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=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_MUTT 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 11E35C43441 for ; Tue, 27 Nov 2018 20:04:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CC9C92086B for ; Tue, 27 Nov 2018 20:04:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="STNZz9N8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CC9C92086B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=oracle.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726349AbeK1HDa (ORCPT ); Wed, 28 Nov 2018 02:03:30 -0500 Received: from userp2130.oracle.com ([156.151.31.86]:45014 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725752AbeK1HDa (ORCPT ); Wed, 28 Nov 2018 02:03:30 -0500 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.22/8.16.0.22) with SMTP id wARK4Obk097519; Tue, 27 Nov 2018 20:04:24 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to; s=corp-2018-07-02; bh=Tv8Yj5RSWnoOnRoxpKd2+JSOo3EZ7RNg13+xVFIk6Wc=; b=STNZz9N8JoWBlgTyE5Izj7x28/xFOWmnwEsq1++N+/BPzQvzOFDKl0SWodigUeG2WwST +GJmoq3k5BkklVevVKYdNyJxti1P7WZGScZrNVc9tjDpdiFYcEfhw3M7qld/JGSw1fva ip0E6GlW2xkm3FZ0x+OOl4bXtxdfjm9GEF1eURvtJ5VmcQbiyPi2eBPxj9b47GGaY6V+ fwnfWPdxyy0cjaea0SfabufEfjW5GhoBeEJstGfwmQWS6dxCgasAIsgOM3TsUnNfcHf2 qQQSPrIJjVLC82LZ8CzPqj4B9Zfu/jgIjejS4xGtuhIIHHUwA59LntoackTaUoqJapAQ uA== Received: from userv0021.oracle.com (userv0021.oracle.com [156.151.31.71]) by userp2130.oracle.com with ESMTP id 2nxx2u6as1-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 27 Nov 2018 20:04:24 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userv0021.oracle.com (8.14.4/8.14.4) with ESMTP id wARK4IW5010370 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 27 Nov 2018 20:04:19 GMT Received: from abhmp0007.oracle.com (abhmp0007.oracle.com [141.146.116.13]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id wARK4IYc002847; Tue, 27 Nov 2018 20:04:18 GMT Received: from unbuntlaptop (/197.157.0.30) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 27 Nov 2018 12:04:17 -0800 Date: Tue, 27 Nov 2018 23:04:08 +0300 From: Dan Carpenter To: Steven Rostedt Cc: Ingo Molnar , linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org Subject: Re: [PATCH] tracing: Fix an off by one in __next() Message-ID: <20181127200408.GG3073@unbuntlaptop> References: <20180620110758.crunhd5bfep7zuiz@kili.mountain> <20181127134412.6f2141d4@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181127134412.6f2141d4@gandalf.local.home> User-Agent: Mutt/1.9.4 (2018-02-28) X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9090 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1811270171 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 27, 2018 at 01:44:12PM -0500, Steven Rostedt wrote: > > Doing the sweep of my INBOX, I came across this patch (it was sent > while I was in the Alps :-) > > > On Wed, 20 Jun 2018 14:08:00 +0300 > Dan Carpenter wrote: > > > The > should be >= to prevent an off by one bug. > > Well, not really. > > > > > >From reviewing the code, it seems possible for > > stack_trace_max.nr_entries to be set to .max_entries and in that case we > > would be reading one element beyond the end of the stack_dump_trace[] > > array. If it's not set to .max_entries then the bug doesn't affect > > runtime. > > > > Signed-off-by: Dan Carpenter > > > > diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c > > index 4237eba4ef20..6e3edd745c68 100644 > > --- a/kernel/trace/trace_stack.c > > +++ b/kernel/trace/trace_stack.c > > @@ -286,7 +286,7 @@ __next(struct seq_file *m, loff_t *pos) > > { > > long n = *pos - 1; > > > > - if (n > stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX) > > + if (n >= stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX) > > We have: > > static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] = > { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX }; > > And > > struct stack_trace stack_trace_max = { > .max_entries = STACK_TRACE_ENTRIES - 1, > .entries = &stack_dump_trace[0], > }; > > > And nr_entries is set as this, and we have after that this: > > stack_trace_max.nr_entries = x; > for (; x < i; x++) > stack_dump_trace[x] = ULONG_MAX; > > Where we set stack_dump_trace[nr_entries] to ULONG_MAX. > > Thus, nr_entries will not go pass the size of stack_dump_trace. > > That said, if n == nr_entries, the second part of that if will always > be true. And this is a bit subtle, so I will apply the patch. But it is > not an off by one bug ;-) Ah, yes. I follow that now. Thanks for taking the time to review this patch. I am optimistic that eventually I will fix how Smatch handles loops so it maybe will be able to figure out that "x <= STACK_TRACE_ENTRIES - 1" but that's probably some time off. regards, dan carpenter