linux-kernel - Re: time: signed integer overflow in ktime_add

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Message-ID: <56618BC5.6020400@oracle.com>
Date:	Fri, 4 Dec 2015 07:49:09 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: time: signed integer overflow in ktime_add_safe

On 12/04/2015 06:49 AM, Dmitry Vyukov wrote:
>> I'm not so sure. It finds real bugs, e.g. 32a8df4e0b33f ("sched: Fix odd values in effective_load() calculations")
>> > was caught by UBSAN
>> > I guess that we could fix most signed overflows simply by casting to unsigned type.
> 
> Yeah, overflows can be just unexpected in some places (not an intended
> reliance on defined overflow). If we want to continue finding real
> bugs, we need to start fixing the false positives.

Right, this check finds real bugs, but quite a few false positives because
we tell gcc specifically that this overflow is really okay, and people have
relied on that.

On the other hand, we can't "fix" the code that's triggering it since nothing
is actually broken - in ktime_add_safe() for example we'd overflow, but then
we're checking for overflow, so the code is perfectly fine.

Maybe UBSAN annotations are the way to go here?


Thanks,
Sasha
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