Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22

From	Andrey Ryabinin <ryabinin.a.a@gmail.com>
Newsgroups	linux.kernel
Subject	Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22
Date	2016-01-22 13:50 +0100
Message-ID	<qTJk6-45P-13@gated-at.bofh.it> (permalink)
References	<qPvfI-Ou-11@gated-at.bofh.it> <qSFBU-HG-33@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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);

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Re: perf/ring-buffer: Undefined behaviour in  kernel/events/ring_buffer.c:685:22 Peter Zijlstra <peterz@infradead.org> - 2016-01-19 15:40 +0100
  Re: perf/ring-buffer: Undefined behaviour in  kernel/events/ring_buffer.c:685:22 Sasha Levin <sasha.levin@oracle.com> - 2016-01-22 00:40 +0100
  Re: perf/ring-buffer: Undefined behaviour in kernel/events/ring_buffer.c:685:22 Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-01-22 13:50 +0100

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