Re: UBSAN whinge in ihci-hub.c

From	Oliver Neukum <oneukum@suse.com>
Newsgroups	linux.kernel
Subject	Re: UBSAN whinge in ihci-hub.c
Date	2016-05-18 12:30 +0200
Message-ID	<rA6TM-Pd-25@gated-at.bofh.it> (permalink)
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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

Thread

UBSAN whinge in ihci-hub.c Valdis Kletnieks <Valdis.Kletnieks@vt.edu> - 2016-05-18 00:00 +0200
  Re: UBSAN whinge in ihci-hub.c Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-05-18 00:20 +0200
    Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-18 09:50 +0200
      Re: UBSAN whinge in ihci-hub.c Oliver Neukum <oneukum@suse.com> - 2016-05-18 10:30 +0200
        Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-18 11:20 +0200
          Re: UBSAN whinge in ihci-hub.c Oliver Neukum <oneukum@suse.com> - 2016-05-18 12:30 +0200
            Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-18 14:30 +0200
              Re: UBSAN whinge in ihci-hub.c Alan Stern <stern@rowland.harvard.edu> - 2016-05-18 16:50 +0200
                Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-18 17:10 +0200
                Re: UBSAN whinge in ihci-hub.c Alan Stern <stern@rowland.harvard.edu> - 2016-05-18 18:10 +0200
                Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-18 19:20 +0200
                Re: UBSAN whinge in ihci-hub.c Alan Stern <stern@rowland.harvard.edu> - 2016-05-18 21:30 +0200
                Re: UBSAN whinge in ihci-hub.c Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2016-05-19 18:30 +0200
                Re: UBSAN whinge in ihci-hub.c Alan Stern <stern@rowland.harvard.edu> - 2016-05-19 22:20 +0200
              RE: UBSAN whinge in ihci-hub.c David Laight <David.Laight@ACULAB.COM> - 2016-05-23 18:10 +0200

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