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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 411C0C433E0 for ; Fri, 8 Jan 2021 00:52:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 097BD23447 for ; Fri, 8 Jan 2021 00:52:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729677AbhAHAwQ (ORCPT ); Thu, 7 Jan 2021 19:52:16 -0500 Received: from smtprelay0041.hostedemail.com ([216.40.44.41]:36068 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727695AbhAHAwP (ORCPT ); Thu, 7 Jan 2021 19:52:15 -0500 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay07.hostedemail.com (Postfix) with ESMTP id 57BD9181D337B; Fri, 8 Jan 2021 00:51:34 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: clam92_5d138c7274ee X-Filterd-Recvd-Size: 5431 Received: from [192.168.1.159] (unknown [47.151.137.21]) (Authenticated sender: joe@perches.com) by omf11.hostedemail.com (Postfix) with ESMTPA; Fri, 8 Jan 2021 00:51:32 +0000 (UTC) Message-ID: <9e111f0f673ae6ced12efc01d32eefe8402c7f72.camel@perches.com> Subject: Re: deprecated.rst: deprecated strcpy ? (was: [PATCH] checkpatch: add a new check for strcpy/strlcpy uses) From: Joe Perches To: Kees Cook Cc: Dwaipayan Ray , Jonathan Corbet , linux-kernel-mentees@lists.linuxfoundation.org, linux-kernel , Lukas Bulwahn , Linus Torvalds Date: Thu, 07 Jan 2021 16:51:31 -0800 In-Reply-To: <202101071310.3AC5F0C4@keescook> References: <20210105082303.15310-1-dwaipayanray1@gmail.com> <50cc861121b62b3c1518222f24f679c3f72b868d.camel@perches.com> <3ffe616d8c3fb54833bfc4d86cb73427cf6c7add.camel@perches.com> <202101071310.3AC5F0C4@keescook> Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.38.1-1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2021-01-07 at 13:16 -0800, Kees Cook wrote: > On Tue, Jan 05, 2021 at 01:28:18AM -0800, Joe Perches wrote: > > On Tue, 2021-01-05 at 14:29 +0530, Dwaipayan Ray wrote: > > > On Tue, Jan 5, 2021 at 2:14 PM Joe Perches wrote: > > > > > > > > On Tue, 2021-01-05 at 13:53 +0530, Dwaipayan Ray wrote: > > > > > strcpy() performs no bounds checking on the destination buffer. > > > > > This could result in linear overflows beyond the end of the buffer. > > > > > > > > > > strlcpy() reads the entire source buffer first. This read > > > > > may exceed the destination size limit. This can be both inefficient > > > > > and lead to linear read overflows. > > > > > > > > > > The safe replacement to both of these is to use strscpy() instead. > > > > > Add a new checkpatch warning which alerts the user on finding usage of > > > > > strcpy() or strlcpy(). > > > > > > > > I do not believe that strscpy is preferred over strcpy. > > > > > > > > When the size of the output buffer is known to be larger > > > > than the input, strcpy is faster. > > > > > > > > There are about 2k uses of strcpy. > > > > Is there a use where strcpy use actually matters? > > > > I don't know offhand... > > > > > > > > But I believe compilers do not optimize away the uses of strscpy > > > > to a simple memcpy like they do for strcpy with a const from > > > > > > > >         strcpy(foo, "bar"); > > > > > > > > > > Yes the optimization here definitely helps. So in case the programmer > > > knows that the destination buffer is always larger, then strcpy() should be > > > preferred? I think the documentation might have been too strict about > > > strcpy() uses here: > > > > > > Documentation/process/deprecated.rst: > > > "strcpy() performs no bounds checking on the destination buffer. This > > > could result in linear overflows beyond the end of the buffer, leading to > > > all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various > > > compiler flags help reduce the risk of using this function, there is > > > no good reason to add new uses of this function. The safe replacement > > > is strscpy(),..." > > > > Kees/Jonathan: > > > > Perhaps this text is overly restrictive. > > > > There are ~2k uses of strcpy in the kernel. > > > > About half of these are where the buffer length of foo is known and the > > use is 'strcpy(foo, "bar")' so the compiler converts/optimizes away the > > strcpy to memcpy and may not even put "bar" into the string table. > > > > I believe strscpy uses do not have this optimization. > > > > Is there a case where the runtime costs actually matters? > > I expect so. > > The original goal was to use another helper that worked on static > strings like this. Linus rejected that idea, so we're in a weird place. > I think we could perhaps build a strcpy() replacement that requires > compile-time validated arguments, and to break the build if not. > > i.e. > > given: > char array[8]; > char *ptr; > > allow: > > > strcpy(array, "1234567"); > > disallow: > > strcpy(array, "12345678"); /* too long */ > strcpy(array, src); /* not optimized, so use strscpy? */ > strcpy(ptr, "1234567"); /* unknown destination size */ > strcpy(ptr, src); /* unknown destination size */ I think that's not a good idea as it's not a generic equivalent of the string.h code. I still like the stracpy variant I proposed: https://lore.kernel.org/lkml/24bb53c57767c1c2a8f266c305a670f7@sk2.org/T/#m0627aa770a076af1937cb5c610ed71dab3f1da72 https://lore.kernel.org/lkml/CAHk-=wgqQKoAnhmhGE-2PBFt7oQs9LLAATKbYa573UO=DPBE0Q@mail.gmail.com/ Linus liked a variant he called copy_string: https://lore.kernel.org/lkml/CAHk-=wg8vLmmwTGhXM51NpSWJW8RFEAKoXxG0Hu_Q9Uwbjj8kw@mail.gmail.com/ I think the cocci scripts that convert: strlcpy -> strscpy (only when return value unused) strcpy(array, "string") -> stracpy(foo, "string") s[cn]printf -> sysfs_emit would leave relatively few uses of strcpy and sprintf variants and would make it much easier to analyze the remainder uses for potential overflows.