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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