Re: [BUG?] ppc64le: fentry BPF not triggered after live patch (v6.14)

[Hari Bathini <hbathini@linux.ibm.com>]

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