From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031606AbeCAO40 (ORCPT ); Thu, 1 Mar 2018 09:56:26 -0500 Received: from mga03.intel.com ([134.134.136.65]:32125 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031346AbeCAO4Z (ORCPT ); Thu, 1 Mar 2018 09:56:25 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.47,408,1515484800"; d="scan'208";a="21323901" Message-ID: <1519916180.10722.317.camel@linux.intel.com> Subject: Re: [PATCH v2 8/9] lib/vsprintf: Remove useless NULL checks From: Andy Shevchenko To: Petr Mladek , Pantelis Antoniou Cc: "Tobin C . Harding" , linux@rasmusvillemoes.dk, Joe Perches , linux-kernel@vger.kernel.org, Andrew Morton , Michal Hocko Date: Thu, 01 Mar 2018 16:56:20 +0200 In-Reply-To: <20180228100437.o4juwxbzomkqjvjx@pathway.suse.cz> References: <20180216210711.79901-1-andriy.shevchenko@linux.intel.com> <20180216210711.79901-8-andriy.shevchenko@linux.intel.com> <20180227155047.o74ohmoyj56up6pa@pathway.suse.cz> <1519752950.10722.231.camel@linux.intel.com> <20180228100437.o4juwxbzomkqjvjx@pathway.suse.cz> Organization: Intel Finland Oy Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.5-1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +Cc: Pantelis, author of %pOF extension (I leave a lot of the message from Petr to give you a bit of context) On Wed, 2018-02-28 at 11:04 +0100, Petr Mladek wrote: > On Tue 2018-02-27 19:35:50, Andy Shevchenko wrote: > > On Tue, 2018-02-27 at 16:50 +0100, Petr Mladek wrote: > > > On Fri 2018-02-16 23:07:10, Andy Shevchenko wrote: > > > > The pointer can't be NULL since it's first what has been done in > > > > the > > > > pointer(). > > > > > > > > Remove useless checks. > > > > > > > > Note we leave check for !CONFIG_HAVE_CLK to make compiler > > > > to optimize code away when possible. > > > > - if (ZERO_OR_NULL_PTR(addr)) > > > > > > This macro matches also values <= 16. > > > > Yes, I know. > > > > This had been discussed with Rasmus and we agreed that printing a > > result > > of kmalloc(0) is rather weird. > > I see > https://lkml.kernel.org/r/1500546142.29303.133.camel@linux.intel.com > There you suggested to move this check into pointer(). But I do not > see any agreement on this. > > > > switch (fmt[1]) { > > > > @@ -1580,9 +1572,6 @@ char *device_node_string(char *buf, char > > > > *end, > > > > struct device_node *dn, > > > > if (!IS_ENABLED(CONFIG_OF)) > > > > return string(buf, end, "(!OF)", spec); > > > > > > > > - if ((unsigned long)dn < PAGE_SIZE) > > > > - return string(buf, end, "(null)", spec); > > > > > > In this case, "null" was printed for ptr < PAGE_SIZE. The same > > > check > > > is also in string() function. > > > > Do we have a uses cases when invalid (non-NULL) pointer is supplied > > to > > print function? > > > > Those call sites have to be fixed. Pantelis, is this check necessary? What are the use cases of node pointer being < PAGE_SIZE? And main question, can it be just (re-)moved to simple NULL check? See below the originate of the PAGE_SIZE check what Petr found. > I am not aware of any. But this patch will make fixing such locations > more complicated. The kernel would crash and might not show any > message. > Is this really what we want? > > Note that it will most likely crash in vprintk_emit() on the line > > text_len = vscnprintf(text, sizeof(textbuf), fmt, args); > > It will be with logbug_lock() taken. The nested printk() messages > will be stored in per-CPU buffer thanks to printk_safe code. > They might eventually be printed by printk_safe_flush_on_panic() > but it is not guaranteed. > > > > > Note that it is not only about the printed value. The pointer is > > > later > > > derefecend. We will start crashing on dn > 0 && dn < PAGE_SIZE. > > > > Yes. > > So, fix the call sites! > > It would be easier if printk() was able to show the message > when hitting this place. > > I did some archaeology. The first check for PAGE_SIZE was added > by the pre-git commit: > > commit 8bcb3ba1dec5749a7f1eed570cb69a20c2e4bd41 > Author: Andrew Morton > Date: Tue Oct 21 18:22:28 2003 -0700 > > [PATCH] make printk more robust with "null" pointers > > Expand printk's traditional handling of null pointers so that > anything in the > first page is considered a null pointer. > > This gives us better behaviour when someone (acpi..) accidentally > prints a > string which is embedded in a struct, the pointer to which is > null. > > > IMHO, it would make sense to hanve this check also pointers that are > being deferred. > > > > > To be honest, I do not feel experienced enough to decide > > > about the preferred behavior. On one hand, it is bad when > > > printk() would crash the kernel. On the other hand, hiding wide > > > range of values under "(null)" string might confuse people. > > > Would it make sense to survive and write different strings for > > > difference intervals? For example? > > > > > > "(null)" for ptr == 0 > > > "(null-16)" for ptr > 0 && ptr <= 16 > > > "(null-pg)" for prt > 16 && ptr <= PAGE_SIZE > > > > > > In each case, this patch changes the behavior and it should > > > be documented in the commit message. > > > > Personally I strongly disagree with blowing code up in such places > > for > > little or none benefit. > > I do not have strong opinion here. I could imagine that this might > save a day to some people. But I have never encountered such a bug > myself. > > To make it clear. Your clean up work makes sense. I just want to point > out that this patch is not as innocent as the commit message suggest. > Also I think that it goes in the wrong direction regarding the > ability to show useful information in a buggy situation. -- Andy Shevchenko Intel Finland Oy