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

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Message-ID: <Pine.LNX.4.44L0.1605191609580.1354-100000@iolanthe.rowland.org>
Date:	Thu, 19 May 2016 16:11:02 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>
cc:	Oliver Neukum <oneukum@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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 Thu, 19 May 2016, Andrey Ryabinin wrote:

> 2016-05-18 22:28 GMT+03:00 Alan Stern <stern@...land.harvard.edu>:
> > On Wed, 18 May 2016, Andrey Ryabinin wrote:
> >
> >> 2016-05-18 19:09 GMT+03:00 Alan Stern <stern@...land.harvard.edu>:
> >> > On Wed, 18 May 2016, Andrey Ryabinin wrote:
> >> >
> >> >> 2016-05-18 17:40 GMT+03:00 Alan Stern <stern@...land.harvard.edu>:
> >> >>
> >> >> > All right, I'm getting very tired of all these bug reports.  Besides,
> >> >> > Andrey has a point: Unless you're Linus, arguing against the C standard
> >> >> > is futile.  (Even though the language dialect used in the kernel is not
> >> >> > standard C.)
> >> >> >
> >> >> > Does this patch make UBSAN happy?  The runtime overhead is minimal.
> >> >> >
> >> >>
> >> >> It does. However, you could fool ubsan way more easy:
> >> >>              u32 __iomem     *hostpc_reg = ehci->regs->hostpc +
> >> >> (wIndex & 0xff) - 1;
> >
> > This probably should be considered to be a bug in UBSAN.  It ought to
> > treat pointer addition the same as index addition.
> >
> 
> It's more like a missing feature. UBSAN doesn't guarantee that every possible
> UB will be detected.
> 
> 
> >>
> >> So it silences UBSAN, but still undefined.
> >> I think it's up to you to decide - more code churn or undefined behavior.
> >
> > Well, I don't want the compiler to eliminate code that's necessary.
> >
> > On the other hand, it's not clear how much we need to worry about the
> > standard.  After all, zero-length arrays are a GNU extension to C.
> > Since the array objects in question are defined like this:
> >
> >         u32             port_status[0]; /* up to N_PORTS */
> >
> > it's hard to guess what the compiler will think about out-of-bounds
> > pointer values.
> >
> > Maybe the best thing to do is eliminate the underflow while leaving the
> > calculation unchanged.  What does UBSAN think about this?  Does it
> > dislike -1 as an index value as much as it dislikes -1u?
> 
> Type of doesn't change anything here.

Okay, then I'll go back to the more verbose but guaranteed safe patch.

Thanks for your comments,

Alan Stern