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 3B1D6C433F5 for ; Wed, 25 May 2022 12:17:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233519AbiEYMRo (ORCPT ); Wed, 25 May 2022 08:17:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33638 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229577AbiEYMRl (ORCPT ); Wed, 25 May 2022 08:17:41 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 1980336318 for ; Wed, 25 May 2022 05:17:40 -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 BE9C51FB; Wed, 25 May 2022 05:17:39 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.0.228]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5A19D3F73D; Wed, 25 May 2022 05:17:37 -0700 (PDT) Date: Wed, 25 May 2022 13:17:30 +0100 From: Mark Rutland To: Masami Hiramatsu Cc: Steven Rostedt , Wang ShaoBo , cj.chengjian@huawei.com, huawei.libin@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, zengshun.wu@outlook.com, Jiri Olsa Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines Message-ID: References: <20220426174749.b5372c5769af7bf901649a05@kernel.org> <20220505121538.04773ac98e2a8ba17f675d39@kernel.org> <20220509142203.6c4f2913@gandalf.local.home> <20220510181012.d5cba23a2547f14d14f016b9@kernel.org> <20220510104446.6d23b596@gandalf.local.home> <20220511233450.40136cdf6a53eb32cd825be8@kernel.org> <20220511111207.25d1a693@gandalf.local.home> <20220512210231.f9178a98f20a37981b1e89e3@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220512210231.f9178a98f20a37981b1e89e3@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 12, 2022 at 09:02:31PM +0900, Masami Hiramatsu wrote: > On Wed, 11 May 2022 11:12:07 -0400 > Steven Rostedt wrote: > > > On Wed, 11 May 2022 23:34:50 +0900 > > Masami Hiramatsu wrote: > > > > > OK, so fregs::regs will have a subset of pt_regs, and accessibility of > > > the registers depends on the architecture. If we can have a checker like > > > > > > ftrace_regs_exist(fregs, reg_offset) > > > > Or something. I'd have to see the use case. > > > > > > > > kprobe on ftrace or fprobe user (BPF) can filter user's requests. > > > I think I can introduce a flag for kprobes so that user can make a > > > kprobe handler only using a subset of registers. > > > Maybe similar filter code is also needed for BPF 'user space' library > > > because this check must be done when compiling BPF. > > > > Is there any other case without full regs that the user would want anything > > other than the args, stack pointer and instruction pointer? > > For the kprobes APIs/events, yes, it needs to access to the registers > which is used for local variables when probing inside the function body. > However at the function entry, I think almost no use case. (BTW, pstate > is a bit special, that may show the actual processor-level status > (context), so for the debugging, user might want to read it.) As before, if we really need PSTATE we *must* take an exception to get a reliable snapshot (or to alter the value). So I'd really like to split this into two cases: * Where users *really* need PSTATE (or arbitrary GPRs), they use kprobes. That always takes an exception and they can have a complete, real struct pt_regs. * Where users just need to capture a function call boundary, they use ftrace. That uses a trampoline without taking an exception, and they get the minimal set of registers relevant to the function call boundary (which does not include PSTATE or most GPRs). > Thus the BPF use case via fprobes, I think there is no usecase. > My concern is that the BPF may allow user program to access any > field of pt_regs. Thus if the user miss-programmed, they may see > a wrong value (I guess the fregs is not zero-filled) for unsaved > registers. > > > That is, have a flag that says "only_args" or something, that says they > > will only get the registers for arguments, a stack pointer, and the > > instruction pointer (note, the fregs may not have the instruction pointer > > as that is passed to the the caller via the "ip" parameter. If the fregs > > needs that, we can add a "ftrace_regs_set_ip()" before calling the > > callback registered to the fprobe). > > Yes, that is what I'm thinking. If "only_args" flag is set, BPF runtime > must check the user program. And if it finds the program access the > unsaved registers, it should stop executing. > > BTW, "what register is saved" can be determined statically, thus I think > we just need the offset for checking (for fprobe usecase, since it will > set the ftrace_ops flag by itself.) For arm64 I'd like to make this static, and have ftrace *always* capture a minimal set of ftrace_regs, which would be: X0 to X8 inclusive SP PC LR FP Since X0 to X8 + SP is all that we need for arguments and return values (per the calling convention we use), and PC+LR+FP gives us everything we need for unwinding and live patching. I *might* want to add x18 to that when SCS is enabled, but I'm not immediately sure. Thanks, Mark.