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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,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 06235C4708D for ; Fri, 28 May 2021 14:53:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D9DD4613E3 for ; Fri, 28 May 2021 14:53:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235997AbhE1OzJ (ORCPT ); Fri, 28 May 2021 10:55:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38624 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235415AbhE1OzI (ORCPT ); Fri, 28 May 2021 10:55:08 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 00C92C061574; Fri, 28 May 2021 07:53:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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; bh=eYPDj+LfIhrv+LaA3M9K/3pjJ6DaX5CeFYMC/mFkVBE=; b=gQ1U2K3Xo+QlISz1KdFvyYrZPb 2mqImu2CRR6ObYKJPMvmMOg/7qtwA9DlvVtyqKYsUFY+L3x3eJ4NpTZG66ztCfFhlim23uBIxdXDZ VoxIcoucNkIsQLnwbk8AHiOx4l0MGYhHcjgyhFR6tXvkoO2mPqN1pSeIIyvDT9EfOibSwfoyEhuC5 MISE/nlH1O+MJaoECgujjrTSwn14YwX3+mzTqtXMu3j3Iq1YPbeMdYizz12fUbsh/NiH4TqEMw8SS lfQzgxngIwHJmBqY4CA+Lcd/WAsbCFNn4A1An8SH2nrtg+P24dR+b303UHZOr8EG5ggdyScucqrDQ 6AmUmNYA==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lmdqz-006iup-0i; Fri, 28 May 2021 14:52:49 +0000 Date: Fri, 28 May 2021 15:52:48 +0100 From: Matthew Wilcox To: Justin He Cc: Linus Torvalds , Petr Mladek , Steven Rostedt , Sergey Senozhatsky , Andy Shevchenko , Rasmus Villemoes , Jonathan Corbet , Alexander Viro , Luca Coelho , Kalle Valo , "David S. Miller" , Jakub Kicinski , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Johannes Berg , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-wireless@vger.kernel.org" , "netdev@vger.kernel.org" , "linux-s390@vger.kernel.org" Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for file Message-ID: References: <20210528113951.6225-1-justin.he@arm.com> <20210528113951.6225-3-justin.he@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-wireless@vger.kernel.org On Fri, May 28, 2021 at 02:22:01PM +0000, Justin He wrote: > > On Fri, May 28, 2021 at 07:39:50PM +0800, Jia He wrote: > > > We have '%pD' for printing a filename. It may not be perfect (by > > > default it only prints one component.) > > > > > > As suggested by Linus at [1]: > > > A dentry has a parent, but at the same time, a dentry really does > > > inherently have "one name" (and given just the dentry pointers, you > > > can't show mount-related parenthood, so in many ways the "show just > > > one name" makes sense for "%pd" in ways it doesn't necessarily for > > > "%pD"). But while a dentry arguably has that "one primary component", > > > a _file_ is certainly not exclusively about that last component. > > > > > > Hence "file_dentry_name()" simply shouldn't use "dentry_name()" at all. > > > Despite that shared code origin, and despite that similar letter > > > choice (lower-vs-upper case), a dentry and a file really are very > > > different from a name standpoint. > > > > > > Here stack space is preferred for file_d_path_name() because it is > > > much safer. The stack size 256 is a compromise between stack overflow > > > and too short full path. > > > > How is it "safer"? You already have a buffer passed from the caller. > > Are you saying that d_path_fast() might overrun a really small buffer > > but won't overrun a 256 byte buffer? > No, it won't overrun a 256 byte buf. When the full path size is larger than 256, the p->len is < 0 in prepend_name, and this overrun will be > dectected in extract_string() with "-ENAMETOOLONG". > > Each printk contains 2 vsnprintf. vsnprintf() returns the required size after formatting the string. > 1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get the reserve_size. In this case, the _buf_ could be less than _end_ by design. > 2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to really fill the space. I think you need to explain _that_ in the commit log, not make some nebulous claim of "safer". > If we choose the stack space, it can meet above 2 cases. > > If we pass the parameter like: > p = d_path_fast(path, buf, end - buf); > We need to handle the complicated logic in prepend_name() > I have tried this way in local test, the code logic is very complicated > and not so graceful. > e.g. I need to firstly go through the loop and get the full path size of > that file. And then return reserved_size for that 1st vsnprintf I'm not sure why it's so complicated. p->len records how many bytes are needed for the entire path; can't you just return -p->len ?