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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,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 5E5F2C43387 for ; Thu, 20 Dec 2018 02:16:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2F25920989 for ; Thu, 20 Dec 2018 02:16:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729976AbeLTCQT (ORCPT ); Wed, 19 Dec 2018 21:16:19 -0500 Received: from mail.kernel.org ([198.145.29.99]:48940 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726604AbeLTCQS (ORCPT ); Wed, 19 Dec 2018 21:16:18 -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 98A9920578; Thu, 20 Dec 2018 02:16:16 +0000 (UTC) Date: Wed, 19 Dec 2018 21:16:15 -0500 From: Steven Rostedt To: LKML Cc: Linus Torvalds , Andrew Morton , Ingo Molnar , Tom Zanussi , Greg Kroah-Hartman , Alexander Shishkin Subject: [RFC][PATCH v2] string.h: Add strncmp_prefix() helper macro Message-ID: <20181219211615.2298e781@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 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) --- Changes since v1: - Sending this time as Bryana's dad - Moved the assignment of the non constant into the non constant block (Pointed out by Joe Perches) - Added "typeof(prefix)" to define the non constant variable (Suggested by Joe Perches) diff --git a/include/linux/string.h b/include/linux/string.h index 27d0482e5e05..a998a53333ef 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); +/* + * 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) \ + ({ \ + int ____strcmp_prefix_ret____; \ + if (__builtin_constant_p(&prefix)) { \ + ____strcmp_prefix_ret____ = \ + strncmp(str, prefix, sizeof(prefix) - 1); \ + } else { \ + typeof(prefix) ____strcmp_prefix____ = prefix; \ + ____strcmp_prefix_ret____ = \ + strncmp(str, ____strcmp_prefix____, \ + strlen(____strcmp_prefix____)); \ + } \ + ____strcmp_prefix_ret____; \ + }) + /* * Include machine specific inline routines */