From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753383AbcAVMtG (ORCPT <rfc822;w@1wt.eu>);
	Fri, 22 Jan 2016 07:49:06 -0500
Received: from mail-wm0-f50.google.com ([74.125.82.50]:36363 "EHLO
	mail-wm0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753402AbcAVMs4 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 22 Jan 2016 07:48:56 -0500
MIME-Version: 1.0
In-Reply-To: <20160119143111.GH6344@twins.programming.kicks-ass.net>
References: <5692C531.1030302@oracle.com>
	<20160119143111.GH6344@twins.programming.kicks-ass.net>
Date: Fri, 22 Jan 2016 15:48:55 +0300
Message-ID: <CAPAsAGyWy0OqB6+NMfeAyNeGLV8oS9MmKQ=GGNiVwAO6VdkP5g@mail.gmail.com>
Subject: Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22
From: Andrey Ryabinin <ryabinin.a.a@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>, Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

2016-01-19 17:31 GMT+03:00 Peter Zijlstra <peterz@infradead.org>:
> On Sun, Jan 10, 2016 at 03:55:13PM -0500, Sasha Levin wrote:
>> Hi all,
>>
>> While fuzzing with trinity inside a KVM tools guest, running the latest -next
>> kernel, I've hit the following warning:
>>
>> [ 3494.030114] UBSAN: Undefined behaviour in kernel/events/ring_buffer.c:685:22
>> [ 3494.030647] shift exponent -1 is negative
>
> That's rb->page_order == -1, which should 'never' happen, curious!
>

It happens if nr_pages = 0:
     rb->page_order = ilog2(nr_pages);