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Date: Wed, 2 Jun 2021 18:01:24 +0200
From: Arnd Bergmann <>
Cc: Markus Wichmann <>, Florian Weimer <>
Subject: Re: Backwards kernel compatibility

On Wed, Jun 2, 2021 at 4:56 PM Rich Felker <> wrote:
> On Wed, Jun 02, 2021 at 01:52:43PM +0200, Arnd Bergmann wrote:
> > >
> > > The main source of overhead comes from the kernel 4.4 which on arm64
> > > produces stack traces when not implemented syscall is invoked:
> > >
> > >
> >
> > That is clearly a bug that was fixed in mainline and backported to linux-4.14
> > but not 4.4 or 4.9. I've sent a manual backport for inclusion in those kernels
> > now.
> Is this practical to hotpatch into kernels on devices that aren't
> readily upgradable?

Including the patch in a source tree is trivial, as it just removes a few lines
of (misguided) output. If you are asking about run-time patching it out of
a running kernel using kpatch/kGraft/ksplice, this would also be doable
by patching out the branch in that function, but the infrastructure for live
patching kernels is likely missing on most of the systems that lack a way to
replace the kernel image, so in practice it would not help.


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