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Subject: Re: [PATCH 4.14 17/89] futex: Prevent overflow by strengthen input
 validation
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Li Jinyue <lijinyue@huawei.com>,
        peterz@infradead.org, dvhart@infradead.org,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>
References: <20180122083954.683903493@linuxfoundation.org>
 <20180122083956.427607580@linuxfoundation.org>
 <20567b49-e0a3-e9e0-d520-74bbfc20d414@suse.cz>
 <alpine.DEB.2.20.1801251502310.2020@nanos>
 <ab877235-a584-1769-a322-f3451bee5ec6@suse.cz>
 <alpine.DEB.2.20.1801251529540.2020@nanos>
 <e08b3b53-9459-35e9-c119-047745603d49@suse.cz>
 <20180125151219.GA16752@kroah.com>
From:   Jiri Slaby <jslaby@suse.cz>
Message-ID: <99cabcaa-8829-d50e-afbc-920d9dbbe903@suse.cz>
Date:   Thu, 25 Jan 2018 16:21:51 +0100
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On 01/25/2018, 04:12 PM, Greg Kroah-Hartman wrote:
> On Thu, Jan 25, 2018 at 03:47:32PM +0100, Jiri Slaby wrote:
>> On 01/25/2018, 03:30 PM, Thomas Gleixner wrote:
>>> So what's the problem?
>>
>> The problem I see is that every stable kernel now requires updated
>> strace with their commit from yesterday to build correctly. In
>> particular, the new stable kernels cause rpm build failures of strace in
>> all our distros (based on those stable kernels). Sure, we can patch
>> strace in every distro every nth kernel update, but it's mere
>> impractical. Kernel should not break userspace, right?
> 
> Well, when userspace is doing something stupid... :)

No doubt... But does that mean we no longer maintain the "no userspace
breakage even if it is stupid" rule?

>> BTW why was the patch applied to stable? We actually do pass
>> -fno-strict-overflow.
> 
> 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.

thanks,
-- 
js
suse labs