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=INCLUDES_PATCH, MAILING_LIST_MULTI,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 EDE54C43387 for ; Fri, 21 Dec 2018 20:55:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BFA132192C for ; Fri, 21 Dec 2018 20:55:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391883AbeLUUzR (ORCPT ); Fri, 21 Dec 2018 15:55:17 -0500 Received: from mail.kernel.org ([198.145.29.99]:60134 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391870AbeLUUzR (ORCPT ); Fri, 21 Dec 2018 15:55:17 -0500 Received: from gandalf.local.home (cpe-66-24-56-78.stny.res.rr.com [66.24.56.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E99A621929; Fri, 21 Dec 2018 20:55:14 +0000 (UTC) Date: Fri, 21 Dec 2018 15:55:13 -0500 From: Steven Rostedt To: Linus Torvalds Cc: Linux List Kernel Mailing , Ingo Molnar , Andrew Morton , Namhyung Kim , Masami Hiramatsu , Joe Perches , Tom Zanussi , Greg Kroah-Hartman Subject: Re: [for-next][PATCH 23/24] string.h: Add strncmp_prefix() helper macro Message-ID: <20181221155513.11450ca6@gandalf.local.home> In-Reply-To: References: <20181221175618.968519903@goodmis.org> <20181221175659.208858193@goodmis.org> <20181221144054.20bdeb33@gandalf.local.home> <20181221153526.5e6055ca@gandalf.local.home> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 21 Dec 2018 12:41:17 -0800 Linus Torvalds wrote: > Parentheses.... ? -- Steve > > On Fri, Dec 21, 2018, 12:35 Steven Rostedt > > On Fri, 21 Dec 2018 12:01:48 -0800 > > Linus Torvalds wrote: > > > > > On Fri, Dec 21, 2018 at 11:40 AM Steven Rostedt > > wrote: > > > > > > > > OK, what about if we just use strlen() and say that this macro is not > > > > safe for parameters with side effects. > > > > > > I think gcc follows simple assignments just fine, and does the > > > optimized strlen() for them too. > > > > > > So why not just do > > > > > > #define have_prefix(str,prefix) ({ \ > > > const char *__pfx = prefix; \ > > > size_t __pfxlen = strlen(__pfx); \ > > > strncmp(str, __pfx, __pfxlen) ? 0 : __pfxlen); }) > > > > > > and be done with it safely? > > > > > > The above is ENTIRELY untested. > > > > > > > At first I thought this would have issues, but with a slight change... > > > > #define have_prefix(str, prefix) ({ \ > > const char *__pfx = (const char *)prefix; \ > > > > > > And the rest the same, it appears to work. > > > > Need the cast because if for some reason someone passed in something > > like "const unsigned char" then it wouldn't work. But that's just a nit. > > > > So something like this then? > > > > -- Steve > > > > diff --git a/include/linux/string.h b/include/linux/string.h > > index 27d0482e5e05..4586fee60194 100644 > > --- a/include/linux/string.h > > +++ b/include/linux/string.h > > @@ -14,6 +14,28 @@ 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); > > > > +/** > > + * have_prefix - Test if a string has a given prefix > > + * @str: The string to test > > + * @prefix: The string to see if @str starts with > > + * > > + * 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 has_prefix(). > > + * > > + * Returns: 0 if @str does not start with @prefix > > + strlen(@prefix) if @str does start with @prefix > > + */ > > +#define has_prefix(str, prefix) > > \ > > + ({ \ > > + const char *____prefix____ = (const char *)(prefix); \ > > + int ____len____ = strlen(____prefix____); \ > > + strncmp(str, ____prefix____, ____len____) == 0 ? \ > > + ____len____ : 0; \ > > + }) > > + > > /* > > * Include machine specific inline routines > > */ > >