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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D5F4C4597@AcuExch.aculab.com>
Date:	Mon, 23 May 2016 15:58:43 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Andrey Ryabinin' <ryabinin.a.a@...il.com>,
	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" <linux-usb@...r.kernel.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Subject: RE: UBSAN whinge in ihci-hub.c

From: Andrey Ryabinin
> Sent: 18 May 2016 13:21
...
> >> $ 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.

The compiler is allowed to generate the pointer and load it into
a 'pointer register'. On a hardware that has fat pointers and where
the hardware validates the bounds (&foo - 1) can fault.

The most recent hardware that does that is probably a vax.
Although I believe amd64 will fault if you load a suitable invalid
value (not a valid pointer) into the fs/gs offset registers.

	David