stack pointer? > > RBP must be used. > > > > Can you teach ORC to understand JIT-ed frames instead? > > > > We could, but it would add a lot more complexity than this. And anyway, > > the frame pointer unwinder would still be broken. > > I disagree. See above. Only ORC is broken. Hence ORC should be fixed. You're clobbering RBP. Frame pointer unwinding is broken. Period. -- Josh