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=-6.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 9D716C2D0A8 for ; Wed, 23 Sep 2020 23:53:12 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3974A2063A for ; Wed, 23 Sep 2020 23:53:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="r2zhV6ER"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="RV5cXfK+" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3974A2063A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=mIDasrtQXbPI5iHoT75SnQLEDRbrhMvBP9SYhPA7Zns=; b=r2zhV6ER8FhrVRjKrqmzMYXm6 sUf8gbaHyJ/K2Uo5LhowvVfDaK9/ukIsTzKqWmVd5HgEhKDIIVGoyK9xNOPpBwIHzqv3HGcP0w6DC BBVH1yz417dC5wAWc0vN1ObTOM30T8p6m9LpGfmKFR1yG/Sj5I9zYWVqR9JmwvFu1HBkXzBqlbEMy DqciqZsMVwb0gr/76/LZUgWyTzhWx8XTZL5DeLkZaqEUHFrD42rcFTYEbYFIuY4ph3sJjEuQIioyu L0oRWJVveqzgxM1mjaFYD8tzOZX1YWWd39bmco8QrLWwdlcu2EvSO+/fVW192sh0R2GkK6Ac1KBvX cgxbaH+5Q==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kLEY9-0000ia-Ad; Wed, 23 Sep 2020 23:51:49 +0000 Received: from linux.microsoft.com ([13.77.154.182]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kLEY6-0000i6-Bc for linux-arm-kernel@lists.infradead.org; Wed, 23 Sep 2020 23:51:47 +0000 Received: from [192.168.254.38] (unknown [47.187.206.220]) by linux.microsoft.com (Postfix) with ESMTPSA id C6A2220B7179; Wed, 23 Sep 2020 16:51:43 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com C6A2220B7179 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1600905104; bh=GqgBG7sD6S7qtNstlI1PIKQbA+siGqVV5q8dqxKHvGg=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=RV5cXfK+8CDkots7pritItEd6mNvifrnS8eLmaxWpuRVt+XnVh2Tz1Us3fIONgAX4 fq5WeRAMXAUcIUU48OhIz2OVF7gsx9JiFKd8W+rta/V3LhcEf/ZVP030TgL2NDuwTs IeJj3m3tJ2yNhp553s/EKSx3XL0QR1MUcpO5F7WI= Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor To: Arvind Sankar References: <20200916150826.5990-1-madvenka@linux.microsoft.com> <87v9gdz01h.fsf@mid.deneb.enyo.de> <96ea02df-4154-5888-1669-f3beeed60b33@linux.microsoft.com> <20200923014616.GA1216401@rani.riverdale.lan> <20200923091125.GB1240819@rani.riverdale.lan> <20200923195147.GA1358246@rani.riverdale.lan> From: "Madhavan T. Venkataraman" Message-ID: <6409d394-9dd2-ae84-88fc-03218515d57d@linux.microsoft.com> Date: Wed, 23 Sep 2020 18:51:42 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200923195147.GA1358246@rani.riverdale.lan> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200923_195146_543306_822015D6 X-CRM114-Status: GOOD ( 24.07 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, pavel@ucw.cz, kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, oleg@redhat.com, mic@digikod.net, linux-security-module@vger.kernel.org, Florian Weimer , luto@kernel.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, David.Laight@ACULAB.COM, libffi-discuss@sourceware.org, linux-arm-kernel@lists.infradead.org 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 9/23/20 2:51 PM, Arvind Sankar wrote: > On Wed, Sep 23, 2020 at 02:17:30PM -0500, Madhavan T. Venkataraman wrote: >> >> >> On 9/23/20 4:11 AM, Arvind Sankar wrote: >>> For libffi, I think the proposed standard trampoline won't actually >>> work, because not all ABIs have two scratch registers available to use >>> as code_reg and data_reg. Eg i386 fastcall only has one, and register >>> has zero scratch registers. I believe 32-bit ARM only has one scratch >>> register as well. >> >> The trampoline is invoked as a function call in the libffi case. Any >> caller saved register can be used as code_reg, can it not? And the >> scratch register is needed only to jump to the code. After that, it >> can be reused for any other purpose. >> >> However, for ARM, you are quite correct. There is only one scratch >> register. This means that I have to provide two types of trampolines: >> >> - If an architecture has enough scratch registers, use the currently >> defined trampoline. >> >> - If the architecture has only one scratch register, but has PC-relative >> data references, then embed the code address at the bottom of the >> trampoline and access it using PC-relative addressing. >> >> Thanks for pointing this out. >> >> Madhavan > > libffi is trying to provide closures with non-standard ABIs as well: the > actual user function is standard ABI, but the closure can be called with > a different ABI. If the closure was created with FFI_REGISTER abi, there > are no registers available for the trampoline to use: EAX, EDX and ECX > contain the first three arguments of the function, and every other > register is callee-save. > > I provided a sample of the kind of trampoline that would be needed in > this case -- it's position-independent and doesn't clobber any registers > at all, and you get 255 trampolines per page. If I take another 16-byte > slot out of the page for the end trampoline that does the actual work, > I'm sure I could even come up with one that can just call a normal C > function, only the return might need special handling depending on the > return type. > > And again, do you actually have any example of an architecture that > cannot run position-independent code? PC-relative addressing is an > implementation detail: the fact that it's available for x86_64 but not > for i386 just makes position-independent code more cumbersome on i386, > but it doesn't make it impossible. For the tiny trampolines here, it > makes almost no difference. > Hi Arvind, I am preparing a response for all of your comments. I will send it out tomorrow. Sorry for the delay. Madhavan _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel