From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756193AbcA2OSH (ORCPT ); Fri, 29 Jan 2016 09:18:07 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:59316 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756150AbcA2OSG (ORCPT ); Fri, 29 Jan 2016 09:18:06 -0500 Date: Fri, 29 Jan 2016 15:17:51 +0100 From: Peter Zijlstra To: Andrey Ryabinin Cc: Sasha Levin , Ingo Molnar , Arnaldo Carvalho de Melo , LKML Subject: Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22 Message-ID: <20160129141751.GA407@worktop> References: <5692C531.1030302@oracle.com> <20160119143111.GH6344@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 22, 2016 at 03:48:55PM +0300, Andrey Ryabinin wrote: > 2016-01-19 17:31 GMT+03:00 Peter Zijlstra : > > 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); Ah indeed. Something like so then. When !nr_pages, both variables should be 0 and are due to the kzalloc() of rb slightly above. --- kernel/events/ring_buffer.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index adfdc0536117..345130705c49 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -746,8 +746,10 @@ struct ring_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags) rb->user_page = all_buf; rb->data_pages[0] = all_buf + PAGE_SIZE; - rb->page_order = ilog2(nr_pages); - rb->nr_pages = !!nr_pages; + if (nr_pages) { + rb->nr_pages = 1; + rb->page_order = ilog2(nr_pages); + } ring_buffer_init(rb, watermark, flags);