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=-8.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=unavailable 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 447B5C433DF for ; Thu, 14 May 2020 00:09:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6AFE7205ED for ; Thu, 14 May 2020 00:09:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733066AbgENAJG (ORCPT ); Wed, 13 May 2020 20:09:06 -0400 Received: from mga03.intel.com ([134.134.136.65]:17408 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732871AbgENAJE (ORCPT ); Wed, 13 May 2020 20:09:04 -0400 IronPort-SDR: Fgv6nAJTFr8zbTKSuaLdqotHV1ZSnv+zkNC8ClukKbJQyDXOV01A6HGGr/z4ybuarxL9uy7u7B W5JY3Pk/6lPQ== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 May 2020 17:09:03 -0700 IronPort-SDR: lhPIPIrnmkX4X+jhi9tu23//ARu70HzakZKd175bTckQmCEPEicCbnJ82WLVhs8UhtedOO2ZQ4 PpMtIYGNj7YA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,389,1583222400"; d="scan'208";a="251434238" Received: from gliber-mobl1.ger.corp.intel.com ([10.249.38.41]) by orsmga007.jf.intel.com with ESMTP; 13 May 2020 17:08:59 -0700 Message-ID: <2a77f3bc9e7ed88343bbd206eaf25f10a697de90.camel@linux.intel.com> Subject: Re: [PATCH v2] tpm: eventlog: Replace zero-length array with flexible-array member From: Jarkko Sakkinen To: Kees Cook , "Gustavo A. R. Silva" Cc: Peter Huewe , Jason Gunthorpe , Arnd Bergmann , Greg Kroah-Hartman , linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org Date: Thu, 14 May 2020 03:08:58 +0300 In-Reply-To: <202005112224.9EFD07F5@keescook> References: <20200508163826.GA768@embeddedor> <202005112224.9EFD07F5@keescook> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.1-2 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2020-05-11 at 22:24 -0700, Kees Cook wrote: > On Fri, May 08, 2020 at 11:38:26AM -0500, Gustavo A. R. Silva wrote: > > The current codebase makes use of the zero-length array language > > extension to the C90 standard, but the preferred mechanism to declare > > variable-length types such as these ones is a flexible array member[1][2], > > introduced in C99: > > > > struct foo { > > int stuff; > > struct boo array[]; > > }; > > > > By making use of the mechanism above, we will get a compiler warning > > in case the flexible array does not occur last in the structure, which > > will help us prevent some kind of undefined behavior bugs from being > > inadvertently introduced[3] to the codebase from now on. > > > > Also, notice that, dynamic memory allocations won't be affected by > > this change: > > > > "Flexible array members have incomplete type, and so the sizeof operator > > may not be applied. As a quirk of the original implementation of > > zero-length arrays, sizeof evaluates to zero."[1] > > > > sizeof(flexible-array-member) triggers a warning because flexible array > > members have incomplete type[1]. There are some instances of code in > > which the sizeof operator is being incorrectly/erroneously applied to > > zero-length arrays and the result is zero. Such instances may be hiding > > some bugs. So, this work (flexible-array member conversions) will also > > help to get completely rid of those sorts of issues. > > > > Also, the following issue shows up due to the flexible-array member > > having incomplete type[4]: > > > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’: > > drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 54 | size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event) > > | ^ > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’: > > drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 102 | sizeof(event_header->event) + event_header->event_size; > > | ^ > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’: > > drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 140 | sizeof(event_header->event) + event_header->event_size; > > | ^ > > scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed > > make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1 > > > > As mentioned above: "Flexible array members have incomplete type, and > > so the sizeof operator may not be applied. As a quirk of the original > > implementation of zero-length arrays, sizeof evaluates to zero."[1] As > > in "sizeof(event_header->event) always evaluated to 0, so removing it > > has no effect". > > > > Lastly, make use of the struct_size() helper to deal with the > > flexible array member and its host structure. > > > > This issue was found with the help of Coccinelle. > > > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > > [2] https://github.com/KSPP/linux/issues/21 > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > > [4] https://github.com/KSPP/linux/issues/43 > > > > Signed-off-by: Gustavo A. R. Silva > > Reviewed-by: Kees Cook Thank you. I applied this patch, will include it to the next PR. /Jarkko