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Subject: Re: UBSAN whinge in ihci-hub.c
From: Oliver Neukum <oneukum@suse.com>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        LKML <linux-kernel@vger.kernel.org>, linux-usb@vger.kernel.org,
        Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Date: Wed, 18 May 2016 12:19:07 +0200
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On Wed, 2016-05-18 at 12:16 +0300, Andrey Ryabinin wrote:
> 2016-05-18 11:18 GMT+03:00 Oliver Neukum <oneukum@suse.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@linuxfoundation.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.

	Regards
		Oliver