From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751313AbdAQKnB (ORCPT ); Tue, 17 Jan 2017 05:43:01 -0500 Received: from mail-ve1eur01on0076.outbound.protection.outlook.com ([104.47.1.76]:60671 "EHLO EUR01-VE1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751137AbdAQKm4 (ORCPT ); Tue, 17 Jan 2017 05:42:56 -0500 Authentication-Results: spf=pass (sender IP is 217.140.96.140) smtp.mailfrom=arm.com; gmail.com; dkim=none (message not signed) header.d=none;gmail.com; dmarc=bestguesspass action=none header.from=arm.com; Date: Tue, 17 Jan 2017 10:42:39 +0000 From: Dave P Martin To: PaX Team CC: Kees Cook , Mark Rutland , , Emese Revfy , "AKASHI, Takahiro" , park jinbum , Daniel Micay , , Subject: Re: [PATCH] gcc-plugins: Add structleak for more stack initialization Message-ID: <20170117104239.GG18838@e103592.cambridge.arm.com> References: 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X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v0HAh7Vg004523 On Mon, Jan 16, 2017 at 08:22:24PM +0100, PaX Team wrote: > On 16 Jan 2017 at 11:54, Mark Rutland wrote: [...] > > I assume that this is only guaranteed to initialise fields in a struct, > > and not padding, is that correct? I ask due to the issue described in: > > > > https://lwn.net/Articles/417989/ > > the 'issue' is that before C11 the standard didn't make it clear that in > case of a partial initializer list the compiler has to initialize not only > the remaining fields but also padding as well. Which version of the C11 spec clarifies this? The one I'm looking at (n1570, Apr 12 2011) says: "6.7.9 21 If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration." What is meant by "the remainder of the object" is unclear. Is this just the tail of the object, or all unitialised members -- which may be sparse in the case of an initialiser with designators? And is padding (and if so, which padding) considered to be part of (the remainder of) the object for the purpose of this paragraph? This can be read with the interpretation you suggest, but the wording doesn't seem rock-solid. For the kernel, I guess it's sufficient if GCC commits to this interpretation though. A related, perhaps more interesting issue is that C11 still explicitly states that padding is undefined after assignment: "6.2.6.1 6 When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values. 51)" "51) Thus, for example, structure assignment need not copy any padding bits." which means that in: { struct foo foo1 = FOO1_INTIALIZER; struct foo foo2; foo2 = foo1; } the contents of padding in foo2 is unspecified. Similarly, piecemeal initialisation of an object by assigning to every member leaves the padding unspecified, e.g.: { struct timeval tv; tv.tv_sec = secs; tv.tv_usec = usecs; } (On most or all arches struct timeval contains no padding, but relying implicitly on this is still a bit iffy.) It's highly likely that the kernel passes objects resulting from one of the above to put_user() and friends. It would be good if we had a way to catch at least some of these. (I don't know whether that falls naturally under this plugin.) Cheers ---Dave IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.