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 93743C433EF for ; Tue, 31 May 2022 01:00:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242177AbiEaBAN (ORCPT ); Mon, 30 May 2022 21:00:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34058 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238350AbiEaBAJ (ORCPT ); Mon, 30 May 2022 21:00:09 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8490D56C04; Mon, 30 May 2022 18:00:07 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 1D02060F15; Tue, 31 May 2022 01:00:07 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50E11C385B8; Tue, 31 May 2022 01:00:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1653958806; bh=jNUPq/Tf7RZLxM01lVMgFQXyzyWU02Zq99rarwAuvzQ=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=H9giW1zQMpscgulPFUwCl5b84ckvXqhU7BmMSZ35f4dwWJM4XXjWrqB5ZMlaH6RiV 2ybrnwbU96e7unajuXF7G7Nw7kQpbO7rowgaOD/vvuo+ptSBgg5TM1+jyJvEljcuoq twANhFTRPo5JIB7eNqXRKu7bbuGIb9KvpjG7weSytHtV4a1H4UXyfRw2zrWyz0MGg9 oyuCy0WYpemdU5SdNE7SszZRhr+diV6HbccanRZOTX6tCOBiKpdYbcAKDqduoIeIlN yptHxjHgvjHc0uARiI/alXiIS0KMWPkRZNYPUhTmKxx3aS3Zp9PGvDHOJ376+JzTg2 VQ7n1OsU6AcVA== Date: Tue, 31 May 2022 10:00:00 +0900 From: Masami Hiramatsu (Google) To: Jiri Olsa Cc: Mark Rutland , 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, bpf@vger.kernel.org Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines Message-Id: <20220531100000.cbd18c4c08eacb67b95fba5b@kernel.org> In-Reply-To: References: <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> <20220530100310.c22c36df4ea9324cb9cb3515@kernel.org> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 30 May 2022 14:38:31 +0200 Jiri Olsa wrote: > On Mon, May 30, 2022 at 10:03:10AM +0900, Masami Hiramatsu wrote: > > (Cc: BPF ML) > > > > On Wed, 25 May 2022 13:17:30 +0100 > > Mark Rutland wrote: > > > > > 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). > > > > I totally agree with this idea. The x86 is a special case, since the > > -fentry option puts a call on the first instruction of the function entry, > > I had to reuse the ftrace instead of swbp for kprobes. > > But on arm64 (and other RISCs), we can use them properly. > > > > My concern is that the eBPF depends on kprobe (pt_regs) interface, thus > > I need to ask them that it is OK to not accessable to some part of > > pt_regs (especially, PSTATE) if they puts probes on function entry > > with ftrace (fprobe in this case.) > > > > (Jiri and BPF developers) > > Currently fprobe is only enabled on x86 for "multiple kprobes" BPF > > interface, but in the future, it will be enabled on arm64. And at > > that point, it will be only accessible to the regs for function > > arguments. Is that OK for your use case? And will the BPF compiler > > I guess from practical POV registers for arguments and ip should be enough, > but whole pt_regs was already exposed to programs, so people can already use > any of them.. not sure it's good idea to restrict it > > > be able to restrict the user program to access only those registers > > when using the "multiple kprobes"? > > pt-regs pointer is provided to kprobe programs, I guess we could provide copy > of that with just available values Yes, ftrace_regs already provides partial filled pt_regs (which registers are valid is arch-dependent). Thus, my idea is changing fprobe's handler interface to expose ftrace_regs instead of pt_regs, and the BPF handler will extract the internal pt_regs. If the BPF compiler can list which registers will be accessed form the user program, the kernel side can filter it. I think similar feature can be done in the kprobe-event (new fprobe event?). Thank you, -- Masami Hiramatsu (Google)