stable: KASan for ARM

[Sasha Levin]

On Mon, Mar 08, 2021 at 12:34:39AM +0100, Michał Mirosław wrote:
>On Sun, Mar 07, 2021 at 10:48:54PM +0000, Russell King - ARM Linux admin wrote:
>> On Sun, Mar 07, 2021 at 05:10:43PM +0100, Ard Biesheuvel wrote:
>> > (+ Russell)
>> >
>> > On Sun, 7 Mar 2021 at 16:21, Greg Kroah-Hartman
>> > <gregkh at linuxfoundation.org> wrote:
>> > >
>> > > On Sun, Mar 07, 2021 at 04:00:40PM +0100, Michał Mirosław wrote:
>> > > > Dear Greg,
>> > > >
>> > > > Would you consider KASan for ARM patches for LTS (5.10) kernel? Those
>> > > > are 7a1be318f579..421015713b30 if I understand correctly. They are
>> > > > not normal stable material, but I think they will help tremendously in
>> > > > discovering kernel bugs on 32-bit ARMs.
>> > >
>> > > Looks like a new feature to me, right?
>> > >
>> > > How many patches, and have you tested them?  If so, submit them as a
>> > > patch series and we can review them, but if this is a new feature, it
>> > > does not meet the stable kernel rules.
>> > >
>> > > And why not just use 5.11 or newer for discovering kernel bugs?  Why
>> > > does 5.10 matter here?
>> >
>> > The KASan support was rather tricky to get right, so I don't think
>> > this is suitable for stable. The range 7a1be318f579..421015713b30 is
>> > definitely not complete (we'd need at least
>> > e9a2f8b599d0bc22a1b13e69527246ac39c697b4 and
>> > 10fce53c0ef8f6e79115c3d9e0d7ea1338c3fa37 as well), and the intrusive
>> > nature of those changes means they are definitely not appropriate as
>> > stable backports.
>>
>> I agree - it took quite a while for KASan to settle down - and our last
>> issue with KASan causing a panic in the Kprobes codes was in February.
>> So, I think at the very least, requesting to backport this so soon is
>> premature. That fix is not included even in what you mention above.
>> Maybe that fix has already been picked up in stable, I don't know.
>>
>> So, we know that there's probably more to getting kprobes working on
>> 32-bit ARM than even you've mentioned above.
>>
>> Is it worth backporting such a major feature to stable kernels? Or
>> would it be better to backport the fixes found by KASan from later
>> kernels? My feeling is the latter is the better all round approach.
>
>I guessed that KASan support code does not pose problems with
>CONFIG_KASAN=n.  If it does, then I understand that this is definitely
>a deal-breaker for stable, and I agree there is no point in further
>discussion. But, if in disabled state KASan patches meet the stable
>requirements, then maybe it is worth the trouble to help those who
>have to stay on a LTS kernel?

Following this logic nearly every upstream feature can be backported to
stable.

You can't have both worlds; you can't stay on an older "stable" tree
while still benefiting from new features that land upstream.

-- 
Thanks,
Sasha