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

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Message-ID: <CAPAsAGwEPOtymzf_YSVpNWyKc353R9QD7rhYL7pR6bUBfa=qHg@mail.gmail.com>
Date:	Wed, 18 May 2016 15:21:07 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>, linux-usb@...r.kernel.org,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Subject: Re: UBSAN whinge in ihci-hub.c

2016-05-18 13:19 GMT+03:00 Oliver Neukum <oneukum@...e.com>:
> On Wed, 2016-05-18 at 12:16 +0300, Andrey Ryabinin wrote:
>> 2016-05-18 11:18 GMT+03:00 Oliver Neukum <oneukum@...e.com>:
>> > On Wed, 2016-05-18 at 10:40 +0300, Andrey Ryabinin wrote:
>> >> 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.
>> >
>> > That is a bold statement. Pointer arithmetic is defined. How can
>> > the computation of an address be undefined behavior while it is
>> > not used?
>>
>> It's defined only if pointer points to array element or one-past-end
>> element. Everything else is undefined.
>>
>> $ 6.5.6.8
>>    "If both the pointer operand and the result point to elements of
>> the same array object,
>>      or one past the last element of the array object, the evaluation
>> shall not produce an overflow;
>>      otherwise, the behavior is undefined."
>
> But we do not care whether the calculation overflows. We don't use it
> at all in those cases.
>

This doesn't make it defined. Also that pointer is unused only if gcc
doesn't optimize away '!wIndex' check.
If it does,  we may actually use it.