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Message-ID: <CAK8P3a1ADtyVkY8zWd=6KhJLR4eWpyfvDuZMzc_EKp4szC_y7Q@mail.gmail.com>
Date: Wed, 2 Jun 2021 18:01:24 +0200
From: Arnd Bergmann <arnd@...nel.org>
To: musl@...ts.openwall.com
Cc: Markus Wichmann <nullplan@....net>, Florian Weimer <fweimer@...hat.com>
Subject: Re: Backwards kernel compatibility
On Wed, Jun 2, 2021 at 4:56 PM Rich Felker <dalias@...c.org> 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:
> > >
> > > https://github.com/torvalds/linux/blob/afd2ff9b7e1b367172f18ba7f693dfb62bdcb2dc/arch/arm64/kernel/traps.c#L369
> >
> > 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.
Arnd
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