linux-kernel - Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring

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Message-ID: <CAPAsAGyWy0OqB6+NMfeAyNeGLV8oS9MmKQ=GGNiVwAO6VdkP5g@mail.gmail.com>
Date:	Fri, 22 Jan 2016 15:48:55 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22

2016-01-19 17:31 GMT+03:00 Peter Zijlstra <peterz@...radead.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);