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Date:   Thu, 25 Jan 2018 16:30:45 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jiri Slaby <jslaby@suse.cz>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Li Jinyue <lijinyue@huawei.com>, dvhart@infradead.org,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>
Subject: Re: [PATCH 4.14 17/89] futex: Prevent overflow by strengthen input
 validation
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On Thu, Jan 25, 2018 at 04:21:51PM +0100, Jiri Slaby wrote:
> > The same reason it was applied upstream, it fixes a reported
> > issue.
> > 
> > Does that mean that all UBSAN overflow error reports are not valid
> > because of how we build the kernel?
> 
> IMO yes, because with the option, signed overflow is not undefined.
> 
> In the long term, it would be nice to get rid of *all* signed integer
> overflows and kill the compiler option from Makefile. Therefore the
> fixes are indeed very valid in upstream.

I actually think the option is unconditionally good. Undefined behaviour
in a language is bad. Sadly C has lots of it, but any reduction we can
have we must take.