[BUG?] ppc64le: fentry BPF not triggered after live patch (v6.14)
Hari Bathini
hbathini at linux.ibm.com
Mon Apr 7 18:16:19 AEST 2025
Hi Shung-Hsi Yu
Thanks for reporting this.
On 31/03/25 6:49 pm, Shung-Hsi Yu wrote:
> Hi all,
>
> On ppc64le (v6.14, kernel config attached), I've observed that fentry
> BPF programs stop being invoked after the target kernel function is live
> patched. This occurs regardless of whether the BPF program was attached
> before or after the live patch. I believe fentry/fprobe on ppc64le is
> added with [1].
>
> Steps to reproduce on ppc64le:
> - Use bpftrace (v0.10.0+) to attach a BPF program to cmdline_proc_show
> with fentry (kfunc is the older name bpftrace used for fentry, used
> here for max compatability)
>
> bpftrace -e 'kfunc:cmdline_proc_show { printf("%lld: cmdline_proc_show() called by %s\n", nsecs(), comm) }'
>
> - Run `cat /proc/cmdline` and observe bpftrace output
>
> - Load samples/livepatch/livepatch-sample.ko
>
> - Run `cat /proc/cmdline` again. Observe "this has been live patched" in
> output, but no new bpftrace output.
>
> Note: once the live patching module is disabled through the sysfs interface
> the BPF program invocation is restored.
>
> Is this the expected interaction between fentry BPF and live patching?
> On x86_64 it does _not_ happen, so I'd guess the behavior on ppc64le is
> unintended. Any insights appreciated.
As Naveen updated in another thread already, this behavior is expected
as ppc64le does not handle it. Will take a stab at fixing it.
>
> Thanks,
> Shung-Hsi Yu
>
> 1: https://lore.kernel.org/all/20241030070850.1361304-2-hbathini@linux.ibm.com/
fwiw, the above patch was necessary for fentry, but the support was
complete with:
https://lore.kernel.org/all/20241018173632.277333-18-hbathini@linux.ibm.com/
Thanks
Hari
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