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=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 C6621C7618B for ; Wed, 24 Jul 2019 15:19:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A5B1721738 for ; Wed, 24 Jul 2019 15:19:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387581AbfGXPTp (ORCPT ); Wed, 24 Jul 2019 11:19:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47626 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387503AbfGXPTp (ORCPT ); Wed, 24 Jul 2019 11:19:45 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A76E6C024AF3; Wed, 24 Jul 2019 15:19:44 +0000 (UTC) Received: from redhat.com (ovpn-123-65.rdu2.redhat.com [10.10.123.65]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1A5D95D9DE; Wed, 24 Jul 2019 15:19:43 +0000 (UTC) Date: Wed, 24 Jul 2019 11:19:42 -0400 From: Joe Lawrence To: Masami Hiramatsu Cc: live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: kprobes, livepatch and FTRACE_OPS_FL_IPMODIFY Message-ID: <20190724151942.GA7205@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.12.0 (2019-05-25) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Wed, 24 Jul 2019 15:19:44 +0000 (UTC) Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org Hi Masami, I wanted to revisit FTRACE_OPS_FL_IPMODIFY blocking of kprobes and livepatch, at least in cases where kprobe pre_handlers don't modify regs->ip. (We've discussed this previously at part of a kpatch github issue #47: https://github.com/dynup/kpatch/issues/47) The particular use case I was wondering about was perf probing a particular function, then attempting to livepatch that same function: % uname -r 5.3.0-rc1+ % dmesg -C % perf probe --add cmdline_proc_show Added new event: probe:cmdline_proc_show (on cmdline_proc_show) You can now use it in all perf tools, such as: perf record -e probe:cmdline_proc_show -aR sleep 1 % perf record -e probe:cmdline_proc_show -aR sleep 30 & [1] 1007 % insmod samples/livepatch/livepatch-sample.ko insmod: ERROR: could not insert module samples/livepatch/livepatch-sample.ko: Device or resource busy % dmesg [ 440.913962] livepatch_sample: tainting kernel with TAINT_LIVEPATCH [ 440.917123] livepatch_sample: module verification failed: signature and/or required key missing - tainting kernel [ 440.942493] livepatch: enabling patch 'livepatch_sample' [ 440.943445] livepatch: failed to register ftrace handler for function 'cmdline_proc_show' (-16) [ 440.944576] livepatch: failed to patch object 'vmlinux' [ 440.945270] livepatch: failed to enable patch 'livepatch_sample' [ 440.946085] livepatch: 'livepatch_sample': unpatching complete This same behavior holds in reverse, if we want to probe a livepatched function: % insmod samples/livepatch/livepatch-sample.ko % perf probe --add cmdline_proc_show Added new event: probe:cmdline_proc_show (on cmdline_proc_show) You can now use it in all perf tools, such as: perf record -e probe:cmdline_proc_show -aR sleep 1 % perf record -e probe:cmdline_proc_show -aR sleep 30 Error: The sys_perf_event_open() syscall returned with 16 (Device or resource busy) for event (probe:cmdline_proc_show). /bin/dmesg | grep -i perf may provide additional information. Now, if I read kernel/trace/trace_kprobe.c :: kprobe_dispatcher() correctly, it's only going to return !0 (indicating a modified regs->ip) when kprobe_perf_func() returns !0, i.e. regs->ip changes over a call to trace_call_bpf(). Aside: should kprobe_ftrace_handler() check that FTRACE_OPS_FL_IPMODIFY is set when a pre_handler returns !0? In kpatch #47, Josh suggested: - If a kprobe handler needs to modify IP, user sets KPROBE_FLAG_IPMODIFY flag to register_kprobe, and then kprobes sets FTRACE_OPS_FL_IPMODIFY when registering with ftrace for that probe. - If KPROBE_FLAG_IPMODIFY is not used, kprobe_ftrace_handler() can detect when a kprobe handler changes regs->ip and restore it to its original value (regs->ip = ip). Is this something that could still be supported? In cases like perf probe, could we get away with not setting FTRACE_OPS_FL_IPMODIFY? The current way that we're applying that flag, kprobes and livepatch are mutually exclusive (for the same function). Regards, -- Joe