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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_2 autolearn=ham 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 AA6EDC432C3 for ; Thu, 14 Nov 2019 16:19:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 718CE20715 for ; Thu, 14 Nov 2019 16:19:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726767AbfKNQT4 (ORCPT ); Thu, 14 Nov 2019 11:19:56 -0500 Received: from mail.kernel.org ([198.145.29.99]:43262 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726528AbfKNQT4 (ORCPT ); Thu, 14 Nov 2019 11:19:56 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9513720672; Thu, 14 Nov 2019 16:19:53 +0000 (UTC) Date: Thu, 14 Nov 2019 11:19:52 -0500 From: Steven Rostedt To: Miroslav Benes Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , X86 ML , Nadav Amit , Andy Lutomirski , Dave Hansen , Song Liu , Masami Hiramatsu , Peter Zijlstra , Daniel Bristot de Oliveira , Alexei Starovoitov , Josh Poimboeuf Subject: Re: [PATCH 09/10] ftrace/x86: Add register_ftrace_direct() for custom trampolines Message-ID: <20191114111952.3eb1a011@gandalf.local.home> In-Reply-To: References: <20191108212834.594904349@goodmis.org> <20191108213450.891579507@goodmis.org> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 14 Nov 2019 16:34:58 +0100 (CET) Miroslav Benes wrote: > On Fri, 8 Nov 2019, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" > > > > Enable x86 to allow for register_ftrace_direct(), where a custom trampoline > > may be called directly from an ftrace mcount/fentry location. > > > > Signed-off-by: Steven Rostedt (VMware) > > [...] > > > +++ b/arch/x86/kernel/ftrace_64.S > > @@ -88,6 +88,7 @@ EXPORT_SYMBOL(__fentry__) > > movq %rdi, RDI(%rsp) > > movq %r8, R8(%rsp) > > movq %r9, R9(%rsp) > > + movq $0, ORIG_RAX(%rsp) > > /* > > * Save the original RBP. Even though the mcount ABI does not > > * require this, it helps out callers. > > @@ -114,7 +115,8 @@ EXPORT_SYMBOL(__fentry__) > > subq $MCOUNT_INSN_SIZE, %rdi > > .endm > > > > -.macro restore_mcount_regs > > +.macro restore_mcount_regs save=0 > > + > > movq R9(%rsp), %r9 > > movq R8(%rsp), %r8 > > movq RDI(%rsp), %rdi > > @@ -123,10 +125,7 @@ EXPORT_SYMBOL(__fentry__) > > movq RCX(%rsp), %rcx > > movq RAX(%rsp), %rax > > > > - /* ftrace_regs_caller can modify %rbp */ > > - movq RBP(%rsp), %rbp > > - > > - addq $MCOUNT_REG_SIZE, %rsp > > + addq $MCOUNT_REG_SIZE-\save, %rsp > > > > .endm > > > > @@ -228,10 +227,30 @@ GLOBAL(ftrace_regs_call) > > movq R10(%rsp), %r10 > > movq RBX(%rsp), %rbx > > > > - restore_mcount_regs > > + movq RBP(%rsp), %rbp > > + > > + movq ORIG_RAX(%rsp), %rax > > + movq %rax, MCOUNT_REG_SIZE-8(%rsp) > > + > > + /* If ORIG_RAX is anything but zero, make this a call to that */ > > + movq ORIG_RAX(%rsp), %rax > > + cmpq $0, %rax > > + je 1f > > + > > + /* Swap the flags with orig_rax */ > > + movq MCOUNT_REG_SIZE(%rsp), %rdi > > + movq %rdi, MCOUNT_REG_SIZE-8(%rsp) > > + movq %rax, MCOUNT_REG_SIZE(%rsp) > > + > > + restore_mcount_regs 8 > > + > > + jmp 2f > > + > > +1: restore_mcount_regs > > + > > > > /* Restore flags */ > > - popfq > > +2: popfq > > If I am reading the code correctly (and I was confused couple of times, so > maybe I am not), this is what makes the direct fops incompatible with > ipmodify and livepatching for now. Is that correct? Actually, it's the fact that the return goes to some unknown trampoline that may do something else as well. > > What are your plans regarding this? I wanted to see what the eBPF folks were doing, and then perhaps allow the ip modify occur too. I could let it happen as well now, and then we can see what the fallout is later ;-) > > Moreover, we could replace ftrace_regs_caller with direct fops for live > patching when this is merged with all arch support we need. After all, all Note, direct call is currently only available for x86_64. > we need is to change the rip, which we could do easily in the direct > trampoline. On the other hand, it would exclude coexistence of a live > patch and a BPF filter (both direct now) on one function. It may also end up being more complex, and not much of a performance benefit. I believe the BPF is injecting programs into the start of functions, but your trampoline for live patching may be not much different than what ftrace gives you today. -- Steve