From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id D73957DE78 for ; Fri, 27 Apr 2018 09:45:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932439AbeD0Joy (ORCPT ); Fri, 27 Apr 2018 05:44:54 -0400 Received: from www62.your-server.de ([213.133.104.62]:47577 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757353AbeD0Jox (ORCPT ); Fri, 27 Apr 2018 05:44:53 -0400 Received: from [62.202.221.10] (helo=linux.home) by www62.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-SHA:256) (Exim 4.85_2) (envelope-from ) id 1fBzvp-0003Xc-3W; Fri, 27 Apr 2018 11:44:45 +0200 Subject: Re: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON To: Leo Yan , Alexei Starovoitov , "David S. Miller" , Jonathan Corbet , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <1524709611-29437-1-git-send-email-leo.yan@linaro.org> From: Daniel Borkmann Message-ID: <275e03a2-b74e-8f60-4ffe-26c9a79fae9d@iogearbox.net> Date: Fri, 27 Apr 2018 11:44:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <1524709611-29437-1-git-send-email-leo.yan@linaro.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.99.3/24517/Fri Apr 27 06:24:46 2018) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 04/26/2018 04:26 AM, Leo Yan wrote: > When CONFIG_BPF_JIT_ALWAYS_ON is enabled, kernel has limitation for > bpf_jit_enable, so it has fixed value 1 and we cannot set it to 2 > for JIT opcode dumping; this patch is to update the doc for it. > > Signed-off-by: Leo Yan > --- > Documentation/networking/filter.txt | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt > index fd55c7d..feddab9 100644 > --- a/Documentation/networking/filter.txt > +++ b/Documentation/networking/filter.txt > @@ -483,6 +483,12 @@ Example output from dmesg: > [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00 > [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3 > > +When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is set to 1 by default > +and it returns failure if change to any other value from proc node; this is > +for security consideration to avoid leaking info to unprivileged users. In this > +case, we can't directly dump JIT opcode image from kernel log, alternatively we > +need to use bpf tool for the dumping. > + Could you change this doc text a bit, I think it's slightly misleading. From the first sentence one could also interpret that value 0 would leaking info to unprivileged users whereas here we're only talking about the case of value 2. Maybe something roughly like this to make it more clear: When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and setting any other value than that will return in failure. This is even the case for setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the generally recommended approach instead. Thanks, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html