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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 4C586C43387 for ; Wed, 19 Dec 2018 21:55:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1C82D20869 for ; Wed, 19 Dec 2018 21:55:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729339AbeLSVzC (ORCPT ); Wed, 19 Dec 2018 16:55:02 -0500 Received: from smtprelay0015.hostedemail.com ([216.40.44.15]:60440 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728285AbeLSVzB (ORCPT ); Wed, 19 Dec 2018 16:55:01 -0500 Received: from filter.hostedemail.com (clb03-v110.bra.tucows.net [216.40.38.60]) by smtprelay06.hostedemail.com (Postfix) with ESMTP id B25DF18223269; Wed, 19 Dec 2018 21:55:00 +0000 (UTC) X-Session-Marker: 6A6F6540706572636865732E636F6D X-HE-Tag: beds66_48a583d208a38 X-Filterd-Recvd-Size: 4543 Received: from XPS-9350.home (unknown [47.151.153.53]) (Authenticated sender: joe@perches.com) by omf06.hostedemail.com (Postfix) with ESMTPA; Wed, 19 Dec 2018 21:54:59 +0000 (UTC) Message-ID: Subject: Re: [RFC][PATCH] string.h: Add strncmp_prefix() helper macro From: Joe Perches To: Bryana Rostedt , LKML Cc: Linus Torvalds , Andrew Morton , Ingo Molnar , Tom Zanussi , Greg Kroah-Hartman , Alexander Shishkin Date: Wed, 19 Dec 2018 13:54:57 -0800 In-Reply-To: <20181219163207.792e0f97@gandalf.local.home> References: <20181219163207.792e0f97@gandalf.local.home> Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.30.1-1build1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2018-12-19 at 16:32 -0500, Bryana Rostedt wrote: > A discussion came up in the trace triggers thread about converting a > bunch of: > > strncmp(str, "const", sizeof("const") - 1) > > use cases into a helper macro. It started with: > > #define strncmp_const(str, const) \ > strncmp(str, const, sizeof(const) - 1) > > But then Joe Perches mentioned that if a const is not used, the > sizeof() will be the size of a pointer, which can be bad. And that > gcc will optimize strlen("const") into "sizeof("const") - 1". > > Thinking about this more, a quick grep in the kernel tree found several > (thousands!) of cases that use this construct. A quick grep also > revealed that there's probably several bugs in that use case. Some are > that people forgot the "- 1" (which I found) and others could be that > the constant for the sizeof is different than the constant (although, I > haven't found any of those, but I also didn't look hard). > > I figured the best thing to do is to create a helper macro and place it > into include/linux/string.h. And go around and fix all the open coded > versions of it later. > > I plan on only applying this patch and updating the tracing hooks for > this merge window. And perhaps use it to fix some of the bugs that were > found. > > I was going to just use: > > #define strncmp_prefix(str, prefix) \ > strncmp(str, prefix, strlen(prefix)) > > but then realized that "prefix" is used twice, and will break if > someone does something like: > > strncmp_prefix(str, ptr++); > > So instead I check with __builtin_constant_p() to see if the second > parameter is truly a constant, which I use sizeof() anyway (why bother > gcc to optimize it, if we already know it's a constant), and copy the > parameter into a local variable and use that local variable for the non > constant part (with strlen). > > Link: http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanussi@linux.intel.com > > Signed-off-by: Steven Rostedt (VMware) > --- > diff --git a/include/linux/string.h b/include/linux/string.h > index 27d0482e5e05..3dc743e3a0ba 100644 > --- a/include/linux/string.h > +++ b/include/linux/string.h > @@ -14,6 +14,27 @@ extern void *memdup_user(const void __user *, size_t); > extern void *vmemdup_user(const void __user *, size_t); > extern void *memdup_user_nul(const void __user *, size_t); > > +/* > + * A common way to test a prefix of a string is to do: > + * strncmp(str, prefix, sizeof(prefix) - 1) > + * > + * But this can lead to bugs due to typos, or if prefix is a pointer > + * and not a constant. Instead use strncmp_prefix(). > + */ > +#define strncmp_prefix(str, prefix) \e > + ({ \ > + int ____strcmp_prefix_ret____; \ > + char *____strcmp_prefix____ = prefix; \ This creates a warning and discards any const from prefix when used like static const char foo[] = "bar"; strncmp_prefix(str, foo); > + if (__builtin_constant_p(&prefix)) \ > + ____strcmp_prefix_ret____ = \ > + strncmp(str, prefix, sizeof(prefix) - 1); \ > + else \ > + ____strcmp_prefix_ret____ = \ > + strncmp(str, ____strcmp_prefix____, \ > + strlen(____strcmp_prefix____)); \ > + ____strcmp_prefix_ret____; \ > + }) > + Perhaps #define strncmp_prefix(str, prefix) \ ({ \ typeof(prefix) p = (prefix); \ strncmp(str, p, strlen(p)); \ })