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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 31B28C433E0 for ; Mon, 1 Jun 2020 07:05:41 +0000 (UTC) Received: from lists.sourceforge.net (lists.sourceforge.net [216.105.38.7]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0421120678; Mon, 1 Jun 2020 07:05:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=sourceforge.net header.i=@sourceforge.net header.b="ElroVHSc"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=sf.net header.i=@sf.net header.b="CdhNnd7g"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="REe9iTIj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0421120678 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linux-f2fs-devel-bounces@lists.sourceforge.net Received: from [127.0.0.1] (helo=sfs-ml-2.v29.lw.sourceforge.com) by sfs-ml-2.v29.lw.sourceforge.com with esmtp (Exim 4.90_1) (envelope-from ) id 1jfeVw-00024c-K2; Mon, 01 Jun 2020 07:05:40 +0000 Received: from [172.30.20.202] (helo=mx.sourceforge.net) by sfs-ml-2.v29.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jfeVv-00024Q-Aa for linux-f2fs-devel@lists.sourceforge.net; Mon, 01 Jun 2020 07:05:39 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sourceforge.net; s=x; h=In-Reply-To:Content-Type:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=zzZbg6vchb4W1EGHL1ltr5VvzmgvvF0bJQ4NBhDXQzM=; b=ElroVHScYlH55dgoYMWsSCdMZI 8C1KF4bnnobLRDxbGmBFDtteoxVDrqa/BhUVh8F3Bt3RQ60+8puPeAh3SxdDfECNYBFQtsLRjhJTL +jgX7RDGDrGoOXRj5KQ5AagSuIRopL0mZPJwDLKx1T86T/OOfkiltQC2zKYKiK32WPDY=; DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sf.net; s=x ; h=In-Reply-To:Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To :From:Date:Sender:Reply-To:Content-Transfer-Encoding:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Id:List-Help:List-Unsubscribe:List-Subscribe: List-Post:List-Owner:List-Archive; bh=zzZbg6vchb4W1EGHL1ltr5VvzmgvvF0bJQ4NBhDXQzM=; b=CdhNnd7gXQ8ToFNWhmsQV4tFwm 2RcsA/6fNSdmCklYG1Wx2OKoRWLfaV6n3EUlq75e1Y80l5oFnYbx+BLl1bWQ41HhxOrtOs9GjV7ZB kWOkiQtbJYQCam1KddcSgK7YTRVD81rwiUF3buLSNxK1PxFI6SX7RG9Zq9ipSeS/l200=; Received: from mail.kernel.org ([198.145.29.99]) by sfi-mx-4.v28.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.92.2) id 1jfeVp-006C9n-Ds for linux-f2fs-devel@lists.sourceforge.net; Mon, 01 Jun 2020 07:05:39 +0000 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (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 CB03D20678; Mon, 1 Jun 2020 07:05:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590995128; bh=of4R5hmBi9J/EzsQd0OEbsARZdgvjho4xeYJhJNt2Do=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=REe9iTIj0rvPdUZ0fAwCFBL25zQSliEg6NDZXCNWm+GxB7ChH+p6T4g9I3JrG/I29 qQY6A4W6JGi89GzFHUrmUmzoBUmPFfUmnIC3GxnOv3Bb6SVKaZraCbH6JGOIVGVCQE JFlH21cNbzl/CZyF/zoYvoP15LkHIVOpCT2qAfTk= Date: Mon, 1 Jun 2020 00:05:26 -0700 From: Eric Biggers To: Matthew Wilcox Message-ID: <20200601070526.GD11054@sol.localdomain> References: <20200530060216.221456-1-ebiggers@kernel.org> <20200530171814.GD19604@bombadil.infradead.org> <20200530173547.GA12299@sol.localdomain> <20200530204132.GE19604@bombadil.infradead.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200530204132.GE19604@bombadil.infradead.org> X-Headers-End: 1jfeVp-006C9n-Ds Subject: Re: [f2fs-dev] [PATCH] ext4: avoid utf8_strncasecmp() with unstable name X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel Rosenberg , stable@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, Al Viro , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, Gabriel Krisman Bertazi Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net On Sat, May 30, 2020 at 01:41:32PM -0700, Matthew Wilcox wrote: > On Sat, May 30, 2020 at 10:35:47AM -0700, Eric Biggers wrote: > > On Sat, May 30, 2020 at 10:18:14AM -0700, Matthew Wilcox wrote: > > > On Fri, May 29, 2020 at 11:02:16PM -0700, Eric Biggers wrote: > > > > + if (len <= DNAME_INLINE_LEN - 1) { > > > > + unsigned int i; > > > > + > > > > + for (i = 0; i < len; i++) > > > > + strbuf[i] = READ_ONCE(str[i]); > > > > + strbuf[len] = 0; > > > > > > This READ_ONCE is going to force the compiler to use byte accesses. > > > What's wrong with using a plain memcpy()? > > > > > > > It's undefined behavior when the source can be concurrently modified. > > > > Compilers can assume that it's not, and remove the memcpy() (instead just using > > the source data directly) if they can prove that the destination array is never > > modified again before it goes out of scope. > > > > Do you have any suggestions that don't involve undefined behavior? > > void *memcpy_unsafe(void *dst, volatile void *src, __kernel_size_t); > > It can just call memcpy() of course, but the compiler can't reason about > this function because it's not a stdlib function. The compiler can still reason about it if it's in the same file, if it's an inline function, or if link-time-optimization is enabled. (LTO isn't yet supported by the mainline kernel, but people have been working on it.) Also, as I mentioned to Al, it's necessary to cast away 'volatile' to call memcpy(). So the 'volatile' serves no purpose. How about using barrier(), which expands to asm("" : : : "memory") to tell the compiler that memory was clobbered? if (len <= DNAME_INLINE_LEN - 1) { memcpy(strbuf, str, len); strbuf[len] = 0; /* prevent compiler from optimizing out the temporary buffer */ barrier(); } I think it's still technically undefined to call memcpy() on concurrently modified memory at all, but I think the above would be okay in practice... Using 'noinline' could be another option. - Eric _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel