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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7FD29C433F5 for ; Wed, 25 May 2022 12:19:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=NhL0Cu/b555HUcjsFodz8zVPJuhNb9cbxF54Q7Uy7sg=; b=T/OVRjKYaFFduU Xe9eQd8Pl29W+R2jLs2kc5Erd90oYct4n+19M5tbOeArSGX6VmpbGTcBYXfmBxBVihK4O82uOUOia +eDB61r0qNYbokimBXuLfTHfTz/pQZcfIlBPuD0TEv0ohRmmBYfnDec+2Rs93LOt5r7MddEuR6QaG 6C/ApTv5A0bmo7bfDbPMeo2Poqu4qnO1lTPj6ik0lyCAaTQyMWNCBgYpVClEAyEbVRQIMRsm9w8O/ WatjvIQw4sGVkHA9UOImC86fv2k7NkLQGxsKgi6rjcGnm4qWZ3pT8Kmrvli4MibZChmub0MEeRNmE +S9vvEGBupe9FzcjD+ow==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1ntpxR-00B4if-1X; Wed, 25 May 2022 12:17:45 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1ntpxN-00B4i4-Kw for linux-arm-kernel@lists.infradead.org; Wed, 25 May 2022 12:17:44 +0000 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-Disposition: inline In-Reply-To: <20220512210231.f9178a98f20a37981b1e89e3@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220525_051742_583840_DA7768C3 X-CRM114-Status: GOOD ( 33.97 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel