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

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Date:	Wed, 18 May 2016 10:18:27 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.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

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?

	Regards
		Oliver