RE: UBSAN whinge in ihci-hub.c

[David Laight]

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