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Subject: Re: Linux 4.9.256
To:     Sasha Levin <sashal@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        torvalds@linux-foundation.org, stable@vger.kernel.org, lwn@lwn.net,
        jslaby@suse.cz
References: <1612535085125226@kroah.com>
 <23a28990-c465-f813-52a4-f7f3db007f9d@scylladb.com>
 <20210208185707.GC4035784@sasha-vm>
From:   Avi Kivity <avi@scylladb.com>
Message-ID: <219a26ec-fd6b-b841-43ef-57e04b417c4e@scylladb.com>
Date:   Tue, 9 Feb 2021 10:53:15 +0200
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On 2/8/21 8:57 PM, Sasha Levin wrote:
> On Mon, Feb 08, 2021 at 05:50:21PM +0200, Avi Kivity wrote:
>> On 05/02/2021 16.26, Greg Kroah-Hartman wrote:
>>> I'm announcing the release of the 4.9.256 kernel.
>>>
>>> This, and the 4.4.256 release are a little bit "different" than normal.
>>>
>>> This contains only 1 patch, just the version bump from .255 to .256 
>>> which ends
>>> up causing the userspace-visable LINUX_VERSION_CODE to behave a bit 
>>> differently
>>> than normal due to the "overflow".
>>>
>>> With this release, KERNEL_VERSION(4, 9, 256) is the same as 
>>> KERNEL_VERSION(4, 10, 0).
>>
>>
>> I think this is a bad idea. Many kernel features can only be 
>> discovered by checking the kernel version. If a feature was 
>> introduced in 4.10, then an application can be tricked into thinking 
>> a 4.9 kernel has it.
>>
>>
>> IMO, better to stop LINUX_VERSION_CODE at 255 and introduce a 
>
> In the upstream (and new -stable fix) we did this part.
>
>> LINUX_VERSION_CODE_IMPROVED that has more bits for patchlevel.
>
> Do you have a usecase where it's actually needed? i.e. userspace that
> checks for -stable patchlevels?
>

Not stable patchlevels, but minors. So a change from 4.9 to 4.10 could 
be harmful.


I have two such examples (not on the 4.9->4.10 boundary), but they test 
the runtime version from uname(), not LINUX_VERSION_CODE, so they would 
be vulnerable to such a change.