From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by sourceware.org (Postfix) with ESMTP id BFE7D386181A for ; Wed, 23 Sep 2020 23:51:46 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org BFE7D386181A 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 Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor To: Arvind Sankar Cc: Florian Weimer , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, oleg@redhat.com, x86@kernel.org, libffi-discuss@sourceware.org, luto@kernel.org, David.Laight@ACULAB.COM, mark.rutland@arm.com, mic@digikod.net, pavel@ucw.cz 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-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-17.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, ENV_AND_HDR_SPF_MATCH, NICE_REPLY_A, SPF_HELO_PASS, SPF_PASS, USER_IN_DEF_DKIM_WL, USER_IN_DEF_SPF_WL autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libffi-discuss@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libffi-discuss mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Sep 2020 23:51:48 -0000 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