linux-kernel - Re: UBSAN whinge in ihci-hub.c

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Message-ID: <CAPAsAGxMnbpzuDGLOLr48qqJyU9GGC1_2TAOr9oA_F7jaupk3w@mail.gmail.com>
Date:	Wed, 18 May 2016 10:40:33 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>, linux-usb@...r.kernel.org
Subject: Re: UBSAN whinge in ihci-hub.c

2016-05-18 1:16 GMT+03:00 Greg Kroah-Hartman <gregkh@...uxfoundation.org>:
> On Tue, May 17, 2016 at 05:52:40PM -0400, Valdis Kletnieks wrote:
>> So, not content in the amount of breakage I generate already, I
>> compiled with UBSAN enabled...
>>
>> The immediately relevant part:
>>
>> [    2.418576] ================================================================================
>> [    2.418579] UBSAN: Undefined behaviour in drivers/usb/host/ehci-hub.c:877:47
>> [    2.418582] index -1 is out of range for type 'u32 [1]'
>
> <snip>
>
> It's a known bug in ubsan,

It's not a bug.  int *p = &a[-1] is undefined behavior. It doesn't
matter whether that pointer dereferenced or not.