[External] Re: Question - Livepatch/Kprobe Coexistence on Ftrace-enabled Functions (Ubuntu kernel based on Linux stable 5.15.30)

[Song Liu]

On Mon, Oct 20, 2025 at 2:31 PM Andrey Grodzovsky
<andrey.grodzovsky at crowdstrike.com> wrote:
[...]
> Song, I identified another issue in pre 6.6 kernel, building
> ~/linux-6.5/samples/livepatch/livepatch-sample.c as ko,
> before insmoding it, bpftrace fentry/fexit fires as expected, after
> insmod, while no errors reported on attachments,
> the hooks stop firing, both if attaching before insmod and if attaching
> after insmod. If i rrmod the ko, existing hooks
> resume working.
>
> ubuntu at ip-10-10-115-238:~$ cat /proc/version_signature
> Ubuntu 6.5.0-1008.8-aws 6.5.3
> Source obtained to build the test module for the AWS kernel from the
> related stable branch -
> https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.5.tar.xz
>
> Let me know what you think.

I tested various stable kernels. I got:

With livepatch, fentry and fexit work on 6.3 kernels.

On 6.4 and 6.5 kernels, the combination stops working since this commit:

commit 60c8971899f3b34ad24857913c0784dab08962f0
Author: Florent Revest <revest at chromium.org>
Date:   2 years, 7 months ago

    ftrace: Make DIRECT_CALLS work WITH_ARGS and !WITH_REGS


On 6.5 kernels, it got fixed by the following two commits:

commit a8b9cf62ade1bf17261a979fc97e40c2d7842353
Author: Masami Hiramatsu (Google) <mhiramat at kernel.org>
Date: 1 year, 9 months ago
ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default

commit bdbddb109c75365d22ec4826f480c5e75869e1cb
Author: Petr Pavlu <petr.pavlu at suse.com>
Date:   1 year, 8 months ago

    tracing: Fix HAVE_DYNAMIC_FTRACE_WITH_REGS ifdef

I tried to cherry-pick 60c8971899f3b34ad24857913c0784dab08962f0
and a8b9cf62ade1bf17261a979fc97e40c2d7842353, on top of 6.5.13
kernel. Then, fentry and fexit both work with livepatch.

Thanks,
Song